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Trojan Mini-Motor

Mini-Motor Ltd notepaper heading


Mark I - brochure


The story begins in Italy, for our purposes, in 1946.  Vincenti Piatti had designed a 50cc engine unit for driving portable lathes and also foresaw the possibilities of this engine power-assisting a bicycle - the Mini Motore.  Italy was suffering the results of six years of war and there was a huge demand for cheap personal transport, fuel being scarce and expensive.

In 1948, George Murray Denton was holidaying in Switzerland and, as he recalled in an article in Classic Bike in 1985, "...I followed a cyclist who not only free-wheeled downhill but shot uphill with no loss of speed and with only occasional pedalling.  There was a device behind the saddle which emitted blue smoke and a noise like a bored-out wasp.  It was an Italian Mini Motore." On his return to England, George found that Trojan of Croydon - remember their two-stroke Brooke Bond Tea vans? - were making them under licence for sale in the UK.  George later became Sales Manager of Trojan and reckoned that, by the time sales were overtaken by the more technically advanced Cyclemaster in 1951, over 100,000 must have been sold.

Mark II - brochure

The Mini-Motor opened up a whole new market in post-war European cycle attachments.  So much so that, in a four page review of cyclemotor attachments in the December 14th 1950 issue of The Motor Cycle, no fewer than ten were on the market with an eleventh due to go on sale the next spring.  The Mini-Motor's weight was quoted as 22lbs, tank capacity 3 pints, price £21.  Roy Morton was present at the "Box Hill Trials" in February 1951 and he summed up his views thus: "These little units take all the work out of cycling, leaving only the pleasure".  The Mini-Motor was as revolutionary in its day as the NSU Quickly would become a few years later.

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Other Mini-Motors

Trojan produced the Mini-Motor in several guises.  As well as the cyclemotor unit, it was sold as a small industrial engine too; there was also a "Mini-Mower" - a bolt-on attachment that converted lawn mowers to power operation - and an outboard motor for small boats.  The industrial version of the Mini-Motor was usually mounted on a steel stand with a rectangular petrol tank above the engine.  Several of these engines had fan-cooling with blades on the flywheel directing air through an aluminium alloy casing that covered the magneto side of the engine.

Mini-Mower press advertisement

A 75cc version of the engine was also produced.  This was advertised as being suitable for powering carrier tricycles.

Trojan made a half-size non-working model of the engine too.  This was used by their salesmen to demonstrate the engine to prospective agents.

The Trojan was not the only Mini-Motor.  Production of the Italian-built machine continued and another version of the engine was made in France.  British and Italian machines looked identical but the French model used a different design of tank, considerably altering its appearence.

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Although the Mini-Motor cycle unit was changed several times throughout its production; its basic design remained the same.  The backbone of the device was the fuel tank.  The tank had a hinge at the front; clamped to this was a rod which, in turn, clamped to the cycle's seat pillar.  A "U" shaped steel hoop fitted over the cycle's rear wheel.  The two prongs of the "U" were secured to the ends of the wheel spindle.  The rear of the fuel tank was attached to this hoop by a mechanism which would raise and lower the tank.  The engine itself was fitted below the tank with the cylinder facing forwards.  A friction roller was mounted directly on to the crankshaft.  When the tank was lowered, this roller would press onto the top of the tyre tread and would drive the cycle forward.  Apart from some pre-production units which were permanently held in contact with the tyre by spring pressure, all subsequent Mini-Motors had some form of lifting/lowering mechanism.  This was usually controlled by a trigger or ratchet lever on the handlebar.  A more unusual version was controlled by a lever to the rear of the machine (If this was supplied, the handlebar lever and cable were not fitted). When the rear lever was fitted the rider had to dismount and raise or lower his cyclemotor while stationary. Strangely, Mini-Motor instruction books printed from December 1949 onwards contain no reference to this rear lever mechanism although there are is photographic evidence that some machines did use it.

Yet another mechanism was fitted to the 75cc carrier tricycle version.  In this case the engine was hingeded at the rear and a long lever raised and lowere the front of the engine.

Original MkI hoop

Several of the design changes made to the Mini-Motor concerned the hoop and the raising & lowering mechanism.  The earliest machines had a hoop in one piece.  This was slotted at its lower end where it fitted over the cycle axle, thus allowing for adjustment to different sizes of cycle wheel.  A rod at the upper end passed between two guide rollers.  The engine crankcase was bolted to the tank with extra-long studs and the rollers were mounted on these studs.  A tension spring pulled the tank down, pressing the drive roller onto the tyre.  The drive could be disenganged by lifting the tank using a handlebar lever and Bowden cable control.  Very few of these early pattern Mini-Motors have survived in their original condition.  As Trojan made each improvement to the design, a conversion kit would also be produced so that owners of older machines could also benefit from the improvements.

Second MkI hoop

The use of a tension spring to hold the roller onto the tyre was found to be unsatisfactory.  The engine unit was liable to bounce up and down by a small amount on uneven road surfaces.  This caused variations in the roller contact pressure and the drive would slip leading to not only loss of driving power, but also damage to the tyre.  An early improvement was to reverse the mechanism so that the cable control pulled the engine down onto the tyre.  A compression spring would lift the engine when the drive was disengaged.

Third MkI hoop

The next improvement in this area was to the hoop.  The new design was bolted together instead of being in one piece; the length of the hoop could be varied with the adjustable end plates.  Because the new hoop was adjusted at the endplates instead of at the rear wheel spindle; the rear wheel of the cycle could be removed, to change a tyre for instance, without having to readjust everything when it was replaced.  The next inprovement was to mount the guide rod on a sector plate so that its angle could be changed.  This made fitting the Mini-Motor easier because it provided a greater degree of adjustment.  The original hoop could be awkward to fit on some cycles, especially if one of the tall pre-war style of roadster bicycles was being used.

Late MkI rear hoopMkII rear hoop

Finally, although the new raising and lowering mechanism was a great improvement on the old one, it still had one minor problem.  If the cycle was being used with the engine disengaged the roller could sometime be bounced into contact with the tyre, particularly if luggage was being carried on the top of the tank.  A new toggle-action mechanism was provided that provided greater support to the tank in the raised position.  Since it no longer used guide rollers on the crankcase studs, this mechanism also paved the way to impovements in engine mounting.

The early models (Mark I & Mark II) had the engine mounted at two points: a single bolt at the top that fitted into a boss cast into the cylinder block and the two studs at the rear of the crankcase.  From the Mark III onwards the engine was held beween sideplates that fitted over the crankcase studs.

There were at least four different designs of handlebar lever used to controlled the engine engagement mechanism.  The various possible combinations of handlebar lever and drive control mechanism meant that a huge variety of different control cables were made.  The fact that conversion kits for the drive control were available exacerbated this problem since a machine with an early lever could have been changed to a later drive control.  In addition, cables were also available in extra long versions for tandems.

The friction drive roller was also subjected to change.  The first models, like most other makes of friction drive cyclemotor, had a roller with deep, straight-cut teeth.  A carborundum surfaced roller was then introduced.  This gave much better grip on the tyre, especially in the wet. However, it also increased the rate of tyre wear.  Finally a steel roller with shallow curved teeth in a herringbone pattern was adopted.

The flywheel magneto fitted to the Trojan Mini-Motor was either a Miller or a Wico-Pacy Bantamag.  Engines equipped with the Miller magneto were identified by stamping a letter "Z" as a suffix to the engine number.

Cylinder assembly

The Mark I Mini-Motors were not originally equipped with decompressors and this could cause problems with starting, especially in the wet.  The roller did not always have enough grip on the tyre to overcome the engine compression.  A special technique was needed to get the engine going.  This was to first engage the roller on the tyre and gently wheel the cycle backwards until the engine compression started to resist.  Next, disengage the roller, mount the cycle and pedal off.  Then engage the roller.  The crankshaft then had to make almost a complete revolution before it hit compression, by which time the combined forces of flywheel inertia and the drive from the tyre were enough to force it over top dead centre.  The introduction of a decompressor on later models made it possible to use a more conventional starting procedure.  Evidently, the decompressor was optional for a while and some Mini-Motors were produced with the cylinder head machined to take one, but with a blanking plug fitted instead.  Again, conversion kits were produced for improving the earlier models

Crankshaft assembly

Although some Trojan publicity referred to a new stronger crankshaft on the Mark II, the only change made was to drill and peg the crank pin to the crank webs.  This change was made to prevent over-enthusiastic mechanics from twisting the crank when changing the roller.

Fuel tanks all had the same basic shape although the change of engine mounting on the Mark III meant changes to the tank brackets.  Early tanks were brazed, later ones were welded.  Some tanks were provided with loops underneath so that luggage could be carried on the tank, held by straps or "aerolastics".  The tanks, up to and including the Mark III, were nearly always painted blue.  At least one machine was finished in white in 1951 and used as a demonstrator and show exhibit.  It is also possible that some were finished in black.  When the Mark V was introduced, it had a new design of tank: it looked basically the same shape but a little more rounded.  The major difference in the Mark V fuel tank was that it was no longer load-bearing.  The engine was mounted on a pressed steel sub-frame and the tank was held onto this frame by three studs.  The mountings for the rear number plate were moved off the back of the tank to brackets on the hoop and the standard colour changed from blue to polychromatic gold.

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The Factory

The Mini-Motor assembly line was on the north side of the main Trojan factory, running west to east.  The production area was about 50 foot square whilst the assembly and test area was about 12 feet wide and about 100 feet long.  Items like magnetos, sparking plugs, carburettors, ball races and piston rings were bought in but the rest of the parts were made on site.


Pistons were first machined on the inside of the skirt and the bottom edge was faced.  Next, locating on to the machines by the inside skirt, the gudgeon pin hole was drilled, then reamed.  Thirdly, the piston was mounted on a fixture in a lathe and was held by a draw bar through the gudgeon pin hole; thus enabling the outside, the ring grooves and the crown to be turned to size, using paraffin as the coolant.  Pin holes were drilled and pins fitted to stop the two rings turning in their grooves.

The crankshaft assembly was made up of four main parts: the magneto side crankshaft, the driving side crankshaft, the crank pin and the con rod.  The two crankshaft halves were pressed onto the crank pin with the roller bearings and con rod in position.  The next operation was to grind the places where the main ball races went on the assembly to the correct diameter and the correct distance between the faces, thus maintaining a fairly tight tolerance.  The crankcase halves were machined in the normal way; so too the cylinders, which were bored and honed.  All cylinders were bored and honed at the Trojan factory and there was a small section that did nothing else.

Moving on to the assembly line itself, studs were inserted using a hand stud insertion tool and the engines were assembled in much the same way.

There was a test rig against the north wall of the building that contained positions for twelve engines.  An engine was put in the fixture and turned by electric motor. After a 20 minute running-in period, the sparking plug was fitted, petrol/oil mixture was turned on to the carburettor and the electric motor was used to start the engine.  The exhaust was connected to an extraction pipe.  After a successful running-in period the engine was removed from the test bed, cleaned and the fuel tank bolted on or, if it was an industrial unit, the frame of whatever the customer had ordered was fitted.  The completed unit was then passed to Stores ready for dispatch.

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Appendix 1 - Summary of model changes

Mark I

Mark II


Mark III

Herringbone roller
Works photo of the herringbone roller

Mark IV

Mini-Motor MkIV
Mini-Motor MkIV
(detail taken from works photo)

Mark V

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Appendix 2 - Engine Numbers

As our club records of surviving machines show, production numbers ran approximately as follows:

Dates* Model Engine number range of known survivors
1949-1950 MkI A193 to A10396
1950-1951 MkII B10575 to B44278Z
1951-1953 MkIII C44604 to C68919
1953-1955 MkV F70806 to F73741
1955-1957 MkV G74390Z to G74434Z, H74807Z
[* Date is first registration date, not date of manufacture]

The suffix letter Z denotes that a Miller flywheel magneto was supplied as original equipment, rather than the alternative Wico-Pacy Bantamag.  Engines with other prefix letters (e.g. "E") were industrial units.

Mark V - brochure

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Appendix 3 - Parts List

Number Description Number Description
4 Crankshaft distance sleeve 5 Steel roller
10 Cylinder base gasket 11 Cylinder block
13 Little end bush 14 Gudgeon pin, std
15 Piston, std 16 Piston ring, std
17 Cylinder head 19 Inlet manifold gasket
20 Inlet manifold (early model) 20A Inlet manifold
21 Exhaust manifold gasket 22 Exhaust manifold
23 Exhaust silencer gasket 24 Exhaust silencer body
25 Exhaust silencer, perforated tube 26 Exhaust silencer, packing
28 Main bearing 29 Crankshaft seal, roller side
30 Roller Washer, 1/2" std flat 31 Roller Nut, 1/2" BSF
32 Washer, plain, 5/16" 34 Washer, plain, 1/4"
35 Screw, exhaust silencer, 1BA × 9/16" cheese head 36 Piston ring peg
38 Washer, spring, 1/4" 39 Cylinder base stud
40 Nut, 1/4" BSF 41 Manifold stud, 1/4"
42 2BA x 9/16" cheese head screw 43 Washer, spring, 2BA
44 Roller guard screw, 1BA × 3/8" cheese head 45 Gudgeon pin circlip
46 Cylinder head stud 47 Sparking plug (Lodge C14)
48 Long stud, rear engine mounting, MkI 49 Roller washer, fibre
50 Crankshaft seal, magneto side 51 Tank pivot pinch bolt
57 Front mounting plate bolt 58 Cylinder head gasket
59 Drive control pinch bolt 61 Cable stop, early MkI drive control
62 Drive control roller (MkI) 64 Front mounting plate, short
65 Front mounting plate, long 60 Set screw
68 Saddle pillar attachment 68 Clip for saddle pillar attachment
70 Tank pivot block 71 Tank pivot bush
74 Tank pivot bolt 75 Drive control spring, early MkI
78 Cable adjuster, drive control 81 Nut 5/16" BSF
85 Spring washer, 1BA 86 Nut, decompressor, 2BA
88 Cable adjuster locknut, drive control, 1/4" BSF 89 Woodruffe key, 3/8" × 1/6"
90 Filler cap washer, cork or leather 91 Petrol tap washer, fibre
92 Flywheel washer, 5/16" spring 93 2BA x 7/8" cheese head screw
94 Drive control roller washer, 2BA plain 100 Engine mounting spacer, fibre
101 Engine mounting washer, 21/64" fibre 102 Rear engine mounting washer, 17/32" fibre
103 Lower plate, top engine mounting 106 Trojan head transfer
107 Mini-Motor transfer 108 Box spanner, roller nut/plug
109 1BA × 3/4" cheese head screw 113 Spring Washer
114 Circlip 116A Compression spring, carburettor pump plunger
117 Air cleaner stop 119 Carburettor fixing screw
122 Plug for carburettor pump 123 Mixing chamber cover
124 Carburettor fixing screw 125 Ring
126 Thottle guide screw 130 carburettor cover
131 Air filter screw 132 Throttle slide
133 Air cleaner cover 124 Air cleaner disc
135 Carburettor body 138 Carburettor pump piston
139 Main jet No 43 142 Plug for filter elbow
144 Banjo union base washer 145 Main jet washer
146 Banjo union top washer 147 Air cleaner filter
148 Carburettor gasket 148 Carburettor pump base washer
152 Throttle cable adjusting screw 153 Carburettor sump screw
154 Carburettor sump washer 155 Cable abutment
156 Solderless nipple 161 Rear number plate (MkI to Mk III)
165 Body, drive control lever, 1st type 166 Lever, drive control lever, 1st type
167 Trigger, drive control lever, 1st type 168 Handlebar clamp, drive control lever, 1st type
169 Trigger spring, drive control lever, 1st type 170 Cable outer stop, drive control lever, 1st type
171 Trigger pivot, drive control lever, 1st type 172 Clamp nut, drive control lever, 1st type
173 Clamp bolt, drive control lever, 1st type 174 Pivot bolt, drive control lever, 1st type
175 Pivot nut, drive control lever, 1st type 176 Spring washer, drive control lever, 1st type
177 Drive control cable outer 178 Drive control cable inner
179 Cable ferrule, drive control 181 Barrel nipple, drive control
182 Tube nipple (fixed), drive control 185 Body, throttle lever
186 Lever, throttle lever 187 Top plate, throttle lever
188 Handlebar clamp, throttle lever 189 Small washer, throttle lever
190 Large washer, throttle lever 191 Spring washer, throttle lever
192 Central bolt, throttle lever 193 Handlebar clamp bolts, throttle lever
194 Throttle cable outer 195 Throttle cable inner
196 Cable ferrule, throttle 197 Barrel nipple, throttle
198 Tube nipple, throttle 206 Washer
207 End stop, early MkI drive control 208 Cylinder head bolt
209 Crankcase bolt 210 Front mounting arm
211 Front mounting clamp 212 Rear engine mounting nut, 5/16" BSF
218 Front mounting bolt 214 Hub bolt
215 Hub screw 219 Special washer
220 Special washer 221 Locking washer, top engine mounting
222 Setscrew, top engine mounting 232 Cylinder head
233 Cylinder 234 Roller spanner
235 Drive control roller nut, 2BA 236 Crankcase gasket (studded type)
237 Crankcase stud 239 Engine mounting washer, plain
240 Side plate 241 Washer
242 Bolt 243 Drive control bracket bolt, MkIII
244 Filter holder 245 Filter gauze
246 Filter spring 247 Sediment plug for carburettor
250 Gudgeon pin, oversize 9/16" (14.288mm) 251 Piston, +.020"
252 Piston, +.040" 253 Piston ring, +.020"
254 Piston ring, +.040" 256 Washer for sediment plug
257 Thrust collar 258 5/16" Thackeray spring washer
259 Cable stop, later MkI drive control 260 Drive control spring, later MkI
261 Retaining plate, later MkI drive control 262 Thrust washer, later MkI drive control
263 End stop, later MkI drive control 265 Lever, drive control, 2nd type
266 Trigger, drive control lever, 2nd type 268 Cable inner stop, drive ctl lever 2nd type (screw nipple)
269 Screwed nipple outer, drive control 270 Screwed nipple inner, drive control
271 Handlebar clamp, drive control lever, 2nd type 272 Trigger bush, drive control lever, 2nd type
273 Pivot bolt, drive control lever, 2nd type 274 2BA plated nut, drive control lever, 2nd type
275 Bolt, drive control lever, 2nd type 277 Trigger spring, drive control lever, 2nd type
278 Cable outer stop, drive control lever, 3rd type 279 Decompressor body
280 Decompressor valve 281 Decompressor cable stop
282 Decompressor spring 294 Main jet, size 39
295 Main jet, size 40 296 Main jet, size 41
297 Main jet, size 42 298 Main jet, size 44
299 Main jet, size 45 300 Main jet, size 46
301 Main jet, size 47 302 Coil & core group (Wico)
303 Coil core fixing screw (Wico) 304 Coil core fixing screw lock washer (Wico)
305 Condenser (Wico) 306 Condenser screw (Wico)
307 Condenser fixing screw (Wico) 308 Condenser fixing screw lock washer (Wico)
309 Breaker arm pivot & fixing plate (Wico) 310 Cam pad bracket (Wico)
311 Cam pad (Wico) 312 Cam pad bracket fixing screw(Wico)
313 Cam pad bracket fixing screw lock washer (Wico) 314 Contact breaker, fixed (Wico)
315 Eccentric screw (Wico) 316 Fixed contact screw (Wico)
317 Fixed contact lock washer (Wico) 318 Fixed contact screw washer (Wico)
319 Contact breaker, sprung (Wico) 320 Breaker arm spring clamp plate (Wico)
322 Breaker arm spring clamp lock washer (Wico) 323 Breaker arm spring washer (Wico)
324 Flywheel (Wico) 325 Magneto cover (Wico)
326 Magneto cover clip (Wico) 327 HT Pick-up (Wico)
328 HT terminal block gasket (Wico) 329 HT fixing screw (Wico)
330 HT terminal fixing screw lock washer (Wico) 331 HT terminal block contact (Wico)
332 HT terminal end (Wico) 333 Rubber dust cover (Wico)
335 HT wire terminal end (Wico) 341 Stator plate (Wico)
353 Drive control guide rod, later MkI on 354 HT lead terminal cover screw (Miller)
355 HT terminal cover (Miller) 356 Terminal cover rubber grommet (Miller)
357 Contact breaker arm terminal screw (Miller) 358 Shakeproof washer for contact breaker arm screw (Miller)
359 Plain washer for contact breaker arm screw (Miller) 360 Contact breaker unit (Miller)
361 Condenser (Miller) 362 Shakeproof washer for condenser (Miller)
363 Magneto cover (Miller) 364 HT coil (Miller)
365 Cam oiler (Miller) 366 Backplate (Miller)
367 2 fixing screws & washers (Miller) 368 Insulating washer (Miller)
381 Drive control sector plate, later MkI 382 Hoop, later MkI
383 Hoop spacer, intermediate/later MkI 384 Csk bolt for hoop
396 Special silencer baffle 397 RH hoop endplate, intermediate/later MkI
398 LH hoop endplate, intermediate/later MkI 399 Drive control bolt
413 Piston .0025" oversize, with gudgeon pin 414 Circlip
415 Piston ring .0025" oversize 420 Cable outer stop (solid nipple), drive ctl lever 2nd type
426 1BA Csk bolt, later roller guard 429 Crankcase gasket (nut & bolt type)
430 Lever, drive control lever, 3rd type (Amal) 431 Body, drive control lever, 3rd type (Amal)
432 Handlebar clamp, drive control lever, 3rd type (Amal) 433 Trigger, drive control lever, 3rd type (Amal)
434 Trigger spring, drive control lever, 3rd type (Amal) 435 Rivet, drive control lever, 3rd type (Amal)
436 Pivot bolt, drive control lever, 3rd type (Amal) 437 Nut, drive control lever, 3rd type (Amal)
438 Handlebar clamp bolt, drive control lever, 3rd type (Amal) 451 Terminal for Lodge sparking plug
452 Tank pivot grease nipple 453 Tank pivot block, later type
454 Tank pivot plate, later type 456 Sparking plug, Champion J8
458 Copper/asbestos washer decompressor 459 Strengthening piece
467 1/16" × 1" split pin, MkII drive control 470 Body, throttle/decompressor control
471 Lever, throttle/decompressor control 472 Top plate, throttle/decompressor control
473 Cable drum, throttle/decompressor control 474 Spring washer, throttle/decompressor control
475 Fibre washer, throttle/decompressor control 476 Plain washer, throttle/decompressor control
477 Cable adjuster, throttle/decompressor 478 Cable adjuster nut, throttle/decompressor
479 Throttle cable outer (decompressor model) 484 Thrust washer, MkII-on drive control
487 Upper pin, MkII-on drive control 488 Lower pivot stud, MkII-on drive control
489 Return spring 491 Cable end stop, MkII drive control
492 Middle pin, MkII drive control 498 Decompressor cable inner
499 Decompressor cable outer 500 Eyelet nipple, decompressor
503 Decompressor cable retaining screw 504 Short stud, MkII rear engine mounting
515 5/16" BSF Simmonds nut for drive control bracket, MkIII 516 Decompressor cover
521 Roller disc 522 Air cleaner gauze
523 1BA Shakeproof washer, roller guard 542 47mm × 17mm × 14mm ball bearing (MkV roller side)
543 Distance piece 562 Spacer washer
563 Piston, +.010" 564 Piston ring, +.010"
565 Piston, +.030" 566 Piston ring, +.030"
567 Dust guard, roller side MkV 598 Crankcase bolt, MkIII on
638 Front mounting stud, MkIII 640 Link plate, MkIII drive control lever
641 Inner cable stop, MkIII drive control lever 642 Top plate, MkIII drive control lever
643 Bottom plate, MkIII drive control lever 645 Bush, MkIII drive control lever
646 Lever, MkIII drive control lever 647 Pivot bolt, MkIII drive control lever
652 Pivot screw, MkIII drive control lever 664 Sparking plug cap
667 RH number plate bracket, MkV 668 LH number plate bracket, MkV
671 Spacer for number plate bracket, MkV 678 Seat pillar clamp half, MkV
679 Pivot block, MkV 680 Seat pillar clamp plate, MkV
681 Pivot block clamp plate, MkV 40017 Sub-frame spacer, MkV
40080 ¼" BSF bolt, MkV rear number plate A3 Piston assy, std
A5 Crankcase Assy (studded, engine < 15258) A5A Crankcase Assy (nut & bolt type, engine => 15258)
A6 Crankshaft/connecting rod assy A7 Exhaust assy
A8 Wico-Pacy Bantamag A9 Carburettor
A10 Roller guard A12 Carburettor jet holder
A13 Carburettor pump knob and spindle A14 Petrol filter
A17 Float and needle A21 Tank, MkI
A22 Rear hoop assy, early MkI A24 Petrol tap
A25 Petrol pipe A26 HT lead (Wico)
A27 Drive control lever, 1st type A28 Throttle lever, complete
A32 Licence holder A33 Horn
A34 Speedometer head A35 Speedometer cable
A36 Drive control cable, MkI, 1st & 3rd type A37 Throttle cable (no decompressor)
A38 Front & rear number plates A40 Front number plate
A41 Magneto withdrawal tool A42 Filler Cap, early MkI
A47 Front mounting assy A48 Special attachment plate assy
A49 Tap filter assy A50 Petrol tap and filter
A54 Drive control lever, complete, 2nd type (Trojan) A55 Drive control cable, MkI, 2nd type, fixed nipple
A56 Decompressor assy A57 Fuel tank, MkV
A59 Drive control cable for tandems A60 Throttle cable for tandems (no decompressor)
A62 Miller magneto A63 Filler Cap, later MkI onwards
A64 Hoop with fixed sector, intermediate MkI A65 Hoop assy, intermediate MkI
MA65 Hoop assy, last MkI (with guide rod) A66 HT lead (Miller)
A68 Stator plate unit (Wico) A69 Stator plate with coil & core only (Wico)
A70 Drive control cable for tandems A71 Spanner set
A74 Piston assy, +.020" A75 Piston assy, +.040"
A76 Piston assy, +.0025" A77 Body, drive control lever, 2nd type (Trojan)
A87 Drive control lever, complete, 3rd type (Amal) A95 Spare parts price list
A96 Cycle unit instruction book A97 Flywheel (Miller)
A98 Back plate & all parts (Miller) A99 Contact breaker assy (Miller)
A100 Front tank pivot assy, later type A101 Contact breaker assy (Wico)
A103 Throttle/decompressor control, complete A104 Throttle cable (decompressor models)
A105 Drive control assy, MkII on (except guide rod & spring) MA105 Drive control assy, MkII on (including guide rod & spring)
A106 Lower scissor arm, MkII drive control A108 Drive control cable, MkII
A109 Drive control cable for tandems, MkII A110 RH upper scissor arm, MkII drive control
A111 Upper bracket, MkII drive control A112 Lower pivot block, MkII drive control
A113 RH upper scissor arm, MkII drive control A114 Throttle cable for tandems (decompressor models)
A115 Decompressor cable A115 Decompressor cable for tandems
A119 Drive operating cable, MkII conversion & Amal lever A120 Drive operating cable for tandems, MkII conversion & Amal lever
A121 Carrier A123 Carborundum Roller
A124 Hoop assy, MkV (including guide rod) MA124 Hoop assy, MkII (no guide rod)
A125 Crankcase assy A128 Crankcase halves assy, MkIII on
A134 Piston assy, +.010" A137 Piston assy, +.030"
A145 Drive control upper bracket, MkIII on A147 Tank, MkIII
A154 Drive control lever, complete, MkIII on A155 Body, MkIII drive control lever
A161 Herringbone drive roller A163 Roller spanner (for herringbone type)
A165 Rear number plate (MkV) A170 Engine mounting subframe, MkV
A9005 Fuel pipe K102 Decarbonising gasket set
K103 Engine overhaul gasket set K104 Magneto spares kit (Wico)
K105 Nut grips K106 Set of dealers' service tools
K107 Drive control mechanism kit (Trojan lever) K108 Drive control mechanism kit (Amal lever)
K109 Decompressor valve & controls with service cyl head K110 Decompressor valve & controls for plugged cyl head

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Appendix 4 - Instructions for the Mark V


Equipped with this unit, your bicycle will be transformed into the simplest motor vehicle on the road, yet it will retain all the advantages of a bicycle.

The engine itself is constructed of a few sturdy parts and requires little attention.  It is, nevertheless, important that it be fitted with care, preferably by a mechanic or cycle dealer.  Alternatively, by following the instructions given in this book the work may be carried out by anyone with ordinary mechanical aptitude.

To increase the pressure of the roller on the tyre; move the
hinge "D" in the direction of the arrow for a small increase, or the
U-shaped fork "H" for larger increases.


  1. Rest the unit on the rear mudguard (if there is a carrier or other fitment over the rear wheel this must first be removed).  Fasten the elbow lug "A" on to the saddle pillar as low as possible, but do not completely tighten the clamp nuts "B." (Fig.2.)
  2. Bring the petrol tank into a horizontal position by adjusting the clamping pieces "Z" on the elbow lug and sliding the hinge "D" up or down the elbow lug as required. (Fig.1.)
  3. Make a mark on the mudguard at a distance of about ¼in. forward of the roller mudguard "E" remove the unit and cut the mudguard in two halves at the mark, allowing the rear portion of the cut mudguard to swing backward, leaving a space between the two ends. (Fig.2.)
  4. Now replace the unit as before and the roller will rest on the tyre.  Move again the sliding hinge "D" to bring the tank afresh into a horizontal position.  Make sure now that the cylinder fins and the inlet tube are at least 1 in. clear of the mudguard.  If the clearance is less, it is necessary to cut away the sections of the mudguard which may endanger the free movement of the engine. (Fig.3.)
  5. Now look at the bicycle from the rear.  Make sure that the tank top is parallel to the spindle of the rear wheel of the bicycle and that the roller rests well on the middle of the tyre.
  6. Now tighten the clamp nuts "B" and sliding hinge nuts "C." (Fig.3.)
  7. Undo the nuts of the wheel spindle and spring the U-shaped fork "H" into position.  Each branch of the fork carries at its lower end an adjustable lug "G." Fix the position of these lugs so that a space of about 1 in. is allowed between the tyre and the fork. (Fig.1.)
    To fit the drive control remove long pivot pin "I" (Fig.4.) and locate drive control mechanism between the arms of bracket "Y."  Align holes in bracket and toggle arms "L."  Insert pivot pin "I" and secure with split pin, at this point observing that the control has been assembled with the cable towards the rear.  Now slacken clamping bolt "O" and pull guide rod "R" through adjustable pivot block "N" until the holes in the lower end of the guide rod align with the holes in the quadrant plate "J" making sure that there is about 1/16 in. clearance between the guide rod and the bracket "Y." (See Fig.4.)
    Secure guide rod to quadrant plate and tighten up wheel spindle nuts.  Slide adjustable pivot block up or down as required to obtain 1/8 in.  clearance between the tyre and the roller with the drive control in the free position.  Tighten up clamping bolt "O."
  8. Now swing the cut-off portion of. the rear mudguard forward until the front edge is beneath the flange of the roller mudguard.  Using the holes in the rear stay as a guide, drill the two holes for the bolts "Q" and fit the bolts, thus securing the rear mudguard.
  9. The controls should now be mounted on to the handlebars of the bicycle, the speed control (THROTTLE and DECOMPRESSOR) on the right-hand side, and the drive control on the left-hand side.  Set the position of the control levers to be convenient, but make sure that the cables have easy sweeping bends and are not led round sharp corners or are "kinked."
  10. The control cables are already coupled up to the appropriate levers, but adjusters are provided at the ends of the cables for the taking up of any slack that may develop in use.  After making use of the adjusters always remember to tighten the locknuts.
    With the cable correctly adjusted, the spring should return the drive control mechanism to the free position.  This can be checked by observing that the three pivot points of the drive control mechanism marked "*" are in line (see Fig.4.).
  11. Make sure that when the engine is engaged on the tyre (i.e., drive position of the handlebar lever) the roller does not slip if the bicycle is pushed.  Also, that when the roller is lifted by means of the drive control lever the tyre must be able to accomplish a complete revolution without touching the roller at any point.  If one of these conditions is not fulfilled, adjust the pressure of the roller on the tyre (see below).
  12. Check that the float chamber of the carburettor is vertical.


In order to obtain good results from the engine, it is very important that the pressure exerted by the roller on the tyre is adequate to obtain a positive drive.

If the engine tends to race when you open the speed control lever abruptly, then the pressure of the roller on the tyre should be increased, this will avoid slip of the roller on the tyre with consequential increase in rate of tyre wear.

To increase the pressure of the roller on the tyre, first check to ensure that the tyre is inflated hard.  If slip still occurs, slacken the nuts "C" on the hinge "D" and slide the hinge downwards in the direction of the arrow, Fig.1.  Then re-tighten the nuts "C."  If a large amount of adjustment is required, the U-shaped stay should be moved down another notch at "G," Fig.1.



  1. First grasp the roller with the special spanner and undo the retaining nut, Fig.5.  It is important that this instruction be followed carefully, as if an attempt is made to unscrew the nut while holding the magneto, damage to the crankshaft may result.
  2. Having removed the nut and washers, the roller itself may now be unscrewed.  It should come away quite easily and in this case it is permissible to grasp the magneto.  If the roller will not unscrew easily it should be soaked in freeing oil and another attempt made after the oil has penetrated into the thread.
  3. Reverse the operations to fit the new roller, making sure that the washers are replaced in the correct order, i.e., the fibre washer "T" should be fitted between the crankcase and the roller "S", Fig.5, and that the roller is screwed right home before applying the locknut.


Normal road speed:
Ignition Timing:
29 degrees before top dead centre, or 1/8in. measured on the piston stroke.
Flywheel Type, Contact breaker gap: .018in.
Type of Plug:
LODGE C14, KLG F50 or F50R (suppressor-type plug), CHAMPION L8.
Plug Gap:
.018in. - .020in.
Petroil (1 part of oil to 20 parts of petrol).
Trojan (made under Dell'orto licence)
9 mm.
Jet size:
45 (for temperate climates)
(Smaller or larger jets may be required for hotter or colder climates.)
Lodge or KLG - PS.



The unit runs on a mixture of petrol (or gasoline) and oil.  First quality lubricant is required and the best results are obtained from the use of "Colloidal Filtrate Oil".  If this oil is not available, a known brand of grade SAE.20 oil should be employed.

The proportions of the mixture of petrol (or gasoline) and oil are 20 to 1 and can be mixed as follows:

3 measures of oil to 1 gallon of petrol.

The correct measure will be found within the underside of the tank filler cap.

It is better to mix in petrol with the oil in a separate container before pouring into the fuel tank, but when this cannot be done it is important to operate in the following order:

  1. Turn off the petrol tap under the tank;
  2. pour in the oil;
  3. pour in the petrol;
  4. shake the bicycle thoroughly before turning on the tap again.


Having turned on the petrol tap. depress the primer plunger "U" on the carburettor (Fig.6.) four or five times; place the mixture control in the "RICH" position (if this is the first time you have started the unit).  Grasp and pull into the ratchet stop the drive control lever (long lever on left-hand side of handlebar) bringing the roller into contact with the tyre.  Move the speed control lever (small lever on the right-hand side of handle-bar) to the right. thus opening the decompressor valve; mount the bicycle and pedal away.  The engine will now be revolved, but will not commence firing.  When speed has been gathered, move the speed control lever smartly towards the left, thereby closing the decompressor valve and opening the throttle halfway.  The engine should now commence firing.  Continue to pedal briskly until the engine is running smoothly, then your speed may be regulated by operation of the speed control lever.

If you wish to use the machine as an ordinary bicycle, lift the roller from the tyre by operating the drive control lever on the left-hand side of the handlebar.  BUT REMEMBER TO STOP THE ENGINE BY SHUTTING DOWN THE SPEED CONTROL LEVER.


During the first four-hundred miles do not put too much strain on the engine.  If the engine tends to slow down under effort open decompressor valve, by means of the control lever. and pedal for a short while until the engine has cooled down.



Adjustments to the mixture may be required to suit varying climates.  For cold climates a richer setting of the carburettor is obtained by substituting the existing jet "V" (Fig.6.) by a bigger jet.  The size of the jet is marked on the outside.  If only a minor adjustment is needed, as, for instance, when changing from Summer to Winter conditions, it can be obtained by turning the air intake "W" in the direction of the arrow marked "Rich."

A weaker mixture may be needed in a warmer climate and is obtained either by substituting the existing jet by a smaller one, or again for a minor adjustment, by turning the air intake in the direction of the arrow marked "weak."

Too rich a mixture is indicated by a pronounced tendency for the engine to four-stroke at slow speeds, or even with an extremely rich mixture at full throttle.  (A two-stroke engine, such as the Mini-motor, is said to "four-stroke" when the exhaust note becomes staccato and uneven.)

Running on too rich a mixture fouls the insulator around the central electrode of the sparking plug which may result in its becoming unserviceable.

Too weak a mixture is indicated by a tendency to run erratically at full throttle (the exhaust note undulates) or, by poor pulling and the fact that the highest speed is not obtainable at the full throttle opening, but with a somewhat smaller opening of the speed control lever.



It is advisable that all adjustments to the magneto be carried out either by a Mini-Motor Agent or by a similarly experienced mechanic, and it is to such as these that the following paragraphs are addressed.

  1. Check, and if necessary, readjust the contact breaker points every 3 months.  The contacts should be cleaned by inserting a dry smooth piece of paper between them and withdrawing same with the contacts in the normally closed position.  Do not allow the engine to run with oil or petrol on the contacts or they will burn and blacken.  If this should occur, lightly polish with a magneto file or piece of smooth emery cloth.
  2. Moisten the cam lubricating pad with a few drops of thin oil every 5,000 miles.
  3. Occasionally clean the high tension lead and insulator, examine for cracks or other damage, and replace if necessary.
  4. If the magneto requires any attention beyond replacement of the contact breaker points or condenser it is recommended that the complete instrument should be sent to your local agent or Mini-Motor Ltd. for servicing.


  1. Before attempting to fit a "Mini-Motor" unit to your bicycle make sure that the spokes of the rear wheel are in good condition and that the wheel runs true.
  2. Check and adjust your brakes carefully.
  3. In the absence of a special "power drive" type of tyre we advise fitting a heavy (i.e., tandem type) pattern tyre, with, for preference, a ribbed tread, to the rear wheel.
  4. Do not attempt to "slip" the drive control lever, as such treatment tends to wear the rear tyre.
  5. If the engine races in wet weather or when climbing a steep hill. close the throttle slightly and assist with the pedals.
  6. Before attempting in any way to dismantle the unit, make sure that you have a new set of gaskets for use when re-assembling.
  7. Remember that while fitting a "Mini-Motor" will remove the hard work from cycling, it does not turn your bicycle into a motorcycle.



  1. Remove the sparking plug, cylinder head and exhaust silencer.  Carefully scrape the carbon deposit from the inside of the cylinder head and from the piston top, using, for example, the blunt side of a penknife or similar article, care being taken not to scratch the surface of the aluminium parts.  Also clear the carbon from the two small recesses on the sides of the piston crown.
    Remove the decompressor valve, clean and reseat if necessary.  Make sure that the decompressor is tightly screwed into the cylinder head, always using a new copper and asbestos washer when refitting the decompressor.
    The cylinder will also have a carbon deposit near the cylinder head joint corresponding to the aforementioned recesses in the piston.  These carbon deposits should also be carefully removed.
    Now, turn the engine until the piston is at its lowest position and, looking into the exhaust port (Fig.7.), you will find that it probably has a carbon deposit around the edges.
    Scrape away this deposit.  Now wipe the inside of the cylinder with a clean rag.
    To remove any particles of carbon which may have fallen in, blow through the exhaust port with a hand pump.  When you are satisfied that the inside of the cylinder is clean, carefully clean the contact surfaces of both cylinder and head and then make a joint with a gold size, shellac or similar compound, and a new gasket.
    When replacing the cylinder head tighten the three securing bolts progressively, then check again for tightness when the engine is warm after restarting, when your service work has been completed.
  2. Oil the Control Cables.
  3. looking at the bicycle from the rear, check that the roller rests well on the centre of the tyre.  If this is not so, loosen Bolt "C" of the sliding hinge and centralise the unit.  Make sure to re-tighten the bolt again. Remove all dirt from between the roller serrations. 
  4. Turn off petrol tap and remove screwed Plug "X" (Fig.6.) from the bottom of the carburettor float chamber.  Turn on the petrol again momentarily to wash dirt out of the float chamber and replace screwed plug.
  5. Check tightness of all nuts and bolts.


NOTE. - We do not normally recommend removing the cylinder barrel when decarbonising the engine, but if it does become necessary to remove the barrel, be sure that two spare piston rings are available against possible breakage, also that new cylinder base and cylinder head gaskets are to hand for use when rebuilding the engine.


The following checks should be made In succession:


  1. If it runs erratically or mis-fires it is probable that the carburettor is delivering a weak mixture.  Turn the air intake "W" (Fig.6.) in the direction of the arrow "rich" and check for an improvement.
    If you are already running with the control in the "rich" position, a larger jet is required.
  2. If the engine loses power and the exhaust becomes very quiet, the cause may be a choked exhaust port (see maintenance A) or a choked silencer (see maintenance F).
  3. If the engine shows a poor compression when turned, it may be that the cylinder head is leaking, or that the sparking plug is not tight, or that the piston rings need to be replaced, or if you have covered a great mileage the cylinder may need re-boring and a new piston fitted.
  4. The roller may be slipping on the tyre because the pressure is not adequate or the tyre is not pumped very hard (see Adjustment of roller pressure.), or because the roller serrations are choked with mud.


The first symptom of roller slip, if and when it does occur, is of the engine going at top speed while the cycle and rider proceed comparatively slowly.  As roller slip can result in reduced tyre life, the following points should be examined at once.

Check rear tyre pressure.  It should be hard but not "solid" (i.e., 30 to 40 lb. per square inch).

Adjust the drive cable to take up slack.  See that it moves freely, that it is well oiled at each end, and that there are no acute bends; the cable should have a wide sweep as it enters the toggle assembly (Fig.4.).  Unlock the toggle action by moving the handlebar lever slightly and then press down firmly on the petrol tank to ensure that the Mini-Motor is pivoting freely; if it is not, the forward hinge lug should be dismantled and lubricated.

With the Mini-Motor in the "off" position, make certain that the gap between roller and tyre is not more than 1/8 in.  If the gap is greater, the unit should be adjusted as described earlier.  Properly adjusted, the "Sure-grip" roller is capable of transmitting the full drive of the Mini-Motor in almost any weather condition.

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Early MkI Mini-Motor

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Parts of this article first appeared in Buzzing magazine in December 1990.  It has been extensively modified since then

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