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Front Mounting For The Mini-Motor

Doug Cobb

With the return of plentiful petrol in the mid 1950s most Mini-Motors (and their ilk) were discarded and consigned to a corner of the garden shed (usually the dampest one!)  Sadly the lazy owners left the mountings on the host cycles - hence the dearth of them we have today.  The MkI engine (number A2746) I purchased in June 1997 was no exception.  Strangely however its original buff logbook issued in Perthshire in 1949 had survived the 40 years of wanderings down to deepest Somerset where I located it.  Incidentally the reason for the purchase was largely nostalgic to bring back memories of the one I had ridden in 1950.  Any rate the engine (duly inhibited with WD40) sat under my bench for a couple of years awaiting its turn for restoration.

The only progress was the acquisition of a 1950 Instruction book from BMS ( in which PI 6 illustration depicts, inter alia, the front and aft mounting components).  When I was ready to commence actual work, logic decided me to tackle the front first - which was fortuitous as when I eventually got around to looking at the rear mounting & lifting gear I had by then joined the NACC and was able to obtain the hoop from our previous Spares Secretary (albeit I had to manufacture the rest of components using the PI 6 illustration as a guide).  Recalling that I had noticed an engine, with its front mounting still attached, sitting on a shelf awaiting restoration at my friend Sammy Miller's museum, I headed that way with rule, micrometer, pencil and sketchpad.

Basically, that mounting was the same as the PI 6 illustration but with a slightly different Motor Pivot Block casting (lower clamp) but I could now add dimensions to form.  When I considered how I was to manufacture the mounting, I concluded that there were three options open to me - namely: casting, fabrication and fashioning from solid - all of which required a certain amount of machining!  Ideally access to a lathe, a mill and a pedestal drill were required.  Unfortunately I don't own a mill but I do own a lathe with a 5" swing, a 5" three-jaw chuck, a 5" four-jaw chuck and a running centre for the tail stock to cover both lathe and mill functions.  So machining was to be no serious problem for me albeit I had to regain the forgotten skills of my youth.  Casting was out of the question as no foundry will produce units on economic grounds (they talk in thousands!)  Fabrication didn't appeal as I would have to be reliant on a welder for his availability and skill, besides which I wanted to be independent.  That left machining from solid - not terribly economic as you are left with more swarf than component, yet satisfying, as all is done in your own garage.

Aluminium Alloy (AA) was the chosen material as it is light, strong, free cutting and it looks good left in self finish - but more importantly, my son has a ready source of supply from a scrap metal dealer who sells rod, bar and sheet by the kilogram.  The only snag is that the offcuts are often minus their colour coding which indicates the alloying constituents.  Bar was used for the main blocks and strip & sheet for the clamp caps.  Besides AA, mild steel rod is required for the sleeve insert and, ideally, phosphor bronze for the bearing in the Pivot Block but, if not available, brass is a reasonable substitute as wear should be minimal.  Returning to the AA, care has to be taken when tapping threads as it easy to strip them - ideally thread inserts should be used.  On the grounds of authenticity I have used BSF form threads (but also because I happen to have a set of taps and a collection of screws, bolts and nuts!)

Having made all of these decisions, the next task, using skills from a previous existence, was to produce a set of engineering drawings (including cutting drawings complete with machining datums).  Much machining time can be saved in this planning stage, eg: by the judicious use of a hacksaw, which also can produce further useful offcuts for other jobs.  Basically there are 3 major components:

  1. the Motor Pillar (Pt No  210) ie: the goose neck which connects the two clamps,
  2. the Motor Pivot Block (Pt No 70) ie: the lower clamp for the goose neck which is of course also the motor pivot (complete with steel liner, phosphor bronze bush (Pt No 71) and clamp cap), and
  3. the Cycle Saddle Pillar Clamp, ie: the upper clamp, for the goose neck, to attach the whole assembly to the cycle saddle pillar.  When machined, this clamp is divided into two halves (Pt No 211) as the last operation.  A clamp cap (Pt No 69) completes this assembly.

Important Notes:

  1. Whilst these part numbers are those from the PI 6 illustration, the nomenclature is my own, as the Instruction Book from which it is taken has unfortunately no cross reference to Trojan part names.
  2. Technically "the lump" is an engine and not a motor but I have used the term in deference to its Trojan title "Mini-Motor".  It is appreciated that the manufacture described above sounds complicated to those without engineering skills or access to machines but unfortunately it is!  But if one is prepared to sacrifice authenticity for simplicity I believe I could design an austere alternative which could be made using basic hand tools - providing outside help was obtained to drill or bore at least the 1" diameter hole and maybe others.

First published, August 2003

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