index   Go to the Archive index

Honda P50

Carl Squirrell

The Honda P50 model (also known as the 'Little Honda') was the first Japanese moped to find its way to our shores, being introduced in December 1966, bringing Japanese 4-stroke technology into Joe Public's garage.  Whether the technical know-how used in the highly successful Grand Prix machines was utilised in this utilitarian machine is debatable, but the quality that we have come to expect from the land of the rising sun was clearly evident, even at this embryonic stage.  Production ran until April 1968 when it was replaced by the PC50 fitted with a similar ohc engine but mounted more conventionally mid-way between the wheels of the machine.

Two versions of the Cyclemaster-inspired P50 were imported into Great Britain: 24,892 of the small 17" wheel Japanese-built machine, which was available in Scarlet (ie: red), Sky Blue or Charcoal Grey, and 1,128 of the larger 19" wheeled Belgium-assembled machine, which was available in grey only.  Both shared the same ohc 42mm × 35.6mm engine mounted within the confines of the rear wheel hub, using a roller bearing crankshaft running on small - by today's standards - 6202 and 6203 bearings.  Lubrication was by splash, stationary engine style, with an oil flinger on the bottom of the con-rod; oil reached the alloy overhead camshaft and rocker arms - mounted on a 6mm steel spindle - by utilising the cam chain and its guide to steer the oil towards the top-end and then via a scroll on the camshaft spindle through the hollow camshaft to the point of contact with the rocker arms.  The inlet valve ran direct in the aluminium cylinder head on most models but on very late versions a replaceable valve guide was fitted, a similar exhaust valve guide being fitted to all models.  The tiny - 15mm - valves had only one valve spring each; these were retained with 'key-hole' type retainers.  The aluminium 3-ring piston ran in a cast steel barrel, the gudgeon pin running direct in the steel con-rod; compression ratio was a reasonably high 9.0:1.  Transmission ran from the crankshaft through a centrifugal clutch to the rear wheel via a series of three sets of chain and sprockets.

Incoming air was filtered through a foam element situated in the pressed steel frame just under the saddle, and the 0.6 gallon (2.5lts) of fuel was stored in the fuel tank fitted above the rear wheel; the resulting fuel/air mixture entered through a down draught carburettor - two types being fitted during the production run - via a rubber connecting pipe to the cylinder head.  The burnt exhaust gases exited through a small bore pipe to the pancake type silencer situated on the offside of the rear wheel.  Ignition was provided by a flywheel magneto - also incorporating lighting coils - with traditional contact points triggering an externally mounted HT coil tucked out of harm's way in the middle of the frame; this provided the spark for a short reach 10mm spark plug.

Cycle parts consisted of the aforementioned pressed steel frame holding compact short arm leading link forks at the front end and a rigid rear end - comfort for the rider's rear end being provided by a height-adjustable sprung saddle.  Cycle style handlebars - again adjustable for height only - carried a twist-grip, front and rear brake levers, a decompressor lever and a press button to activate the electric (AC) horn (mounted on the handlebar stem); all grips and levers were finished in black to contrast with the white head lamp shell, mudguards, chain guard, carburettor cover and front carrier.  A cycle-style chain wheel and free wheel complete with spring loaded tensioner was fitted to enable starting and pedalling - in case of the petrol running dry or, heaven forbid, engine failure!  Pedalling was achieved by flicking a lever on the engine unit to disengage the engine drive.  Starting could also be accomplished by running alongside the machine and dropping the decompressor lever - handy if the free wheel failed!

Lighting was basic, a single 10W beam at the front and tail light only at the rear (although a twin filament bulb was fitted in the rear light as foreign - to us - models had a stop light fitted activated by the rear brake cam within the rear wheel) operated by a simple two position switch on the rear of the head lamp.  A speedometer driven from within the front wheel hub and mounted in the head lamp shell recorded speed only, and not mileage covered, on its face.  Wheels consisted of an aluminium front hub laced to a 32 hole chrome steel rim housing a 3½" internally expanding drum front brake; the rear 40 hole rim was laced to the large aluminium centre used to house the engine unit, an unusual externally contracting 3¾" brake was fitted within the hub centre.  Tyres were 2.00-17 for the front, running at 19psi, and 2.25-17 for the rear, running at 27psi, for the small wheel version and corresponding 2.00-19 and 2.25-19 sizes for the larger Belgium-built version.

Optional parts available at the time were: a front carrier mounted wire basket fitting kit, a bracket to hold an auxiliary front cycle battery lamp, an exhaust pipe protector and a winker kit including battery and rectifier for charging from the flywheel magneto.


Valves burn out due to the inlet rubber pipe perishing, the original replacement part is now obsolete but can be cheaply substituted with a short length of car heater hose; the inlet valve itself can be replaced with an exhaust valve for a longer life as the exhaust valves are made of better material to withstand the heat produced by combustion - the inlet valve being cooled by incoming fuel mixture.

The camshaft and the rocker arms can wear prematurely, although this is usually due to the 0.7lts of 20w oil not being changed often enough and care must be taken when starting from cold to ensure that oil has reached the top end before the engine fires up.  It is wise to turn the engine over a few times with the decompressor lever pulled in to circulate the oil.

The piston and rings sometimes fail, again usually due to lack of lubrication, however the cylinder can be rebored (up to +1.00mm, ie 4 oversizes) to utilise later PC50K1 pistons.

Brakes, especially the front, are somewhat marginal in terms of performance, the front brake needing the brake shoe pivot and cable lubricating regularly and the shoes and brake hub need to be 'deglazed' from time to time; the rear brake is likely to suffer from neglect being difficult to gain access to, but once the engine unit is removed from the frame it is really quite straight forward to remove the wheel and attend to the brake mechanism in much the same way as the front wheel.

The speedo drive is quite fragile and often defies all attempts to make it perform, regardless of how much time and money you spend on it.

Silencers rust and like most parts are difficult if not impossible to locate.

All the cables are quite long and need regular oiling, especially the choke cable which is of the piano wire type and difficult to replicate.

The brake levers soften to something akin to one of Bertie Basset's Liquorice All-sorts and for efficient(ish) braking are best replaced with late PC50 alloy blades.

The twist-grip mechanism needs greasing to ensure a smooth action, although there is always quite excessive free play at the grip.

The centre stand pivot holes often 'oval-out' but this is easily rectified by opening out to 12mm or similar and using an appropriate bolt, either with a Nylock nut or drilled for a split pin.

Front fork bushes wear rapidly if not greased using the four nipples fitted, although these are easily replaced and currently readily available, albeit quite expensively, being common to parts fitted to later PC50 models.


...  is relatively simple; the aluminium engine cases respond well to bead blasting and the frame and associated parts are easily restored to their former glory by sand blasting and painting, although replacement decals are very hard to locate.  Fortunately wheel rims usually clean up well, because new replacements are very difficult to find, especially the rear - being a 40 hole as opposed to the more commonly used 36 hole - also the spoking pattern on the rear is peculiar to this model as are the very short spokes.  Plastic parts respond well to a good application of Jif - the kitchen cleaner not the pancake lemon juice!

Engine rebuilds are reasonably straightforward; the only special tool needed is a flywheel extractor - the same as a Mobylette pulley remover.  The cylinder head is easily removed; however do not forget to carefully remove the 3mm pin securing the camshaft spindle before removing the camshaft and note the position of the camshaft at TDC on the compression stroke.  Most camshafts have two blind holes which align with the gasket face but some sprockets are marked with a pointer which is set at 12 o'clock at TDC with the engine upright.  Valves are easy to fit and remove but fiddly to reseat; a short length of 3/16" ID rubber tubing slipped over the valve stem and attached to a piece of similar size tubing fitted to an electric drill is a somewhat crude but effective alternative to the traditional palm-aching method of valve grinding.  An exhaust valve can be substituted for an inlet valve and, being made from better material, is likely to give longer service but resist the temptation to fit an inlet valve in place of the exhaust valve as major problems are likely to occur.  The cylinder has a generous chamfer to aid fitting of the piston and refitting is not a problem but do ensure that the piston rings are fitted correctly; ie the ring marked 'T' is the top ring, 'N' is the middle ring and the lower ring is the oil ring, all markings must face upwards, never reuse piston circlips as these are readily available, being used on current machines.

Engine casing screws sometimes prove stubborn but patience, a good fitting screwdriver, an impact wrench and, sometimes, a little heat will always ensure that the unit comes apart with no damage to the mating surfaces.  If you keep the transmission chains and sprockets in order when you strip the engine then reassembly is easy; if you don't it's not impossible but you will need a parts book.  The castellated nut securing the clutch assembly can be carefully removed using a punch - for ease of assembly this can be replaced with a hexagon nut and matching tab washer from the PC50K1 model.  Check the condition of the pivoting clutch shoes, especially the pivot points, as these do wear on high mileage engines; if you do have to replace the main bearings note the position of the cam sprocket on the crankshaft as this affects the valve timing.  Only use good quality bearings - not Eastern European products - for a trouble free rebuild and lastly make sure that you fit the spacer between the crankshaft cam chain sprocket and the clutch body the right way round: wide flange towards the clutch, otherwise the cam chain won't fit round the sprocket.  The cam chain is best renewed if possible and the aluminium rivet holding the cam chain tensioner can be ground off and driven out; the resulting 4mm hole can then be tapped to 5mm and fitted with a 5mm set screw or Allen screw - with a drop of Loctite if you're the nervous type.

No gasket sealant is required on any part of the engine unless the gasket mating surfaces are badly mutilated, but the surfaces must be absolutely spotless with no old gasket material remaining; a coating of grease on both sides of the gasket will ensure oil tight joints and make for easy disassembly should the occasion arise.  Similarly a coating of grease or Copperslip on all screw threads into alloy casings works wonders.

Spares can be found cheaply at Autojumbles but be prepared to rummage through a pile of parts that no one else considers worth a look; some smaller dealers still have parts on their shelves, especially gaskets, valves and other consumables.

All in all the P50 offers an interesting and reasonable alternative to the usual restoration projects undertaken by moped enthusiasts

First published - June 1996

index   Go to the Archive index