Plunger clutch dimensions

[Chris]

Hello,
I'm a new motorcyclist and have a 1951 A10 plunger. The clutch chainwheel (sprocket) seems rather loose. I've checked the centre nut and it was tightened. I have removed the clutch and see that the looseness comes from the clearance between the chainwheel bush, and the boss and thrust plate sandwich. The workshop manual says if the wear exceeds .0015 in the bush or centre should be replaced, but doesn't give the dimensions! Does anyone know what the dimensions should be?
Additionally, the workshop manual says the rollers should be 1/4 x 1/4, but most suppliers are selling 1/4 x 15/64. Does anyone know which they should be?
Chris

[Minto]

Hey Chris,
Welcome to the most helpful and friendly bike forum ever.
I'm no expert but have very recently rebuilt my engine (52 plunger). Do you mean that there is play between the duplex sprocket and clutch or just that the whole clutch wobbles? If it's the latter, I believe that's quite usual, mine had a lot of play, and still has more than I would like but not as much as before.
I'm sure someone with more knowledge and experience will be along shortly.
Cheers
Jase

[Chris]

Hi Jase,
Yes the whole of the outer clutch wobbles as it is bolted to the sprocket. Seemed strange given the thought that has gone in to the design of the engine and gearbox.
Chris

[Minto]

Yes, it surprised me too.
Jase

[Swarfcut]

Chris, Welcome to the Forum.

  The design of the clutch means that the chainwheel will wobble, simply because of the narrow track of the rollers, and the need for a running clearance between the front face of the thrust washer and the back of the chainwheel. If it's any consolation, they all do that, and seem to survive with no problems. The Plunger 6 Spring Clutch is considered to be one of the better designs, before BSA cheapened it for the S/A models, then revamped it at the latter end of production. From an engineering point of view, lateral support is minimal, so the basket rocks.

 A few things to check.

   With the clutch centre removed, pull off the thrust washer. This should reveal two hardened semi circular abutments, which form a support ring for the thrust washer. They sit in a shallow circular groove in the gearbox mainshaft. On my bike they were missing, the thrust washer running on the heads of the rivets holding the oilseal carrier.

    Reassemble the thrust washer and abutment  ring halves, trick is to stick them in place with a dab of grease, once the thrust washer is replaced and seated, they can't escape. Add the clutch centre, tighten down with the nut. This assembly should end up nice and solid. Then try a selection of rollers in the gap between the thrust washer  and the clutch centre. They should fit without binding, the important dimension is the 1/4" diameter, the overall length of the roller is not so important, as long as it does not exceed the size of the gap and bind. The originals are 1/4 x 1/4" and there are 18 of them. Part number is 26-650.

   If all seems in order,  try the clutch centre  and rollers assembled into the chainwheel.  Any slop here is due to age and wear. The usual cure is new parts, even then the improvement will be marginal and most clutches will give satisfactory service with plenty of rock and slop.  Change the rollers as a cheap improvement, assemble them with grease. Even with close fitting rollers there will still be rock,and grinding material accurately from the rear face of the clutch centre to narrow the gap is possible but carries risks of causing more problems

 One plain clutch plate is slightly thicker than the rest and this goes into the basket first.  Clutch is designed to be run dry, primary oil level is critical, plenty on this elsewhere in the Forum.

 Tell us how you came to be the owner of a very early machine, head over to the introduction section.

 Swarfy.

 

[Chris]

Hello,
Thank you both for the welcome.
As an update, I have now checked the dimensions on my boss and chainwheel. The spigot on the boss is 0.26" long, whilst the thickness of the chainwheel bearing surface is 0.25" thick, consistent with the 10 thou feeler gauge measurement of the clearance when assembled. The rubbing surfaces are all flat, as is the thrust plate which suggests that these are the correct dimensions. Like this one expects an end to end wobble at the sprocket of about 30 thou which I measure on the bench. My impression was that it was much greater before I dismantled it. Next port of call is the split washer locking the thrust plate. Maybe the ID is a bit too large, as the lip in the gearbox main shaft is not large.
Chris