The BSA A7-A10 Forum

Technical => Clutch, Primary, Gearbox => Topic started by: Steverat on 13.11. 2018 18:29

Title: What are the mainshaft split collets 67-3251 for?
Post by: Steverat on 13.11. 2018 18:29
I'm getting close to reassembling Josef's gearbox and find this item in the exploded diagram on the parts list (#74 on plate 3) but can't see it in my box of bits. What is it for please and where does it sit?

Title: Re: What are the mainshaft split collets 67-3251 for?
Post by: muskrat on 13.11. 2018 18:52
G'day Steve.
The sit on the mainshaft behind the clutch thrust washer that the rollers bear on. So it stops the whole clutch from moving inboard.
Title: Re: What are the mainshaft split collets 67-3251 for?
Post by: Swarfcut on 13.11. 2018 19:17
Hi Steve, as Musky says they support the thrust washer behind the clutch.  You are looking for a couple of square section semi circular insignificant looking bits of metal!

  They are part of the primary drive rather than part of the gearbox.  Your gearbox mainshaft should be splined (rather than the later keyway and taper S/A shaft).  Before you build up the box, make sure the mainshaft has a good undamaged location groove for the collets. The location is not very deep and there is a lot of load on this area when the clutch centre/mainshaft nut is tightened. The gearbox side of the groove area should be adequate enough to offer good lateral support to the collets.

   New collets come as a single ring with two thinner sections which must be broken to allow the collets to be fitted...always seems a shame to break it but there is no other way.

  In theory there are various thicknesses of thrust washer available to enable accurate chain alignment, but these tend to be Unicorn Parts...heard about but never seen.

  On my Flash, the collets were missing, and for a while the clutch support thrust washer had been running on the heads of the rivets that secure the mainshaft/ chaincase oilseal. The clearance here is very small, I thought this was how it should be until I looked in the parts book and realised the previous owner had  lost them.

   Bolt the gearbox to your crankcase, stick the collets into the location groove on the mainshaft with a dab of grease, then slide on the thrust washer. The collets should sit nicely in the circular groove in the back of the thrust washer. The washer will lie close to the back of the chaincase. Then assemble the clutch inner and outer along with the primary chain and cush drive sprocket and sleeve. Grease the clutch rollers as lubricant and also to keep them in place as you assemble.  With the big nut tightened, the chainwheel  should rotate smoothly with no tight spots but expect some lateral shake in the clutch chainwheel. If you are lucky, chain alignment will be fine.

Title: Re: What are the mainshaft split collets 67-3251 for?
Post by: Steverat on 14.11. 2018 11:17
Thank you both for your good guidance. Now I have been told of course it is easy to see from the diagrams and the shaft where they have to go. Nevertheless it is not trivial, as you say.