I'm about to assemble the sprocket on the driveshaft on m20 1942.....found out that the frontcush touches and will probably eat the aluminium casing and also need to align front sprocket with the larger sprocket on the clutch to prevent the primary chain from derailing...... has anyone hints about seizes and type of material where to make a bushing from? I messured 24.9 mm driveshaft...correct?
BSA made three different sizes of the spacer to set the sprocket alignment and two sizes of shims.
The spacers are:
66-2528 - .320" thick.
66-2529 - .288" thick
66-2530 - .256" thick.
The shims are:
66-1707 - .003" thick
66-1708 - .004" thick.
The spacers and shims are made of hardened steel. I would check with Draganfly, Russell Motors or one of the other dealers who have M20 parts.
You could make the spacers if you have access to a lathe and can harden the part yourself. But you going to need a surface grinder to get both sides of the spacer in perfect alignment.
Blimey Bruce! What exotic parts list have you got? Mine only shows one spacing washer/shim 66-1707 at .032" ??
I think the loose spacer has mostly been replaced by a machined in one piece spline and spacer 65-2534. But I have worked on engines with a loose spacer. Cheers Ron
The crankshaft sleeve shown by Ron is the best option for an M20...This is, in fact, the earlier part...Post war BSAs with a 2 lobe shock absorber and some 4 lobe assemblies use the seperate spacer....I've never come across an original hardened spacer as described by Bruce and would question whether that was the case...In fact wear of the original parts can be a problem that in severe cases can cause a clearance problem where the spacer fits into the crankcase...
That aside, I also make hardened spacers when using the 2 lobe shock absorber unit... With the 4 lobe set up I always use the one piece crankshaft sleeve as pictured..If needed the spacer can be made from silver steel (tool steel in the U.S.) and is then very simply flame hardened by heating gradually to bright red and then quenching rapidly in cold water (oil is only needed as a quenching medium for larger parts to slightly slow the contraction rate)...I don't temper the part back at all after hardening... Ian
The 2 and 4 lobe sprockets are completely different....(as is the matching part of the cam, the spring, the nut and the crankshaft sleeve)...The 2 lobe sleeve has wider splines externally, the sprocket cam surface is wider and only has 2 'high points' not 4, and the cam form is more gentle than the 4 lobe type...It's worth noting that the 2 lobe cams were also fitted to the A7 nd A10 but for those models the crank mainshaft is a bigger diameter so those sleeves won't fit on the single cylinder cranks...
The most common type for the singles is the 4 lobe type and it's probably best to stick with that type as there are far more second hand parts available...The 2 lobe version was only fitted to later, pre alternator, swinging arm model OHV singles and the crank sleeves in particular are much less common for that reason....Ian