Britbike forum Forums Thread Like Summary - Crankcase repair.

I've been racking my brain trying to work out exactly how this happened and the only thing I can think of is that I screwed up! I pulled away from a stop sign and just after I changed into second and opened the throttle there was a noise and the engine stopped. Since I was just around the corner from work and the bike was still rolling I coasted down the road, round a round about and into the yard where the problem became somewhat apparent.



That was just before Christmas. So during the two week holiday I took the gearbox and primary apart, found some interesting things whilst I was in there. The clutch basket fell off when I removed the primary cover, the centre nut had come undone and the key had sheered. I had no symptoms of this before hand so I can only assume it came loose when the gearbox locked up? Also found that the clutch centre had split open and was being held together by the steel plates.

Looking at the chain I could find no evidence of rubbing and it had come apart at the master link. I found the master link in road. This leads me to believe that I somehow managed to not fit the spring clip properly. After 30 years of fixing motor cars this leaves me somewhat disappointed in myself!

Anyhow, nothing for it but to fix it. Once I got the chain free I was left with quite a hole.



I had a strip of 1/8 inch aluminium left over from making the front exhaust strap used in 66. After removing the jagged edge left by the break, I cut and shaped a piece of plate and epoxied it in place.



Then mixed a little aluminium weld through primer with some matte black to get a half decent colour match and touched it in. After using the bike mostly every day since the start of January it's starting to age nice enough.



I may yet muck around with the diagonal join but for now its good enough.

Well Mr Swede, I can say that the chain was most definitely not worn. As for wheelies, I may not have been trying but since this bike sounds as good as it looks ( Yes I am biased ) giving it a decent handful is somewhat required!

Hi R Moulding, Beautiful job on repair!

No telling why master link came off. I doubt you put clip on backwards. Of course that break was from the wadded up chain. Good you didn't go down when it happened.

Regarding loose clutch nut. The taper in hub & on main shaft should be wiped with clean dry cloth. New lock nut is strongly suggested. A small amount of loctite I feel is advised. Holding cush hub with tool made from 2 old clutch plates & 18" long 1/4x1" flat bar for handle. Use torque wrench to manual spec. The nut won't come loose. I've seen many come loose by not following this.

Broken center of cush hub is not common, but certainly happens. In the one case I personally worked on the cush rubbers had gone very bad, so vane was going metal to metal. On the few others I've heard about rubbers were very bad also. How does the metal to metal slamming effect clutch hub taper/shaft/key? Can that slamming whip/break chain? Could it zig zag chain hard enough for momentum to pop clip from master link?? I can say for sure hard slamming of parts can do really odd things. I don't know, but it's possible that was root cause of it all.

Here's the weird part. On the bike that had broken cush hub vanes I could not hear or feel a problem. I've ridden bikes including mine that I'll hear a strange loud double clack when cresting certain steep hills, then rolling off throttle, then back on. Clack, Clack like that. Sort of like chain hitting chain guard but sharper. Each case was cush rubbers.

The new cush rubbers have very short service life. Often only a few years & less than 5k miles. This time I used the polyurethane rubbers. About 25-50% stiffer, but supposedly much more oil resistant. Only 1300 miles on them so far. Riding bike I can't feel or hear any difference from the normal black ones. The made in Uk seem to last even less than made in Taiwan. I've changed many. None of the black ones last good at all.
Don