Primary gear bush failures.

Although not of immediate interest to many road-runners, mainly accorded to the racing scene, it seems to be a perplexing problem to a very large number of folk around the world. So I’m having a pop at trying to solve the problem wholesale here.

Distilling the myriad of ‘the symptoms go like this’ descriptions down from the various languages it was put to me in (some were highly entertaining where more than a smattering of ‘sign language’ was incorporated) – the end result was always the same. The bushes at one end or other, and sometimes both, had failed in their duties. Incidentally – some of the confusion when trying to sort the problem descriptions was down to miss-understandings about which end of the primary gear is which. To put the record straight, the end nearest the engine is the FRONT end. Consequently the end nearest the flywheel is then the REAR end.

The two biggest outstanding symptoms were severe oil leaks onto the flywheel/clutch assembly, and difficulty/impossibility in selecting gears. Strangely, these problems were still suffered immediately after refurbing the offending article, using ‘modifications’ suggested by some of the many Mini ‘specialists’ out there who all but guaranteed it’d cure the problems! Some bought new gears from other ‘specialists’ who make their product ‘special’ by using ‘specific machining detailing’ to ‘cure the problem’. Unsurprisingly these didn’t work either. So how’s this happening, and what’s the solution?

Front bush damage is caused by it becoming loose, spinning between the gear and crank, and generating an enormous amount of heat. This ruins the bush, destroying all clearances, and allows excessive amounts of oil to pass – both through the now much larger clearance between crank, bush and gear and past the primary gear seal. The latter happens because the primary gear wobbles about excessively so the sealing lip on the seal can’t do its job. Slightly less severely super-heated bushes cause them to move outwards, jamming the primary gear between the retaining clips and the thrust washer and crank shoulder. This is why gear selection becomes difficult/impossible. The primary gear won’t disengage drive from the engine, and is the main reason why the rear bush gets it’s thrust lip broken off. Even if the loose front bush isn’t immediately apparent. This lip does break off on it’s own though, but for the same reason all the other problems occur.

The ‘miracle cures’ to this have been legion over the years. Despite much nose-tapping and eye-winking, almost all solutions revolve around two themes - running a much bigger front bush to crank clearance and/or welding it to the primary gear. As many can attest to, even this doesn’t work. That’s because the cause isn’t being addressed. And that’s EXCESSIVE HEAT.

It’s generated by the slipping clutch – be that when gear changes are made, getting off the line, or badly set-up clutch. Magnified by the use of cerametalic plates. The slipping causes friction, generating a huge amount of heat. This spreads through the plate and into the primary gear. When the heat level becomes excessive, the bushes pinch on the crank, are grabbed and spun. This is magnified by using desperately-lightened pressure-plates in conjunction with the cerametalic plate where heat generated by the clutch isn't efficiently/effectively dissipated. The heat simply bleeds way into the primary gear. The cure? Initially and mainly - reduce the heat level. Simply achieved by either boring holes in the clutch cover (‘wok’), by welding on a suitably sized and positioned duct. (whichever, always cover with meshing to deter foreign objects from joining the fray), and use a sensibly dimensioned pressure-plate. Alternatively – further proper development. This is something I am currently looking in to – so keep your eyes open for the results!

The crank definitely needs a grind. Some people including nb racers have ground it down to 1.500" then used a standard 998 primary gear.

I have used floating bushes on a few cars no problem, but find they are always too long as supplied and need trimming.
Edit
These weren't turbo, blown or race motors.

Yes grind it to 1.500" same as the rest, keep 1275 thrust washer, use a 998 primary gear, everything will fit as normal.

However if you have A+ drop gears you would need an A+ 998 primary gear.

The crank definitely needs a grind. Some people including nb racers have ground it down to 1.500" then used a standard 998 primary gear.

I have used floating bushes on a few cars no problem, but find they are always too long as supplied and need trimming.
Edit
These weren't turbo, blown or race motors.