Primary Gear - Bush problems.

21 November 2000

Although not of immediate interest to many road-runners/street-burners, mainly accorded to the racing scene, it seems to be a perplexing problem to a very large number of folk around the world, and has burned up plenty of telephone time.

Distilling the myriad of ‘the symptoms go like this’ descriptions down from various languages it has been put to me in – 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 misunderstandings 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 most of 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 and the 'wobble' draws oil down the primary gear/crank to leak out where the retaining C-clip is. The deteriorated bush can also 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 (see 'Flywheel - Pre Verto types' for further information). Magnified by the use of sintered/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, grabbed, and spun. Disaster. Results as above. The cure? Initially and mainly - reduce the heat level. Simply achieved by checking clutch drive strap position, boring holes in the clutch cover (‘wok’), or by welding on a suitably sized and positioned duct. Whichever, always cover with meshing to deter foreign objects from joining the fray!

I also highly recommend using the later primary gear oil seal as fitted to the injected Minis - it is of better quality and design than the more common 'red' seal.

There is an alternative method that seems to work extremely well - but it is still in the 'development' stage at the moment.

Useful part numbers:

13H2934	Standard 'Red' primary oil seal
LUF10005	Injection primary oil seal
CE12	Clutch oil seal fitment tool
22G109	Rear bush, primary gear all engine sizes except very early ones
DAM8889	Front bush, primary gear Big bore engines
DAM8887	Front bush, primary gear Small bore engines except original oil fed type