Gearbox - Up-rating drop gears

ST TRANSMISSIONS & JACK KNIGHT

Same idler/input as above

Separate set from above 2.

Gearbox - Up-rating drop gears.

24 October 2000

Terminology –

Drop Gears	Transfer gears (primary, idler and input gears)
Large-bore	Refers to anything based on a 1275-type unit
Small-bore	Refers to anything based on 850/998/1098 units

The standard drop gears are fine for practically all road use - almost irrespective of power output. Despite what many folk believe - they are more than strong enough, and will perform perfectly well if correctly set up. That means getting the idler and primary gear end floats right, and using new bearings for the idler gear at each re-build. Simply following the methods outlined in the relevant workshop manuals will achieve these simple goals. There are two problems with standard drop gears - the main one is the helical cut of the teeth, the other a very limited selection of ratios. The helical-cut teeth are essentially power absorbing - both from increased metal-to-metal contact through having a greater tooth engagement area, and from side loads applied by the helical-cut. The limitation in ratios is mainly a one-to-one ratio. The only exceptions being the later 'economy' primary gear used on a very limited number of A+ 1275 units, and an 'economy input gear' sold by Jack Knight in the eighties. The former had 30 teeth instead of the 'standard' 29, the latter a fudged tooth profile to fit with the idler gear.

To solve both problems, straight-cut versions are available. It has to be said here - contrary to popular knowledge, the straight-cut gears are no stronger than the helical standard versions, all things being equal (i.e. correctly fitted).

Straight-cut drop gears come in a variety of ratios from a couple of manufacturers/suppliers. As they supply drive from crank to gearbox, they can be used to fine-tune the actual FD. The accompanying table gives the gory details. It’s these that are largely responsible for the whining howl emitted by race Minis. Music to some folks ears and desired by many in street used/tuned variants. Obvious from the table is the fact that the 'Trannex' range has a very broad option for ratios as they revolve around a 'common to all' idler gear. Their manufacture is far superior to any other, with proper gear-ground teeth, minimum back-lash and rigorously maintained manufacturing tolerances. No surprise then that this is the type supplied by Mini Spares/Mania.

STRAIGHT CUT DROP GEAR RATIO AVAILABILITY CHART
	NUMBER OF TEETH ON GEAR
TYPE	PRIMARY	IDLER	INPUT	RATIO		COMMENTS
TRANNEX
	22	30	25	1.136	- 1
	24	30	24	1.0	- 1
	23	30	23	1.0	- 1	Extra strong input gear
	24	30	25	1.0416	- 1
	23	30	24	1.0434	- 1
	22	30	23	1.045	- 1	Extra strong input gear
	23	30	25	1.0869	- 1
	22	30	24	1.09	- 1
	24	30	23	0.958	- 1

NOTE; All Trannex ratios use same idler gear, so are totally interchangeable with each other. Not interchangeable with other manufacturers. Superior over-all quality makes these Number One.

NOTE: Realistically not interchangeable between manufacturers due to tooth finish, so not recommended. ST transmission gears are generally a better product than the Jack Knight ones in terms of finish, fit, and longevity.

Drop gears can be used to fine tune final drive. To assess actual FD, just multiply FD by the drop gear ratio. So a 3.44FD using a 1.0434 drop gear set would give - 3.44 x 1.0434 = 3.59 FD. A glance at the FD table will show this can be used to achieve ratios not available off the shelf.

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 its 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!

Part numbers:

DAM9373	'Economy' primary gear, A+ 1275 type only
C-STR122	Large-bore 22-tooth Trannex s/cut primary gear
C-STR123	Large-bore 23-tooth Trannex s/cut primary gear
C-STR124	Large-bore 24-tooth Trannex s/cut primary gear
C-STR222	Small-bore 22-tooth Trannex s/cut primary gear
C-STR223	Small-bore 23-tooth Trannex s/cut primary gear
C-STR224	Small-bore 24-tooth Trannex s/cut primary gear
C-STR30	Pre-A+ idler gear
C-STR30A	A+ idler gear
C-STR30T	Pre-A+ idler gear with taper-roller bearing conversion
C-STR30TA	A+ idler gear with taper-roller bearing conversion
C-STR230	23-tooth input gear
C-STR240	24-tooth input gear
C-STR250	25-tooth input gear