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Saturday 28 January

Gearbox Repair

I couldn't get 3rd gear - the gear lever just refused to go in but all other gears were fine. I got the gearbox off and found the gears, bearings, selectors, etc all looked fine with no scoring and little signs of wear. I connected up the gearchange rod and had a look at what was going on. On close inspection I found this little gem jammed neatly between the case and the 3rd/4th selector lever:



This is almost certainly a tooth from a planet wheel. But now I could get 3rd gear and the gearbox appeared to be working great!

There was no sign of a missing tooth from any of the gears which left me a bit puzzled. However I did notice a large amount of play in the output shaft which seemed to be coming from the diff:

So the diff was dismantled:



It seems to be a well-known fact that the diff pin is the weak link in a Mini drivetrain and just to back this up, have a look at this:



The diff pin was very badly worn. Guessworks do an upgrade kit (planet wheels, upgraded pin, washers) for £55 so I bought and fitted this.

On the broken tooth, I think a previous owner must have broken a planet gear and had it changed without checking the gearbox for any debris. It might have lain there for years before jamming itself in the selector lever.

I reassembled the diff and gearbox and everyhring was much tighter. There was about 10 degrees of free play at the output shaft before I upgraded the diff now there's probably about 1 degree. If you ever need to take the engine/gearbox out of your Mini, its worth having a look for free play in the output shafts. If there's more than say 2 degrees, its worth taking the diff apart to check the diff pin and replace/upgrade if necessary.

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