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M20 Gearbox rebuild - wisdom required!

I will shortly be stripping and rebuilding the gearbox as the main case seems to have had the primary chain try and cut through it at some point.
What should I look out for and are there any improvements in terms of bearings etc to be had?
Many thanks in advance.

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Re: M20 Gearbox rebuild - wisdom required!

Hello Scott,

There's a wealth of information on this site, in the technical section, which amply rewards the careful reader! You'll find a complete description of how to overhaul a gearbox, and it's excellent.

There are also some fascinating threads on the advantages of sealed bearings, and of different lubricants.

Take a look. Really, there's so much that I doubt anyone can reduce it to just one reply, like this one. My approach is always to read as much as I can first, and resist the urge to take everything apart until I've done the reading. Then take lots of pictures and make sure you know where things like shims come from, and which way round. Parts diagrams don't always show this clearly enough for my taste.

Best wishes,


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Re: M20 Gearbox rebuild - wisdom required!

Pay close attention to how the shafts are shimmed. Significant wear on the shifting forks means it was shimmed incorrectly the last time it was apart.

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