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Gearbox Bush Fitment / Machining


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#1 MacFreuden


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Posted 05 August 2019 - 01:58 PM

Hi all, am rebuilding my 1380cc engine and want to do it right, have got the gearbox in pieces at the moment, have bought new bearings, synchros and some gears as they were chipped amongst other things.... Have struggled to find definite answers to these questions however....


Do the diff side cover bushes need to be reamed to the CV joint / Pot joint ?


Does the replacement Top Hat bush (sold by minispares code 22G109) need to be reamed mainshaft and to what tolerance would this be?


Does the diff cage and crown wheel bushes need reaming/ machining? 


Does the reverse gear bush need reaming?


Do the camshaft liners / bearings need reaming / machining?


Finally does anything else also need reaming / machining done to it on the engine build that you know of as I would like to take it to a machine shop to have it all done in one go if possible. Also if anyone knows the tolerances that these should be machined/reamed to for a fast road engine and not race that would be great! If I've missed key info out sorry but am an optimistic amateur at this lol









#2 ACDodd


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Posted 05 August 2019 - 03:44 PM

Reaming is problematic in that it's fixed size and does not centre the bore.

Reverse gear bushes can be reamed. I bore these on a for the same reasons as above. I aim for 0.0005" larger than the shaft that it runs on. I can do these.

Primary gear bushes need line boring to size. I use a lathe. You need a much larger clearance on these aim for 0.005" clearance on the crank tail. I also do many of these.

Diff crown wheel and cage bearings also need sizing on a lath. I do these too. Aim for 0.0005 to 0.0008" clearance over the shaft that runs in them.

Diff side plates need boring on a.lathe. both myself and John at Guessworks keep these on the shelf as exchange items. Aim for 0.00075 to 0.001" clearance over the pot joint that runs in them.

Cam bearings come pre finished however you may need to ease the fit once installed to ensure friction free fit. I can supply and fit these too.


Edited by ACDodd, 05 August 2019 - 03:58 PM.

#3 MacFreuden


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Posted 26 September 2019 - 05:39 PM

Great info thanks!

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