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MG MGB Technical - Overdrive/Non-overdrive Gearbox update

I need to repair a 3 synchro with overdrive gearbox which has a broken 1st gear or layshaft gear broken - not had a chance to strip the box out yet. I have an opportunity to purchase a non-overdrive 3 synchro gearbox and looking at the parts book it looks like I can just swap the main shaft and all its associated bits from the broken overdrive box to the non-overdrive box hence converting that into an overdrive box. The alternative, if the layshaft sizes/bearing count are the same is to use the parts needed from the non-overdrive box. Am I correct in my assumptions?
Graham Gilmore

Graham - Your assumption is correct. This is how I converted a Magnette box to overdrive. Cheers - Dave
David DuBois

Just make sure of the years of the transmissions. The later 3 synchro trans used a larger layshaft to reduce layshaft failures in earlier boxex. RAY
rjm RAY

Graham-
RJM was right to point that out to you. However, it is possible to modify the earlier three-synchro transmissions (found with the 18GB 74,529 and earlier engines) in order to install the stronger, larger-diameter layshaft (second motion shaft) assembly by boring out and line reaming the layshaft (second motion shaft) mounting holes to .6693” +/- .0005”. (17.0002mm +/- .0127mm). The best, and easiest, way to do this is to use an “end bell reamer” that is used for reaming the holes in the end bells of electric motors. This is a piloted, adjustable reamer that is adjustable (over a small range) to give the proper diameter and clearance. To keep things in line, it pilots in one hole while it cuts the other. They are commonly found at shops that rebuild electric motors, and the technicians there should do the job for a nominal fee.
Stephen Strange

Thanks for the input chaps. I have concludd that the way to go will be to get a non-overdrive box and use the mainshaft from my broken one. Fortuantely the instructions for doing this are here http://www.octarine-services.co.uk/INSTRUCT.HTM Iwill of course be looking at the layshaft diameters/bearing count.
Graham Gilmore

This thread was discussed between 20/06/2010 and 23/06/2010

MG MGB Technical index

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