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MG MGB Technical - Transmissions: 18G vs 18GB

I've always presumed that a 3-sync o/d transmission (originally fitted to a 3-main engine) that I've rebuilt will fit a five-main 18GB (earliest of the five-mains). Recall that the 18GB was originally mated to a 3-sync transmission. Did they make any changes in the mating of transmission and engine when they went to the 18GB?

I'm almost positive that these are fully interchangeable; that there were no changes in the mating. But can somebody put my mind at ease?

Thanks,
Allen
Allen Bachelder

Hi Allen:

The 18 G and GA 1st motion shaft has a smaller diameter than the GB, different crankshaft. You can configure the gearbox either way by replacing the 1st motion shaft to your own need. I think there is a special spigot bushing to allow the 18G and GA gearbox to mate to a GB crankshaft.

Thanks: Rich Boris 67 B roadster
Rich Boris

Rich is correct. Moss lists all the bushes in the B engine section - and they do like to charge for the "special" one! Everything else is the same.
FRM
FR Millmore

Thanks guys,
Now I recall the spigot bushing business. Several years ago I found a 3-main o/d transmission (culmination of 5-6 year's search before that!) and rebuilt it to go with the 5-main engine for my '65 B resto project (and, yes, the 5-main is the correct engine for this car). All of a sudden I woke up in a cold sweat a couple nights ago with visions of mismatched flywheels, flywheel bolt patterns, backplates, and starter positions exploding in my head.

So I had to write. Thanks for putting my mind back at ease.

Cheers,
Allen
Allen Bachelder

Allen,

If you are replacing a 3 sync non OD box have a quick look at where the G box cross member mounts to the chasis and check that there are an extra 2 holes on each side. The X member needs to mount about an inch further forward for the OD box. Originally cars had holes for both positions, but my 67 BGT had been restored and the extra holes were not added when the chasis section was replaced. This has caused me no end of problems when fitting my OD box to the car!

Cheers
Iain
I D Cameron

Thanks Iain,

I always wondered what the extra set of holes were for. This car has them! Whew... (That part of the frame is one of the few spots on the car that didn't need replacing.)

Cheers,
Allen

Allen Bachelder

This thread was discussed between 07/06/2007 and 08/06/2007

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