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MG MGB Technical - Flywheel size

Following up my "5 main engine on 3 main gearbox" post: Some people say that the flywheels on the 3 main G and GA and the early 5 main GB engines have different diameters, and that this makes a conversion difficult.

Is this true?

Tore
Tore

Tore

You need the backplate and flywheel to match so you can fit a starter motor. I think the flywheels are the same diameter until you get to the pre-engaged later starter motors. Beware a Marina flywheel which is different size to both the MGB ones.

Peter
Peter Burgess Tuning

Hi Peter,

Then there is a good chance that the backplate on the early 5 main bearing engine is compatible with the gearbox for the 3 main?

Tore
Tore

I think so but dont quote me on it.

Peter


Peter Burgess Tuning

Tore, if it is an early MGB engine (18GB) which was originally mated up to the 3 synchro box, then yes, it will be. Peter is right about the flywheel diameter. In fact they all have the same ring gear so the diameter must be the same or it wouldn't fit!

If the backplate is off a later engine than that, then all the holes match up except for the two starter holes and the one by the slave cylinder. However the main problem will be that unless you also have the flywheel and starter from the same engine then you'll have a mismatch there. And even then there may be an issue with fitting the larger flywheel into the 3 synchro bellhousing - that's one I haven't ever tried.

If you can't get a backplate to match, you could have your 3 bearing one bored out to take the seal. However, that would make it difficult to use with the 3 bearing motor again.

Peter - re the Marina flywheel ... I had an idea they were the same diameter as the 3 synchro B, but that could be my memory. It could also be because I got confused when I aquired the engine from one of the works Marinas that was brought here for Rally of NZ in (I think) 1972, and the flywheel from that engine is still on my own car. But when I think about it, by the time I got the engine it had been mated up to an mga twin cam close ratio box (I got that too :-)) so the flywheel might have been changes as part of that.
Paul Walbran

Thanks to both of you. The ring gear proves it, of course! I have checked with some parts lists now and notice that a larger flywheel is used from ´67, so I guess my informants are mixing up 3 and 4 synch versions of the 5 main engine/gearbox combinations.

Complete peace of mind now...

Tore
Tore

This thread was discussed between 20/06/2014 and 23/06/2014

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