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MG MGB Technical - Cracked main bearing caps?

Anyone had experience with cracked main bearing caps needing replacement like these?
Mike


Mike Ellsmore

Yes
Been there a few times Mike, always No2 or 4 and if it's No4 the webbing in the block will probably be cracked as well from the corner of the cap seat cutout in the block on the camshaft side up through the webbing
I've got a theory--
If it's No2 it's a balance problem in the flywheel or clutch and if it's no4 it's the front balancer causing it, why it's on the opposite end to the problem I don't know but that seems to be what happens-----or coincidence-?
Also, I notice your centre cap is on bum around, but maybe it was tunnel bored that way round when them special caps were fitted up---might pay to check that out
willy
William Revit

Lol, perhaps in Oz back and front are the other way round, like up and down??
Chris at Octarine Services

Thanks hawk eye. It isn’t my engine but one of the local MGA register guys who is doing a rebuild. I’ll get him to check but i do know it was line bored.
Mike
Mike Ellsmore

Not seen a cap cracked but we lost two factory forged cranks on one B engine , snapped at the rear of no 4 main cap on both occasions, turned out the ring gear had moved and was eccentric to the vertical plane and first time it had gone back in place so we did not see it. My fault for not putting one of our lightened flywheels on, trying to save customer money, always costs me to learn! I do not reuse unknown flywheels anymore!!!!! I have seen the same on a three bearing midget race engine (not my build) where the ring gear ran eccentrically on a posh aluminium flywheel, cracked just in front of centre main. We recognised this from the B engine experience and looked at the ring gear straightaway. We use TTV steel flywheels when we can.
Peter Burgess Tuning

That wrecks my end for end theory then--
Interestingly,thinking back, I used to look after the engine in a friends Mustang racer, a 289 with 347 stroker crank
On these Windsor motors the crank is externally balanced with the flywheel and front balancer both being out of balance to compensate for the crank. We (I) lost two almost new cranks ,breaking just in front of No2 main
Measuring from the centre of the balancer to the centre of the front main and then from the main back to the break ,the distance was exactly equal, so decided the balance weight of the balancer was causing it to fatigue sitting up in the revs all the time as it does- Paid out the big bucks for a Callies internally balanced crank and zero balanced the f/wheel and balancer and it's been bulletproof for the last probably 5 years racing
William Revit

Of interest Mike what crank was in when the caps broke?
Peter Burgess Tuning

Peter, I’ll check with the owner and get back to you.
Mike
Mike Ellsmore

Hi Peter, This block was purchased bare from a guy who had imported a wreck from America, no idea what crank was in it there but obviously had had a hard life.
Mike
Mike Ellsmore

This thread was discussed between 23/06/2020 and 03/07/2020

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