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MG Midget and Sprite Technical - CRANKSHAFT MAIN bearing diametric clearance.

CRANKSHAFT MAIN bearing diametric clearance.

The BMC book says, 0.0010" to 0.0027"

Mine is 0.0025" on the FRONT bearing.

Anybody else here measured theirs, preferably just after a crank grind?
Lawrence Slater

Hi Lawrence

Looks ok to me, I would be happy with that. The clearances vary with the bearing and with the crank grind hence the tolerances given.
I had a B crank ground too small once and had a clearance of 3+ thou so I had the crank redone . Then I discovered some folk in the USA have run 3.5 thou clearance with no probs! I stick within the book numbers cos I am a coward!

Peter
Peter Burgess Tuning

How are you measuring it Lawrence? Is it the crank that's worn or the bearing? These engines usually work ok even out of tolerance so I can't see it being a problem.
I had a crank reground that ended up right on the limit and apart from slightly lower oil pressure it was OK.
john payne

Ok. Thanks Peter. From a general running point of view, I am happy. But this relates to something else that few people are interested in.

Hi John.
I'm using a slim feeler gauge on the exposed front journal/bearing. 0.0025". That's the max size I can get in. I know I'd be better with a 2" micrometer, but I haven't got one and the cranks in the engine, which is still in the car at the moment. :).

It's VERY interesting -- to me at least -- that you mention the oil pressure being slightly reduced. That's exactly the confirmation I was looking for, and it ties in also exactly with something I've noticed.

Cheers.
Lawrence Slater

Probably not the best way of measuring it but not many options with it in the car! If you had a DTI you could put that on the top surface of the front of the crank and then try and push it up from the bottom maybe.

I think the oil pressure was down by about 10 to 15 psi on my other similarly built engine, it never really gave more than 60 psi when cold and dropped off to about 50 when hot. That's at driving speeds.

So if your front main is worn then you would perhaps get more oil thrown out is I guess where you're heading. Never had any oil burning probs and I had the front breather connected to the carb. It was a HIF though if that makes any difference. Unfortunately I can't remember for sure if I replaced the cam bearings though I usually do. (Yes, that proves I've been doing some reading!)
john payne

I use Plastigage - you can get some on EBay for about a fiver.
Fergus Pollock

Cheers Fergus. That stuff does look good. I think I'll invest.

John. " -- 10 to 15 psi --". Yes indeed, quite interesting. If you've a mind to, you'll be able to read my thinking on the other thread. ;).
Lawrence Slater

X 2 ... for the plasti gage stuff, its the only way to measure that spec.

lawerwnce....

I dont know, I think your looking up the wrong skirt, if it were just warn bearing shells, you could replace the oil with a thicker grade and increase your oil pressure, but to do this with your situation would only exacerbate your situation

Prop and the Blackhole Midget

Wrong thread Prop. :).
Lawrence Slater

This thread was discussed on 02/02/2015

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