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MG Midget and Sprite Technical - Measuring the crankshaft

I'm just checking the parts of the spare engine prior to the rebuild. I put the dial gauge on the crankshaft main bearings, and found on rotation a maximum of 0.03mm variation. This was on the centre main bearing journal without the shell in place so is a measure of how straight the crankshaft is. Is this within limits? The other journals show 0.01 and 0.02mm variation. Obviously I don't want to spend money on a regrind if it's not necessary. It is 10 thou undersize at present.
Les Rose

Les
naturally 0 is best but, most engine shops run on the one thou rule for comercial work so if it spins within one though and size and ovality are within one thou it's a goer
Measuring like you have done supported on the end bearing shells doesn't really tell if it's actually got 0.03mm ovality or 0.03mm bend (0.015mm off centre)
If you have measured the other 2 while they were still sitting on their shells then they would be ovality measurements
Really, you need to have a good measure up with a mic. and check for wear/ovality--it could well be a few thou undersize ?
Also it depends on what you're intending to do with the engine---If it's for a racer you need to be looking for perfectly straight/round/size, but for a road car, as long as the journals are up to size with less than a thou ovality you should be good to go

willy
William Revit

Thanks Willy

I forgot to say that I also did measure the centre journal with the shell in place, and the variation was 0.02mm. My micrometre is not big enough and my ancient manual vernier gauge only measures to 0.01mm. I did get this crankshaft measured by Steve Harris (remember him? Mini racer from 1970s), and he pronounced it fine. I think it will be good for a road engine.
Les Rose

Les---If Steve Harris says it's ok then it's ok
Is he still doing a bit, I thought he'd retired/sold up a few years back-
willy
William Revit

I last spoke to Steve Harris in the summer last year when I wanted to book a rolling road session. He said he was winding up and had handed over this work to a younger chap. Still using the Janspeed facility. To be honest I would rather have had Steve do it. I think he must be fully retired now.
Les Rose

The average individual can only accurately measure the rod and main journals. This is done by the use of either a micrometer or a dial caliper and the method is to check the journals for out of round condition by measuring once, then measuring a second time at 90 degrees to the first measurement.

One of the important things to note is how straight the crankshaft is. To do this, you need to use a dial indicator (clock gauge), mount the crankshaft on V blocks, and dial the indicator in on the front main journal, the check each of the other journals to see if there is any out of alignment detectable. This is best done with something like a milling machine or a special jig/fixture to allow the crankshaft to be mounted and the table it is mounted on moved to allow each journal to be checked for alignment. That is the only accurate method of determining straightness that the average machine shop could afford. A specialty shop might well have some form of electronic measuring system now. If so, I have never seen one is use, nor do I know anyone who has.

Les
Les Bengtson

This thread was discussed between 26/10/2022 and 07/11/2022

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