Hi guys. So I’m rebuilding the engine in my MK1, just got the engine torn down and set about measuring things, except I ran into a problem. I have the Rod Grainger manual, and those factory workshop manuals some nice chap uploaded some years ago, and they both quote 1.9661-19667 inches or 49.938-49.956mm for a main journal on the crank.
Well when I measured it with a micrometer, I got more like 1.974 inches, and after going to get some calipers to make sure my micrometer wasn’t knackered, I got something like 50.15mm, which converted is about the same as the imperial micrometer I have.
What’s going on here? Has anyone had this before? If anything I expected it to be below the tolerance range. It’s definitely a MK1 block, but the engine was installed by an unknown shop before I bought the car.
Obviously I’m doing this before I buy bearings, if I buy standard bearings they’re gonna be too tight, all I can think of is to get my machine shop to grind the crank to factory specs, just wondered if you guys had any input?
Hi Biggles - are we talking virgin shaft or one that has been fitted complete with bearings previously?
If the latter as I expect, I would forget the measurements as obviously incorrect somewhere, proven.
Shafts wear so no possibility of yours adding volume unless serious profile damage and material redistribution. The bearing would be standard and currently fits.
To be fair, the difference between your measurment and the accepted range quoted is very small so I would suggest there is an error in one of the figures/measurements somewhere.
Well error in measurement is an obvious one, but I’ve had the other two guys in the shop have a go and they get the exact same reading, I measured it a few hundred times it feels like!
I can’t imagine why it’s over, it’s definitely a MK1 block from an MX5, but this is an engine I’ve never taken apart before so clearly assumptions cannot be made. I doubt I’ll find undersize bearings, so I’m probably just gonna get the machine shop to grind it down to spec.
Just wondered if anyone had ran into this before or heard of it, I’d be fascinated to know if someone had seen it before and found out the answer
By all means investigate the procedure/cost of machining the shaft down but evidence of a previous, presumably standard bearing fitting seriously suggests to me that there will be no issue with a similar new bearing - what to lose by simply trying a standard new bearing?
Cause I don’t wanna order a set of bearings and find they’re wrong, after they’re out of the packaging and gotten oily and stuff.
I need to find some specs for the bearings or go measure the oil clearance with the old bearings and see if I can figure out what size they are. I would have dug deeper but it was getting late and after being stumped by the main journals being oversize I just gave up for the weekend.
I think you have manufactured this problem with the help of others from the sound of it.
For some reason you want to fit new bearings and have already proposed to machine down the shaft to accomodate them. You buy bearings and i am wrong - mysteriously and seemingly impossibly the shaft has added volume, so your initial idea of machining the shaft becomes a reality.
Better to try a new bearing, if that is the way you are going than machine shaft to twilight zone measurements and then find the bearing is too loose.
Whatever Biggles, the mind is beginning to boggle.
I would simply refit the existing shaft and bearing assuming is does not show signs of wear - this is an MX5 not a Ferrari.
What? No I’m saying all documentation points to a size, and my crank is larger than that, when you’d think that coming out of an old engine, it would be in spec or smaller.
I was just wondering if anyone had any idea why, or had been faced with such a problem before.
Also there’s no mystery involved, I have an in spec micrometer, and have a verified measurement that is noticeably over what is documented.
Everything was at the same temperature, the micrometer was verified on a 1 inch and 2 inch standard, and we also used vernier and digital calipers and they all spat out the same result. We also measured every main journal and got the same result within a thousandth.