Hi can anyone confirm if the nominal diameter for a 1275 crank mains are 1.9411 to 1.9417 if the current bearings are 30 thou. The bearings I have are quite scored so I want to replace but not take the crank to 40 thou if I can avoid. Although it could do with a polish.

Plastigauge with old bearings indicated .002 to .003 . I’m thinking a fresh set of bearings may pull that tolerance in. Haven’t checked yet for roundness or roughness but just need to get my details sorted before a trip to the crank shop.

1275 mains diameter 2.0005 - 2.001 in my book.
For Cooper S.
So
For -.030" bearings should be 1.9705 - 1.971.

If you can feel a step on the centre main where the bearing groove was, I would regrind it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks dr mini. It does feel like a record to an extent. It’s slightly rough but not to the extent a finger nail gets caught up . Is 40 thou bad news for an s crank? It’s a progressive rebuild with the block at 60 thou bore size already I think this needs to go up a rebore too.

Here’s the mains as they are , is it best to start with a polish and recheck with the plastigauge , assuming it’s in round still. I’m okay with the regrind but this is my first attempt at rebuild so haven’t any precedent to compare against so want to echo check this out with the experts in the field who’ve seen it all before.

I’d just take it to get measured, crack checked & ground to size. It looks a bit worn to me in those photos

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I agree.
Crack test it first.

-0.040" is fine provided they put a nice fillet radius on it. As for any regrind.
Not sure where you are..
St George Cranks in Seven Hills Sydney do nice work.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Ok great thanks for your feedback, it’s good to validate some personal thoughts and I will go down that path of a professional service. Did a home dye penetrant test with a kit from Sydney Tools earlier just on the journals and it showed nil evidence, also the ping with a tap on the flanges was sharp and consistent too. Apart from wear it seems to be sound.

Needs Speros services St George Cranks Does top work .Needs grinding mate if im not too blind yet