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This is related to align boring an S block.

Is there a factory spec for the diameter of the main bore in the block? Can’t find anything in workshop manuals.

I find specs in bearing manufacturer’s catalogs, however there’s a few differences among their values. I’d like to target whatever original factory dimension and tolerances were used.

For an 'S' and 1275 (up to 1983) Blocks the Main Tunnels were 2.1460 to 2.1465. These suited Original Bearings AEG158.

There was a change in spec (that catches many out it seems) in 1983, when they went to the Plain lower half shell. This maybe the other number you are seeing?

Thanks for the response. Those dimensions are the predominant ones I've found, with one exception.

Vandervell 2.1458-2.1462 [.0004” range] from https://www.mgexp.com/article/vandervell-bearings.html
ACL 2.1460-2.1465 [.0005 range]
King 2.1460-2.1465 [.0005 range]
Glyco 2.1460-2.1465 [.0005 range]

It's interesting that Vanderwell, who I would have thought was the "gold standard", has a dimension that is shifted and with a smaller range than the others. Guess I'll ignore that and just stick to the 2.1460-2.1465 value.

By the way, a good friend and prolific Mini engine builder, said he saw a video by a Mini specialist who made a point of having lost several blocks due to align boring to incorrect published dimensions. My friend can't remember where that video is online or exactly who created it, but that led to my interest. If anyone can recall that same video, I'd love to know.

-Gregg

ACL in their former name, Repco Engine Bearings, were OEM to our local production.

I agree with your thinking on Vandervell, they were spot on, however I found about 15 years ago (maybe more?), they seemed to drop off. All the same, one would like to think their published info would be good.

I'm of no doubt there's been loads of blocks lost over the years to poor machine shop practices !