Kiwi. Thanks for that link. All the thinking was done by men in white coats and as long as the journals and rod eye conform to the published finished size, the running clearance is pre set in the manufacture of the shells.
With multi layer shells we always assumed that when the silvery surface had gone and "It was through to the copper" the shells were badly worn, even if running fine, and needed replacement. Back then the parts were readily available and cheap enough. I wonder now if we wasted a lot of money, replacing parts for the sake of it.
Out of interest I dug out a set of MCA shells, plain white box, Labelled CB 2-B 67-0320 -20 MCA.
On the base of the pack another label says AEB 2024 020 E SA
This label is typical of AE/Glacier Bearings, and in my catalogue, B 2024 is indeed the reference for a set of Big End Bearings for the BSA A7/A10 small journal crank. Suffix SA denotes Steel Shell, Reticular Tin-Aluminium Lined according to the prefix/suffix section. Listed as suitable for A7, A10, Star Twin & Ariel Huntmaster.
The shells themselves are stamped 67 0320 -20 D HT G This is the BSA Part No. for a Small Journal Crank, in this case at 20 Thou undersize.
More usually the aftermarket shell is stamped with a Glacier reference number, not a vehicle manufacturer's OEM Part Number. For this bearing application the Glacier shell number is GS 8748 LC. In my catalogue only LC bearings are listed, being "Steel Shells. Copper-Lead Lined with Lead-Tin Overlay"
So on the face of it in this case the MCA shells are a repack of BSA OEM shells. They are steel backed shell with a single bright silver metallic layer.
In my youth an Urban Myth was that BMC 803cc A Series engines used the same shells!
Swarfy.
