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I have a set of 4 ring pistons (part number AEG 034120 stamped into crown) out of my S that need +20 thou rings. We measured the top compression ring and got a width of about 1.1mm and the oil ring is near as makes no difference to 4mm. The options I see on Minispares have compression rings with a width of about 1.5mm. Can someone advise me what I should be looking for?

The original BMC Cooper S Pistons had a 3/64" Ring Thickness, this translates near enough 1.19 mm. The Oil Scrapers were 5/32".

Grant Rings have the Ring Set you are after, P/N P1715 +0.020" for Plain Rings or C1715 for Chrome. These are supplied with 3 Compression Rings and 1 Scraper per Piston and supplied as a set of 4 Pistion Sets (so 12 Compression Rings, 4 Oil Control Rings).

I was reasonably sure that Cooper S's were fitted with 3 Ring Pistons.

Thanks Spider.

Follow on question...excuse the ignorance.

I have found two sets of rings which were sold to me as the correct size and thickness. Problem is both sets are for 3-ring pistons (2 compression & 1 oil) My pistons are 4-ring pistons.

Can I get away with using some of the compression rings of the second set to complete a set of 4-rings? I see the packets are labeled "top groove" and "second groove".

Rings are designed and made to be used as a pack. Mixing and Matching is a bad idea. It can result in a smoker or something that will wear in very short time (that's how I found out long ago )

Why would you want to use those ancient 4 ring pistons?

@Larry...because there's nothing wrong with them (low mileage motor) and I don't like throwing away money needlessly (?)