I've made a pretty big mistake, I accidentally ordered 0.020" overbore pistons without rings because I didn't realize that the same model number was offered both with and without rings. This is a big problem because overbore rings could be hard to find locally, especially in an imperial size, and I can't afford another 2 weeks or another rush shipment cost.
0.020" is 0.508mm. Are these sizes intercompatible in terms of rings?
When ordering replacement pistons and rings for my ka24e they were used interchangeably.
RockAuto lists .020 as 5mm oversized and .040 as 1mm oversized.
Check end gap and party on!
OK that's good news...er, well, less terrible news at least. Tomorrow the hunt begins.
The difference between 0.5mm and 0.020" is about 0.0003", on a metric engine I bet you're probably getting 0.5mm no matter which you order. Gap them and go.
Do the plus size rings actually get built to a larger diameter (so a standard ring would have less tension than a +.020) or is it strictly a case of being manufactured for a theoretically correct gap with no file fitting?
I've noticed that non file fit rings tend to be on the loose side, gap-wise.
Hunt's over, no luck, gotta order some 
Good thing I didn't just go ahead and order the rings RockAuto said would fit, turns out the 1.5/1.5/2.8 ring thicknesses are for bigport pistons. Smallport pistons including the TC2989s I have use 1.2/1.5/2.8, and to get those I have to order '90 Prelude 2.0 pistons...so after checking the local Honda dealership, I'm ordering a set of ITM 0216262.
Knurled wrote: Do the plus size rings actually get built to a larger diameter (so a standard ring would have less tension than a +.020) or is it strictly a case of being manufactured for a theoretically correct gap with no file fitting? I've noticed that non file fit rings tend to be on the loose side, gap-wise.
While I'm sure that there are some discount rings that don't do this all quality rings are manufactured to have the correct ring tension when installed in the specified bore. Non file fit rings a speced on the loose side because tight is really bad and discount rings also tend to be file fit so you may be seeing a little of both with those.
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