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Rule D.1.j. Engines may be bored to a maximum of .040 inch over standard
bore size. Factory replacement pistons or their equivalent
with the exception of diameter shall be used. Cast or forged
equivalent pistons shall provide the same dome/dish/valve relief
configuration, ring groove width and spacing, pin height relationship,
weigh no less than the factory standard bore pistons.
Piston rings are unrestricted.
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I don't think I could meet those requirements by machining one of the "usual" aftermarket pistons. Rings are free, so that at least is not a problem.

Thanks for pointing me at rivergate, Tom. I'll check them out.

Jeremy, would you mind letting me know what are the companies you've found that do 0.040 over B18C1 pistons? I'd like to at least try them and see if they've got a B17 application.

Thanks!