Bore size is 3 9/16 or 3.560

The ring thickness is: Top:         .058" 1/16 or 1.5 MM
Second:     .058" 1/16 or 1.5MM
Oil ring:     .125"1/8   or 3.0 MM

Onan used two ring styles on the P224G, NHE &NHM engine.

P224G Specs B, C & D & T260G with originalstyle pistons the compression ring thickness is .078" and the ring setpart number is 113-0296.