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13 JGM has opted for Speed-Pro forged pistons and an 0.030-inch overbore from an original standard bore block. JGM has done mock-up and found, based on calculations, they will get approximately 11.5:1 compression. These domed pistons deliver an 11.5:1 compression ratio when combined with 88-91cc Medium Riser chambers. Although we see rings rolled on in a lot of shops, JGM uses a ring expander for uniform expansion without ring distortion.