Black powder coated cylinders. No need to bore-out the cases. With 3.927" bore S&S feels this is the largest bore possible that can be safely used with stock bore crankcase spigots. With a set of S&S .585 cams; S&S teardrop air cleaner and S&S mufflers the 106" S&S test bike showed a very noticeable 60 to 91 horsepower jump. The forged pistons requires no flywheel re-balancing and valve pockets are enlarged to acommodate oversize valves and higher lift cams. Fits stock 4-3/8" stroke 96" Big Twin models and increases capacity to 106". 10:1 CR with stock 89cc cylinder heads and 10.7:1 CR with S&S 79cc heads. Note: S&S does not recommend boring OEM cylinders to a bore this large because of possible failing of structural integrity. Just one piston oversize (917501) available for this kit.