11.1:1 compression ratio. Piston kit includes forged pistons; rings; wristpins and clips. Crankcases do not require boring. Big bore 4.250" cylinders are required; OEM cylinders can not be bored to 4.250". 107" to 124" pistons come with 1.143" deck height and are compatible with the OEM 107" 4.375" stroke. Note: For 4.250" big bore cylinders only see S&S 574121 (black wrinkle) or 547122 (black wrinkle with higlighted fins). Note: Gaskets are sold separately as 591055 head gaskets; 591054 base gaskets or 591056 head & base gasket kit. Note: S&S recommends an S&S 550 series cam; which are available in both chain or gear drive. Note: In any and all applications the technician must be able to judge components for their suitability in combination with the other engine components; prior to installation. It is of importance that a knowledgeable mechanic will install & make adjustments as required. The rider must follow standard break-in and riding procedures for overhauled engines.