High port flow gives great performance now, and more later when additional upgrades are done. Heads are fully assembled with springs and valves. Compression ratios vary depending on cylinder heads chosen, pistons used and engine displacement; see Compression Ratios chart for specifications. 79cc heads work great with stock motors for 88" to 103" motors; 89cc heads work best for motors over 103" or with high-compression pop-up pistons. 79cc have 1.940" intake and 1.575" exhaust valve and springs for up to .640" lift; 89cc have 2.000" intake and 1.605" exhaust and .640" springs. Heads are machined for compression releases unless noted otherwise (sold separately); S&S rocker boxes required when using S&S compression releases. Do not include rocker arm components, intake manifold or gaskets. Heads include four long headbolts, and reuse four long OEM headbolts. Accept OEM intake manifolds. Made in the U.S.A.. Must use S&S rocker boxes (PART #90-4110) if S&S compression releases are used.. Not compatible with Twin Cooled heads..