Designed specifically for circle track racers, this SB Chevy aluminum crank hub is NOT a harmonic damper. Hub is significantly lighter than a harmonic balancer and dramatically reduces rotating weight for quicker acceleration off the turn. Hub has engraved timing marks at 0° and from 30°-42°, is 4-3/8" in dia., features (3) 3/8"-16 accessory threads and weighs just one pound. Fits all internally balanced small block Chevy engines. Designed for internally balanced engines only and must be balanced with rotating assembly