These dual roller chain sets feature billet gears with hardened teeth. The crank gear has nine key ways for adjusting cam timing in 2, 4, and 6 degree increments (advance or retard). They offer greater strength and reduced chain stretch over stock chains to maintain accurate cam timing, and offer less resistance which aids in decreasing oil temperatures and increasing HP.  They require no modifications to install and will fit under the existing stock timing chain cover without any clearance issues.