Standard cam sprockets only allow for the position of the camshaft in respect to the crank, i.e. the cam timing, to be adjusted in increments of 10 degrees - by moving the cam sprocket forwards or backwards by a tooth.

For engines using race cams this does not give enough accuracy for cam timing that will keep the engine safe and give peak performance.

A basic solution for this problem is to use an offset dowel to give additional cam rotation but a much better, more adaptable solution is to fit vernier cam sprockets.

Vernier sprockets have an inner wheel which is bolted to the camshaft and an outer toothed wheel which is driven by the timing chain. The two wheels are bolted together so that when the timing chain turns the outer wheel the inner wheel, and thus the camshaft is also turned. However the two wheels can also be unbolted allowing them to move independantly. This allows fine adjustment of the camshaft positioning without turning the crank, i.e. accurate cam timing.