Custom-machined set of steel main caps, featuring a 4-bolt center main. A must for high-revving race engines! We've been racing on our design of steel caps for over 20 years without a problem. The two outer bolts (included) of the center main are 7/16", and no thrust washer is cut into the cap. This cuts down on machining costs necessary to align the thrust surfaces. Fits all 1300 A-Plus or Austin America blocks. Line-boring is required for installation.