Part and parcel of the advance curve process is limiting the amount of the curve. This is a simple task in an MSD distributor. Underneath the advance assembly plate is a locknut and washer (they're fastened to a post on the bottom side of the distributor shaft). When the hardware is removed, you'll find a bushing. It's removed and a new one (larger or smaller) is installed. The smallest bushing (red) allows for 28 crankshaft degrees of advance. The silver bushing allows for 25 degrees, and the black bushing-the largest overall-allows for 18 degrees of advance.