The cast distributor gear on the left is good when used with cast camshafts (normally found in flat tappet, low lift valvetrains). But if you’re using a steel camshaft, you will need to swap it out for a brass gear like you see on the right to prevent premature wear. These also happen to be for a Ford distributor which uses a small roll pin to hold the gear to the distributor shaft. While swapping them out, it’s also a good idea to drill out the hole in the distributor shaft to take a beefier roll pin (center) which is used in Chevrolet distributors. This reduces the chances of the roll pin shearing off and leaving you without spark (or a functioning oil pump).