[blykins]

It hasn't worn, that's just the size of that particular shaft. The part of the shaft that interfaces with the block (goes down into the hole to contact the pump shaft) is always going to be the same diameter, no matter what. However, there is a difference in the diameter of the shaft based on whether or not it's a factory or aftermarket distributor. That difference will be up much higher, where the gear rides.

I'd stay away from both bronze and polymer gears.

It's pretty clear cut: if you have a flat tappet camshaft, you'll want to use a cast iron gear. If you have a billet core roller cam, use a steel gear.

If you're unsure about which inside diameter to get, you can measure the diameter of the shaft and get an approximate diameter, but the only perfect way is to press the gear off and measure the shaft where the gear rides.

I get to do this task very frequently and it's a pain in the hindend, but if you do change gears, here are a couple of pointers:

1. Do not assume that you can press the new gear on, line the holes up, and everything will be fine. There is a dimension on how far the gear needs to be pushed onto the shaft and you will have issues if it's not in spec. If I remember correctly, that spec is 3.045-3.050" (on an FE) from the bottom face of the gear up to the distributor housing flange.

2. I use Mallory or Crane gears. Mallorys will come with 3/16" roll pins and the hole is probably 1/8" on the shaft. You will need to get the gear pressed onto the shaft on the correct dimension, then drill through it all with a 3/16" drill and knock the pin in.