Cobra#1 Pictures of the gear would help, Here are some failures of why
1 Wrong gear material to camshaft.
2 Too much endplay of camshaft causing gears not to mesh correctly. Need to measure this before reassembly of motor, could happen again.
3 IMP DON'T enlarge the pin for the distributor. This just gives less material of the shaft to turn the gear with if it runs the
oil pump. If the motor has a different oiling system,( drysump or crank driven) there should be no problem with needing a bigger pin.
Failure of this pin in an FE distributor happened in my car. I have a 100 pound spring and cold run 135-138 psi at idle. 15-40 Rottela
oil 13 quarts. The problem was not leting the
oil warm up before running. Let car run at 1,100 rpm for 2-3 minutes and drive it. Pressure is under 90. I tryed a bigger pin and broke the shaft at the hole. Also this pin breaking may save your motor from metal getting into it and eat itself alive.
I have seen this.
4 There is no oiling to this part of the motor and these 2 gears rely on oil spray, mist, or splash to lube the area of the gears. If you have a oil galley behind the distributor you can put a .005"-010" hole in the plug. You will loose 5-8 psi of pressure depending on hole size. This will give you oiling to the part of the motor and help stop wear of the gears.
5 Alignment of the distributor assembly into the block and contact of the 2 gears to be 90 degrees and centered. The block may need to be remachined and have a spacer installed to center the distributor square over the camshaft.
6 Check 2 things before reassembly of the motor, pull the oil pan( I know this is a pain in the ass) find the other parts of the pin and check for damage. Cut open the oil filter and again check for metal or any weird partials. Motor should be ok.
Unless you have or find all 3 parts of the pin DON'T run this motor, unless you have a spare. We did this at Run&Gun 3 years ago with installing a new pin to make a couple of runs, On the 2nd lap the car quit. The piece of pin from the oil pan got through the screen and locked up the oil pump. No damage to the motor except for a 6,500 rpm backfire.
Any way good luck with your repair. I have tryed to cover all ends. Rick L.