OP: Did you use any lube at all before placing the gear onto the shaft? Was it extremely difficult to put on?
I do not know if the gear is too small to work, but maybe a gear puller at your local autoshop might be able to get it off, or heat the gear with a torch and see what you can do.
Quoted from johnwartjr:The originals might last 10 years.. but the repro plastic gears on the market are not as well made as the originals, some failing within days of installation.
A +1 to this. Broke 3 repros in 3 months on my FH. Put the original back on, been going strong for 5+ years. Still hoping Pinbits does a Delrin gear set sometime soon. Was thinking about buying one of the metal gears mentioned by the OP, but not at 90+ bucks on ebay.