Ok you legends, here's one for you.

The brass gear is meant to be a direct replacement for the shitty plastic one which cracked.

The original one spins freely on the spindle, the brass one is tight and binds up after six or seven turns.

So it seems the thread is close, maybe not cut correctly, maybe not the right thread at all.

Got hold of a 1/2-10 ACME tap, it goes through both of them without any binding at all, they both rock a little on the biggest part of the tap.

The ISO trapezoidal tap in the closest size is even looser.

Measured the ID of the original, was 11.3mm, the new brass one was a bit smaller, drilled it out with an 11.3mm bit, followed up with an 11.4mm bit just to be sure. Still binds up on the spindle.

The shop that supplied the brass gear is not really helpful, won't tell me the thread spec, says that the spindle must be damaged, which I don't agree with, since the original gear spins freely.

They say use lapping paste and work the new gear back & forth to grind the thread. I don't think this will work...

Any suggestions? Or am I fucked?