Originally Posted by LOSI123
if you have a metric 3mm tap its easy to tap out an imperial alum pinion to accept a 3mm grub screw.
That would be at best a bodge to "get home".
1. If the pinion is hard steel is not easy to tap (if possible at all). You will most likely destroy both tap and pinion.
2. If the pinion is aluminium, you will be able to cut the thread but it won't hold for long (if at all). I have done it, it's a waste of time and it will let go at the worst time.
The problem is the M3 thread pitch is 0.5mm i.e. finer than both 5-40 and 4-40 imperials. It is also shallower (if that makes sense; I mean the depth of each thread is less than the imperial sizes; think about it: to cut a thread at M3 you need a hole at dimaeter 2.4mm. That leaves only 0.6mm for the thread on the diameter. The thread depth is half that, i.e. 0.3mm). This means what you will do is basically scratch the imperial thread to a M3 thread (pitch/depth) which is not sufficient.
On top of that, some materials give in after a while even when they start with a M3 thread from factory (some of the pinions I bought have stripped the thread after taking out and tightening the grub screw a number of times) so this thread is at best fragile (in normal aluminium). Overtighten it and you'll strip it straight away. Proper materials (good duraluminium, aircraft grade, etc) are good, but not too many manufacturers do use them. Steel holds very well if it starts with a M3 from factory but even then I wouldn't overtighten it. Hard steel is very good but then you might strip the grubscrew.
If you just want to finish a race, you can bodge a thread, but then use loctite if you want to get somewhere. If you're looking for replacements, Robinson black pinions 64 pitch are very good quality and really smooth finish so they last forever and don't chew the spur either. Kawada is the next best thing, their dural being a bit softer. The Kawada spurs however are on the par with the Robinson pinions.