Dont you even try it. It will easily damage the spur gear and you'll have extremely poor speed if as you said the hole is eccentric. TAKE IT TO A LATHE SHOP AND RELIEVE YOUR MIND.

I think mike has a shim you can use to put a 5mm pinion on a smaller shaft. If he doesnt it would be a good product idea.

I have a 6mm to 5mm reducer, but haven't found a 5mm to 4mm reducer yet. I am going to get a few 4mm bore pinions made the next time I get some done (very soon).

Hey Taz, you're a swissy?! You get PM!