Have been looking at replacing the water pump impeller shaft, I pressed the shaft out of the f3 impeller and a stock mc21 impeller (mc21 top, f3 below):



It looks like they started life as the same blank part and have been machined to suit, unfortunatley the small spigot at the end which runs inside the crankcase is smaller on the 21 shaft! I will have to try and source the proper nf5 part or maybe try and find a stock mc16 impeller.

I'm going to have the knurled section machined on this one to suit the f3 gear anyway. I can then use the f3 transmission on my 21 cases if need be, it even has the helical gear so I can run it with the standard oil pump

I tried it in some 21 cases and everything seems to line up a treat.