Thanks Fontyyy - just the sort of information I am after, from someone who has first hand experience.

I wanted to build a track only bike - and with 2 '21's in the shed, another one seemed a bit ho-hum. Time for something a little different.

So, I now have much of the late model NF5 parts, mostly stuff that involves little engineering should it need to be changed out, if you get my drift. The bottom end is where I thought the incompatibility issues would be the greatest.

I'm not too keen on a complete NF5 motor, solely due to the crank availability and service life issues - I know they're closer to MC18's - but MC21/28 cranks are still available.

Still planning, hunting and hoarding at this stage. I definitely don't want to slap it together, but just make sure I'm on the right track.