I'm in the process of rebuilding the bottom end of my mc18. I had a big end bearing fail and a piston was distroyed in the process. So, I have a spare crank, new bearings and new seals. Problem is, the spline on the end of my spare crank ( where the primary drive gear goes on) is shorter than the crank that came out of the motor. Also, the primary drive gear is thinner and smaller in diameter than the one I took off the crank

Primary drive gear




Ends of the two crankshafts (broken one first)




Why are they different? They both have the same bearing and seals, they both fit into the cases perfectly, both primary drive gears fit on the end of the crank and the taper on the flywheel ends are the same as well. I am worried that the smaller drive gear wont engage properly on the clutch gear. Can I use this spare crank or do I get the original one rebuilt, assuming that the pins are all ok? Off to go clean more parts. Thanks heaps for any help, appreciate it!

R2K Vs R5K ? just a guess.
I know most if not all the NSR cranks have subtle differences between them.
Some one here should be able to tell you what two cranks you are likely to have. fonty knows everything doesn't he?

Yeah, did cross my mind, maybe I could buy a different primary gear, because the thin one doesn't engage on the clutch hub real well............ will have to do some research!

First thing I would do is pull that collar off the cranks and switch them around. Looks like it'll be good to go if everything else fits.

When you engage the wide primary drive gear with the driven gear n the clutch you'll see that it's far wide than it needs to be any way. In moments of madness I machine the width down, which obviously lightens the gear, but not all the way to centre, just leave meat on the bolting faces. Must give a whopping 0.01% improvement. Maybe.

Ahh thanks HEAPS matt! Was stressing for a minute there! Now, how do you pull the collar off? Is it tight? Thanks for all the help guys.

Yes it'll be fffffffffff........ully tight.

Need a puller to get that puppy off.

A heat gun on there for a while would help.

Ahh, might have a trip to an engineering shop in the morning then. Should be interesting! Might just get the primary drive gear cut down a tad. Would be the easier of the two options methinks given the lack of specialty shops near me.