Shorten MK III Crankshaft?

Has anyone shortened a MkIII crank after ditching the sprag? If so, any comments after the fact?

I have cut a couple of them off for racebikes and cut new threads. I got rid of the alternator too. Nothing too special. Jim

I just did mine last month. I cut both cheeks down to fit the MKII spec and then cut the output shaft. It was pretty easy. I like having an extra stud holding it all together.

Cutting the cheeks would be the time to put a radius at the base of the PTO shaft to eliminate that weak spot - 1st place to break. Very good idea.

jseng1 said:

Cutting the cheeks would be the time to put a radius at the base of the PTO shaft to eliminate that weak spot - 1st place to break. Very good idea.

Click to expand...

that's what I did.

Jseng1 can you elaborate on the PTO radius. I'm building a lightweight crank flywheel from EN24 and whilst I've heard of this mod I've yet to see one done or even know the dimentions required. I presume you cut a larger transition radius between drive nose and cheek but thinking about it it would mean undercutting the crank cheek. Is this thinking correct? Sorry to hijack thread but gotta strike whilst the irons hot!!

Thanks guys,

Did you use the stock retaining nut or drill tap the PTO for a bolt. Seen both done, what's better?
That thing just seems to stick out for miles. I'm trying to slim the engine/primary down but need to keep the alternator for running lights.

Jim,
Any thoughts for balance factor for a P11 with 850 leaning forward? looking for smoothness in the mid-range.
I'ld love to splash out for an offset crank and/or your rods and pistons but life has been getting expensive lately so will have to wait for next time around.

Will

Cranks:
To cut a .090" radius into the PTO shaft joint by thinning the cheeks of an electric start crank you will have to cut into the cheek just enough to clear the radius of the super bearing inner race - it wont' take much. I have only cut this radius into non-electric start cranks - see photos. Its tricky because you must make a radius cutting tool and then be careful to keep it moving sideways so the entire tool doesn't dig in at once or it will chatter. Then polish with a fine dremel tool type stone and follow that with folded wet or dry paper. Never reduce the diameter of the PTO shaft. I have never broken a lightened radiused crank with the lightweight pistons. The crank (salvaged from junkyard) in the photo is from my monoshock road racer which was frequently revved into the valve float range and now its in my café bike as in my piston demo video at jsmotorsport.com.

Featherbeds:
I don't recommend stock heavy pistons in a solid frame - especially nothing bigger than a 750 - no balance factor will make it smooth. Its just not a good ride. You need the lightweight pistons to keep your teeth in your mouth - its worth the $.

Jim






Below is a photo of the first lightweight pistons back in the 1980s (850cc)


The 320lb 850 monoshock as I raced it below

Thanks Jim,

I'll give it a shot with 80% BF. Not for daily use. This will be a shake down build. I expect to redo a few things in a few years as funds allow.

Will

I would keep the BF down to 70% max.

The 80% balance factor has been used and handed down and I've used it myself but its too high in my opinion.


Jim

That's what I was after
Thanks again Jim, I'm sure that I'll be picking up a set of your rods and pistons in a few years, just can't swing it now.