KZ650 Modified clutch basket durability

I've been reading up on the 6 speed transmission swaps for the kz650 and the only part that I'm hesitant about is modifying the clutch basket to fit the shorter output shaft of the kz550 transmission.

www.kzrider.com/forum/11-projects/602462...-mod?start=40#737632

As his build is still in progress I can't really ask how it's holding up in the long run. Anyone have any constructive thoughts on this, or better yet actual experience? Maybe someone ran a supercharger on a 550 engine and the clutch basket splines didn't fail. Am I just being too paranoid?

The 550 bottom end went on to be the GPZ600 water cooled, so plenty of headroom there and clutch basket spline failure I've never heard of on these.

I considered the 6 speed mod but decided against as the 650/750 have nice torquey motors so a close ratio box maybe less important than the rpm power tune of the 550's.

That link is an amazing build thread, one of the best on the forum.

I agree on the close ratio nature of the 6 speed vs 5, the reason I started giving it serious thought was the numbers he shows from gearing commander. It looks like the final product is almost a stock 5 speed with an overdrive gear.

aplaceinspace42 wrote: I agree on the close ratio nature of the 6 speed vs 5, the reason I started giving it serious thought was the numbers he shows from gearing commander. It looks like the final product is almost a stock 5 speed with an overdrive gear.

The top gear (5th) ratio for the KZ750 is .88, so it's an overdrive ratio.

The KZ550 6th gear ratio is .85, so basically the same.

If the gearing ratios don't pan out and I just wind up having to shift more often I'll just have to laugh at myself until I find the ambition to tear the engine down and swap back to a normal five speed.

In the mean time I need to decide which spare clutch to tear down and lathe to fit.

I can't help but chime in.

Caveat: My thread on how I did the 6-speed swap is meant to be a "This is how I did it, not necessarily how it should be done." and is a totally unproven series of steps. As you pointed out, there is a risk in machining material off the inner hub, and I have no idea how it will hold up.
I just like to document my steps because when I read about folks doing this swap, they rarely explained how they did it, much less provided pictures. So maybe you and others can learn from my triumphs and mistakes.
But from all that I read, all those who had done the swap said it drove like it was meant to come from the factory with six speeds. So since I was splitting the engine cases anyway and I had multiple 550 transmissions laying around, I decided to give it a try.

First to consider...
The kz650 does have the same 2.55 primary drive ratio as the 750 going into the transmission, but my build is so different from stock, that you should run the specific numbers of YOUR kz650 in gearing commander several times to make sure it is possible to get the rpm-to-speed benefits you want.
As you mentioned, I wanted to end up with the exact same rpm-to-speed ratios as the original 5-speed but with a sixth overdrive gear. And I had to use very specific front and rear sprocket tooth counts, rear tire circumference, chain pitch that combined with the 6-speed transmission to end up with what I wanted.
You will have to get a new front sprocket as your 650 will not work, which means you will need a new chain and rear sprocket to go with it. You should switch to a 530 (or maybe 520 if you can) chain as your choices of offset sprockets will be more varied. (plus its just free power to lighten the rotating mass of the chain) So you have to change the chain pitch number in gearing commander or all your numbers will be way way off.
630 Chain Pitch- .75
530/520 Chain Pitch - .625
So for example, you may find that with your type of rear wheel that you can't get a wheel tall enough to make the numbers you want, so do lots of research.

As for machining the clutch hub, I chose to machine extra space for a beefy washer to distribute the force of the lock nut to a greater area.
I'm still not sure my decision was the right one, but ai didn't like that the lock nut was chamfered on the back so much that only a small amount of surface area was exerting pressure to the clutch hub. The washer that comes stock on the clutch is a warpy washer to help keep the lock nut from backing off. (mine was not a warpy washer and had just a plain one, yet the nut was still so tight I bent my clutch holding tool when removing said nut.) I'm just relying on proper torque, blue locktight and prayers to keep the lock nut from backing off.

As of yesterday, I have my engine back together ( for the second time) and took it for a 5 mile spin around the subdivision and the transmission shifts like butter, but I need to get plates and insurance before I can do a proper road test. So stay tuned.

I converted to the six speed about 5 years ago. I left the washer off the clutch hub and thread locked the nut. No issues. My B1 Z650 is bored to 810 and I have a turbo on it now. I have 530 chain and sprockets are 16 & 38. I'm pleased with the 6 speed.

That is surely empirical evidence that I didn't need to use a washer, but I was definitely to scared to not have the locknut screw all the way on.
If I was to do it again, I would still mill the hub 3mm to let the nut thread all the way on, but leave the washer off.
But Its good to hear from another satisfied 6-speed user.