Lengthen forks for zx to kz swap

I read somewhere on here but can't find it that you can add washers to lengthen forks when rebuilding them.
I'm using 1991 52mm zx7 forks on my 80 kz1000 .
The zx7 forks are 3" shorter but have been sitting with low oil for decades

You can lengthen ZX7 forks internally by a maximum of 32mm with a screw on extension on the end of the damper rod.
See attached pics.

The extension is drilled lengthwise to allow the flow of damper oil to the forks compression adjuster.




You can also gain another 20mm by extending or modifying the fork top nuts to allow the legs to drop the forks through the yokes.
Any more than this and the fork legs tapered outer profile will start to run out of the bottom yoke .

The next option after this is stepped yokes or fit some longer ZX9R - B usd's which fit straight in the 7's yokes and will also take the exact same mods as done to the ZX7's.

my mistake. already answered.


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Cool, thanks for the info.
Is the screw on extension something I can buy or is it a custom made parts?

The ones in the pics I made myself. but any decent machine shop or home machinist could knock some up no problem.

Looking at it now, why can't I just use a spacer on the bottom and a longer bolt?

Bulletkz wrote: Looking at it now, why can't I just use a spacer on the bottom and a longer bolt?

You mean instead of the extension ?
You could but the bolt will still have to be bored out down it's length and cross drilled like the original for the passage of damping fluid.
Not really the best or strongest stable way to do it as the long hollow bolt will a weak point compared to the thicker walled extension but it will work.

I spaced it up with a 3/4" nut and bought a longer bolt. Worked out good.

Is this on USD forks with the compression damping adjuster on the bottom of the leg ?
If so you need the bolt drilled both down it's length and crossways just like a brake banjo bolt to allow the passage of damping fluid and the nut you have used may probably need a copper sealing washer between it and the bottom of the damper also if that is the case to prevent the bypass of oil.

Hey zed1015, hopefully youll see this...

Just curious if you could spill more details on the top cap 20mm extensions? Did you just weld a donut onto the top of the stock cap and lathe it back down? Or is it a whole new billet piece? Billet would make more sense I guess ....

I have some zx9r forks and I'm 20mm of fork length short from stock ride height. Which method would you recommend?

Saablord wrote: Hey zed1015, hopefully youll see this...

Just curious if you could spill more details on the top cap 20mm extensions? Did you just weld a donut onto the top of the stock cap and lathe it back down? Or is it a whole new billet piece? Billet would make more sense I guess ....

I have some zx9r forks and I'm 20mm of fork length short from stock ride height. Which method would you recommend?

I would use the damper rod extension, Very simple and effective especially if you only need 20mm or so and I have done the same on my ZX9R forks that are on the Drouin bike..
The top extensions were not done by me.
Someone had welded 55mm hollow alloy slugs (doughnuts) on to the top of the original caps on some short ZX7R USD's.
They had done a nice job but you couldn't access the damper and preload adjusters.
I machined off 35mm to rectify that and also because it pushed the tapered lower USD leg profile out of the bottom yoke and then reclaimed the length on the damper rod.
This put the ZX7 forks around the same length as stock ZX9's

I fitted the ZX9R forks on mine but they were still too short,so I had these made up;

They use the original fork tops/caps trimmed down and threaded into an extension tube which is in turn threaded into the top of the original fork tube. This means you can keep all the adjustment control of the original forks.