Z1 vs KZ900 rotor compatibility

Good day folks. I'm a new member on the forum. Not new to wrenching on bikes, but new to the Z1. I bought a 1973 Z1 from the fella who owned it since 1979. Had been riding it up until this season. He said it had no issues. Well, after chasing down a ground fault issue with the starter, I got it running. Had a terrible low rpm knock, under 1000-1200rpms. I isolated it to the left side, and hoped it was a starter clutch issue. Sure enough, all 3 bolts bolts were loose and the locating pin had sheared off. The hole for the pin is wallowed out in the rotor, but also the alloy rotor casting has separated from the steel. I have a 1976 KZ900 parts bike and before tearing into it for the rotor, wanted to ask if they are interchangeable. I only ask because they carry different part numbers. Thanks for your time.

Simple answer is YES!.
All 903cc fours have the same KEYED rotor with 8mm retaining bolt so are interchangeable.
All 1015's fours ( except 1 short run model ) have the same rotor but it has no keyway and a 10mm retaining bolt.
Either type of rotor can be fitted to the 1000 crank but only the keyed type to the 900.
This is because the 8mm bolt on the 900 crank is not capable of being torqued enough to hold the rotor from being spun on the taper without the woodruff key whereas the 10mm bolt on the 1000 is.

Thank you, Zed! I pulled it and installed it. Knock went away as expected. When comparing the rotors the KZ rotor is just a little bit shorter than the Z1.

zed1015 wrote: Simple answer is YES!.
All 903cc fours have the same KEYED rotor with 8mm retaining bolt so are interchangeable.
All 1015's fours ( except 1 short run model ) have the same rotor but it has no keyway and a 10mm retaining bolt.
Either type of rotor can be fitted to the 1000 crank but only the keyed type to the 900.
This is because the 8mm bolt on the 900 crank is not capable of being torqued enough to hold the rotor from being spun on the taper without the woodruff key whereas the 10mm bolt on the 1000 is.

Hi zed1015,
Then the depth and angle of the taper on the 900 and 1000 rotors are the same? If so, any reason why a 1000 rotor could not be used on a 900 crank if a keyway was milled into the rotor's taper?

Good Ridin'
slmjim & Z1BEBE

slmjim+Z1BEBE wrote: Hi zed1015,
Then the depth and angle of the taper on the 900 and 1000 rotors are the same? If so, any reason why a 1000 rotor could not be used on a 900 crank if a keyway was milled into the rotor's taper?
Good Ridin'
slmjim & Z1BEBE

Both rotors are identical apart from the key slot so yes ! a 1000 rotor could be used on the 900 if a slot was broached into it.

zed1015 wrote:

slmjim+Z1BEBE wrote: Hi zed1015,
Then the depth and angle of the taper on the 900 and 1000 rotors are the same? If so, any reason why a 1000 rotor could not be used on a 900 crank if a keyway was milled into the rotor's taper?
Good Ridin'
slmjim & Z1BEBE

Both rotors are identical apart from the key slot so yes ! a 1000 rotor could be used on the 900 if a slot was broached into it.

OK, thanks! That simplifies things.

Good Ridin'
slmjim & Z1BEBE