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Hello everyone, I've been looking at my project recently and couldn't find out why there was a rear rotor/caliper on my bike when I couldn't find any other KZ400's with rear calipers, upon looking at the rims they appear to be from a 1977 Yamaha XS750. Has anyone ever heard of someone doing that conversion? Everything seems to be there and the factory KZ400 caliper seems to line up on the front rotor as well, I like the benefits of having a rear caliper instead of the drum assembly just thought I'd post on here and see if it's worth completing the setup. I'll need to track down a rear master cylinder assembly if I do but I'm wondering if there will be more fabrication needing to be done than it's worth.

As far as the rear brake goes, you will need to weld a mounting plate for the rear master cylinder and may need to make a small modification to the brake lever. I see that the torque arm is welded to the swing arm. I suspect that will cause some binding as you move the axle to adjust the chain. The arm should pivot at both ends. Before going any farther with the brake, I would make sure that the front and rear sprockets line up perfectly so that the drive chain will not be put in a bind, or rub on the frame or shock absorber.
If you like the design and the rims will allow you to use modern tires, I bet it will be worth the trouble, but only you can decide for sure.

Thanks! Not too sure if I still want to go this route or find original rims to put back on and save the hassle, I bought the bike as you see in the pics and 2 totes of parts so I'm almost wanting to put factories back on and save these rims until I know what I'm dealing with. There are brand new tires on it but I'll have to see if they are the same size to just swap over. The totes of parts look like KZ parts and the only Yamaha parts appear to be the rims so far, even the brake lever doesn't appear to be for the XS.

After you mentioned the part about the welded bar, I went and had a look, it appears the whole swing arm was from a XS750 as well, the shock mounts are different from the original KZ400 swing arm that thankfully was in one of the totes that I will swap back over. Looks like I might need to look for an XS750 project now as I have a significant amount of parts for it now.

I may be wrong on this but were not all XS750s shaft drive? Your swingarm and wheel might be from an XS500 which were plentiful and not as well regarded as the XS750.

You're right, I had an XS750 that was shaft drive, I was just going by the year and typed in yamaha rims and that was what came up. I'll look into the XS500 and see if that gets me anywhere.

Edit: Upon searching for 77 XS500, that definitely looks like the rims and swing arm and it appears that the torque arm is supposed to be welded, either way I think I'm going to reinstall the factory one and work of the easier to find KZ400 parts.