For the 1991 model year, the mounts on the frame for the linkage dog bone changed as did the swing arm, linkage dog bone, and shock. I knew the frame mounting had changed but wasn't aware of how much the rest of the components changed. Here is a photo of the 2 dog bones side by side; 1990 on the left and 91+ on the right.
![Image](https://bannedcr500riders.com/images/auto/1fb5e95f17117594f61aea56d2a7c1eb.jpg)
1990 dog bone, KZ4 casting number:
![Image](https://bannedcr500riders.com/images/auto/2332868e7a813f5cddae4ddd71685b60.jpg)
1991+ dog bone, KZ3 casting number:
![Image](https://bannedcr500riders.com/images/auto/0e127ba9b6a22dba8da243ee8d08a526.jpg)
At first I thought I could retain the 1990 swing arm and just swap the dog bone and shock. The dog bone bolts right up but when I went to bolt the shock up, I couldn't get the hole to line up because the shock was hitting the swing arm. That's when I realized the swing arm changed slightly; here is a photo of the 1990 swing arm on the right and a 1991 on the left:
![Image](https://bannedcr500riders.com/images/auto/233f6777d7c8fba9f77c87eec2e5badc.jpg)
1990 swing arm close-up:
![Image](https://bannedcr500riders.com/images/auto/12e11163a3e8ffdf0456cdb3f3d49330.jpg)
1991 swing arm close-up:
![Image](https://bannedcr500riders.com/images/auto/e8f0956d8da8bcafe64c94ae988742b0.jpg)
I also have a 1995 swing arm for my other CR500, and comparing it to the 1991 they are almost identical. The only changes were to the sizes of some of the linkage bolts; the 1991 and 1995 linkage components had the same bolt center to bolt center measurements and 1995 was the last year there was a revision made to the rear suspension. I bolted up the 1991 swingarm and linkage to the 90/98 mongrel along with a 1998 shock (an upgrade since it has both high and low speed compression adjustments) and will follow up with a complete 1998 front end as well once the temp in the shop drops enough to be bearable. With the longer 1991 linkage dog bone the height of the rear end of the bike dropped quite a bit (sorry, did not take these measurements but it's a significant difference).
Hopefully someone else will find this info useful!