With the different tread to sidewall ratio options available it's easy to have the same size measured diameter whether 18" or 17" so rim size isn't a way to judge swingarm clearances. You can have 17" tires that are bigger than many 18" tires . it's the aspect ratio sizes you need to look at aka-130/80, 120/90, 110/100 ,ect.....
It your running a bigger width tire a person can use the longest chain that will put the axle near the end of adjustment , adds a little to the wheelbase , which these bikes need, plus gives another 8-12mm swingarm width where the tread will run . I have my '86 like that to fit a 150/70-18 Shinko 705 tire in there . A good quality DID VMX chain and you don't need to adjust slack fo most of it's life so the lack of adjustment room is a non issue.
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'86 XT600/3TB motor,,, '88 XT600, ,,'92 XT600e,,, '08 Husky TE510,,, '05 DRZ463,,, '95 KLX650 ,,,'96 KLX650
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