Do yourself a favor and remove the tank. Then the shock will come out by removing the two mount bolts. With shock out hook a tiedown over the seat to the swingarm and pull it up until the rear axle, the swingarm pivot bolt and the countershaft sprocket are all in a straight line. This is the tightest place for the chain. Adjust it with a 1/2 to 1 inch of freeplay at this point. Now reassemble every thing and look at the adjustment. Find a reference mark you can remember and use this for the tightest point in the future. It will be a lot looser than you think it should be. Better to err too loose than too tight. Tight chains destroy wheel bearings, sprockets and sprocket bolts, rear hubs, transmission bearings and even engine cases.
Howard
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01 KTM 520EXC
97 KTM 620RXC
98 BMWR1100RT