Depends on how and where you want to ride. My 05 660 would burn the 26" Mudlite XTR's I was running. I just added 16g greaseless roller weights. It did have out of necessity (OEM CDI bricked) a Dynatech CDI in it that opened up the performance some. However for shets and giggles you could start with 2mm shims and I would jump to the purple spring.I have an 04 grizzly 660 with 26" mud lites everything else is stock, would I benefit from a shim mod if so how much should I shim it?