Originally Posted by Steev
I've used a SRAM 8 speed grip shift to shift a 7 speed Shimano cassette with a Shimano derailler. If you set up the cable tension with the shifter in the middle and the chain on the middle cog so that the spacing error is evenly spread, it will mostly work, but I never found it very satisfactory. The difference in spacing is 0.2mm between 7 and 8 speed (according to Sheldon Brown). I'd say get the right shifter for the purpose.
Gotcha. I guess it's kind of like using a Shimano 7-speed cassette w/Suntour 7-speed shifters. I'm doing that on my girlfriend's bike and it's very satisfactory (the 0.2 mm difference is only in the upper 3 cogs, weirdly enough).
Also, going from 7 to 8 speed, I believe is where the jump from 126mm spacing to 130mm spacing happens, so the frame may need to be cold set for this.
Yep, to run an 8-speed cassette you need 130 mm spacing supposedly. However, there's a much simpler alternative: just squeeze the dropouts open by 4 mm to get a 130 mm hub into a 126 mm frame. I've done this on both aluminum and steel frames for now 1000+ miles, with no ill effects!