7/8 speed compatability MTB
 

I destroyed an old acera 7 speed rear derailleur this weekend and am wondering if I can replace it with an 8 speed XT I've got. Will this work?

Thanks

If you can run in friction mode, definitely yes, assuming the replacement has enough chain wrap. For index shifting, probably yes.

Yup, you can run 8 speed. You could run a 9 speed if you wanted to. 9 speed or 8 speed compatability in a deraileur means it will shift for nine speeds, but if you don't use them all thats fine. My buddy put a 9 speed LX on his 1988 bike, and it works just fine.

My attitude on such situations is if I have a part I'll stick it on to see if it will work. Theoretically, because 8 speed cogs are a little closer together than 7 speed, one would assume you might have problems with indexed shifters, though as John E says, shifters that have a friction mode can work with many things. But it just might work. I have read that some Shimano 7 sp mtb shifters actually have an 8th click and will work with 8 sp systems. Hey, if you already have the part you can find out in about 10 minutes whether it will work or not. Even if it doesn't work 100 % it might work well enough to let you use a few gears so you can ride while you figure things out.