"Russell Seaton" wrote:

I have a 1991 Shimano Deore DX rear hub. 7 speed cassette. I
want to convert the bike to 8 speed cassette. Will an 8 speed
cassette fit onto the same hub as a 7 speed cassette?

I know I will also have to get new 8 speed bar end shifters and
a new chain.

An 8-speed cassette is a few mm broader than a 7-speed one, so you can't
put an 8-speed cassette on your current freehub. You would need to replace
the freehub body with an appropriate 8/9 speed model.

http://www.sheldonbrown.com/k7.html#transplant

Another option would be to make use of the fact that eight ninths of a
9-speed cassette will fit perfectly on a 7-speed freehub. You'll need
9-speed shifters of course. When the time comes to replace your rear hub,
you can then make the transition to 9-speed.

http://www.sheldonbrown.com/k7.html#up7

HTH

James Thomson