I am about to get a 10spd hub and have a 9spd cassette so will this fit ok or will I need a spacer or new cassette?
Who says it is 10 speed?
Shimano used to make 10 speed hubs (2004 - 2007) with taller splines to prevent steel cogs biting into aluminium freehub bodies. It didn't catch on, and they reverted to the shallower splines. So all Shimano / SRAM freehub bodies are now the same. (Unless you go to 11 speed Dura Ace, which, I think, just needs a spacer to be compatible with 8/9/10 speed cassettes.)
So, no problem (unless your hub or cassette is Campagnolo, which is another story).
Thanks. It's shimano as from a friend so reliable source. Good I only need a spacer.
You'd only need a spacer for a 7 speed cassette. 8, 9 and 10 speed cassettes all fit on the same freehub because they progressively narrower chains and spacing between sprockets.
just for reference a Campagnolo hub that takes a 10 speed cassette will also take a 9 (and an 11 for that matter).