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Fairly certain there wasn't a spacer there when I took it off, but could someone in the know tell me for sure.
Will it be immediately obvious if I do it wrong or will it be one of those issues that just lingers until you're 20 miles from home before jamming up?
Its probably 10 speed AW. You can get 11 speed freehubs for Pro2 evo, but they are aftermarket replacements. 11 speed freehubs are shipped as stock on Pro4. Pop round with it if a problem.
There isn't a spacer on mine.
No spacer
Depends on the exact age of the hub, if it is one of the 'end of line' pro 2 evo's with the 40t engagement then it may have an 11sp freehub fitted. Chances are that if there was no spacer there when you took the previous cassette off then you won't need one. It should be obvious in that the 13t and 15t cogs will feel loose when the lock ring is tightened if the spacer is needed and missing.