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would it make much of a difference running 29er forks instead of 27.5 as I have a set of 29er forks but a 27.5 Â frame ?
Depends on the travel. The number you need to think about is the axle to crown distance. If this is similar to the fork the bike was designed for then it should be fine. Offset will probably be a bit larger on the 29er fork but you may not even notice that.
If you already have the forks and the frame, you might as well try it and see.
Whats the worst that can happen?
All things being equal it'll be 19mm longer, not a huge deal really. (yadda yadda BB height yadda rip off head tube even though the increase in leverage is equivalent to wearing a half filled backpack yaddda slack seat angle etc).
It is for a slack head angle frame anyway, just looking at using them rather than buying again
Go for it, and if anything about how it rides upsets you, swap them out.