18" bmx cranks size

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5/19/2014 2:45 PM

I'm thinking about upgrading the cranks on my sons 18" bmx. It currently has 140 mm stock cranks which are pretty heavy.
Was thinking of going to 165mm (either bsd or something similar) but I also have a set of 175 flys lying around that I might try.
I'm guessing that the flys would be lighter than the stock 140s but am not sure if the 175s would be too long.
What problems could the 175s cause if they are too long for a 18"?