Bicycle Mechanics - Need to know what size allen wrench to use to get crank arms off

my bottom bracket "unspun" and i need to get them off to fix it. i had an allen wrench that fit but i seemed to have lost it. so i wanna go pick up a new one without having to buy a whole new set. the cranks are single speed FSA's and it's bigger than 6mm if that helps

8mm

+1 -- It's 8mm, but if an 8mm key isn't readilly available a 5/16" (7.94mm) will do in a pinch.

Mine is 10mm

I'd strongly recommend an Allen socket to use with a torque wrench. Getting cranks up to specified torque is pretty critical from what I've heard (though I've always brought the torque up to spec and never had a problem, so I haven't ever seen insufficient torque kill a crank).

Here's a good unit, assuming that your cranks use 8mm and you have a 3/8" torque wrench (like the bigger Park Tool wrench):

http://www.mcmaster.com/#8367a45/=2aser4

To get the cranks off, you need a crank puller:

http://www.lickbike.com/productpage.aspx?PART_NUM_SUB=%272430-00%27

Best of luck.

Mine is 10mm
Is it an FSA? All of the Shimano cranks I've ever seen are 8 mm but I came across one requiring a 10 mm and, IIRC, it was an FSA.

Perhaps you can just measure across the hex opening?

Park Tool makes a nifty tool that they say will take off the bolt on cranks. It is 8mm. Since I bought that tool - I haven't seen a single 8mm bolt on a crankset. Only 10mm. I suggest getting a full set of metric allen-wrenches. I even have a 7mm - though I've never used it!* LOL!


* - inside joke for mechanics

I even have a 7mm - though I've never used it!* LOL!

* - inside joke for mechanics

Ever had to work on the early-90's Campy self-extracting cranks*? That's where the 7mm comes in handy.



* - well, neither have I. :D

Word.*

* - I didn't actually buy the 8mm socket individually, I saved myself the aggravation and bought a full set! :D

My SRAM cranks use an 8mm. I recommend an 8mm socket allen head. But if you don't want to do that an small pipe or hollowed out lever will help you with an allen key.

I initially said mine was 10mm. I just changed my crank and bottom bracket this weekend (with the cassette and chain) and I was wrong, it's not 10mm, it's 8mm. It's a Truvativ, both, the old and new.