You shouldn't have to use a wrench to shim the gas cylinder. The wrench is only for holding the gas cylinder while you tighten your gas plug. I don't even use one. I just hold onto the cylinder with my hand and tighten the plug with a combo tool.

When your shimming the cylinder the gas cylinder lock should just stop at the 5 o'clock position and then tighten by hand to the 6 o'clock position. You don't need to crank the lock down. Thats according to "The Complete m14 Assembly Guide"