Join Date: Jun 2011
Location: Sunnyvale, California
Bikes: Bridgestone RB-1, 600, T700, MB-6 w/ Dirt Drops, MB-Zip, Bianchi Limited, Nashbar Hounder
Nitto's specs for M6 pinch bolts is around 5 N-m (around 4 ft lbs = 48 in-lbs). But for M8 pinch botls, it's 13 N-m or around 10 ft-lbs. If you have a 6mm allen wrench bolt, you will need to check if the driver hole is 6mm allen but the threaded bolt might be bigger. That is sometimes true with single clamp quill stems.
To give you an idea of the "feel" of tightness, with nominal 2 inches of leverage of an allen key wrench, you will need around 24 lbs of force to tighten to achieve 4 ft-lbs. If you have a longer reach allen with 3 inches, then 16 lbs of force, etc. For 10 ft-lbs (i.e. 120 in-lbs) you can see how tight that can be.
Note: Before you go over torquing something to compensate for slipping, I'd recommend cleaning the clamp surface first of any thing that might act as a lubricant, and then carefully greasing the threads on the clamp bolt but not getting that grease into the clamping surface.
Most mechanical bolt designs for metal parts have at least a 2-to-1 safety factor. But you shouldn't have to go past spec to have it hold for you. If you over torque, the most likely damage would be to strip the bolt, shear off the head of the bolt, or strip the nutted threads. In some cases, you could flex and crack the clamp.