Originally Posted by FlyingPickel
FYI: The Rule for proper thread engagement into aluminum (top fork clamp) is 1.5 x Diameter. If your using M12 bolts (12 mm dia) you need a minimum of at least 18 mm of thread engagement to ensure the bolt will fail before the threads strip out of the aluminum. If you don't currently have enough thread engagement you should get some longer bolts.
In Terry's case it was the bolt that broke suggesting he had the correct amount of engagement (or used a thru bolt with a steel nut). Also, in the case of blind hole you also need to be careful to have enough extra hole depth so that the bolt does not bottom out before achieving the proper clamp load on the joint (generally indicated by achieving the proper amount of bolt torque during assembly). Again this did not appear to be an issue in Terry's case as it appeared he was able to remove the broken bit without a great deal of trouble.
At somebody already mentioned Stainless Steel bolts are not usually considered high strength. Bolts marked as 10.9 DIN are higher strength (approx equivalent to a Grade 5 in the USA), and probably a good option for clamping Aluminum parts together.
A couple of other things that can contribute to bolt failure are not having clean parallel surfaces at the clamp joint, and not having enough clamp load (a.k.a. bolt torque). Bolts are designed to act like springs and if the clamp load is to low the bolt goes through a high number of repeated stretching cycles during use. The result is early material fatigue of the bolt (like bending a paper clip back and forth until it breaks), and possible fretting and wear to the mating surfaces being clamped.
Great ride report Guy's.
+1 People tend to believe that a high tensile bolt is better but the application is to be considered first. If the bolt is to have any side force on it you are better with a grade 5 which will bend and not snap as high tensile will.