How many miles are on the vehicle? Are the bolts your purchasing the same length as the original one? I'm assuming your talking about the lock bolt of the alt. belt tensioner? How do the threads look where it gets threaded into? The bolt is actually snapping and its happened more than once? The next bolt you buy make sure its grade eight... Perhaps your over tightening the belt...the deflection should be around half an inch for every foot of free span... Do you have any bent pulleys or anything?