While working on a client's bike the other week I found a flaw which could have conceivably resulted in a nasty accident for him. Note that he had been riding the bike with no indication of a problem right up until then.
Take a look at these photos:
As you can clearly see, the stem clamp failed when I tried to remove it...and not at one bolt but at all four!
Your immediate reaction to this might be "I'll be sure not to buy that brand of stem" but the truth of the matter is that this brand of stem is excellent and I have used them many times on many different bikes with no problems. The issue actually is all about torque.
Modern lightweight equipment is made to be assembled with very specific torque guidelines (level of tightening). In this case all four bolts were over-tightened, setting up stress risers in the metal which resulted in the hairline cracks around the area where the bolts seat and eventually a complete fracture. The problem would not have occurred if whoever did the initial assembly had observed the torque value printed or engraved into the body of the stem (or the instructions which came with it) and had been sure not to exceed it.
I've met a number of people over the years who think they can do safe assembly without using a torque wrench but why take the risk? Using a good torque wrench can help to prevent a lot of potential problems and is absolutely essential with carbon bikes and components.
