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Torque wrench clicked at 40 on a stem bolt and I still snapped it, so who is actually wrong here
Had a customer's bike up on the stand at my shop in Sacramento on Tuesday, installing a new stem, and my 3/8 torque wrench clicked right at 40 inch pounds, but the bolt still sheared off in my hand. Half the guys I know say I was an idiot for not greasing the bolt threads first, and the other half say the wrench is out of spec and I should have it checked. I ended up drilling the stub out and chasing the threads for 45 minutes while the guy waited, which was fun. So what is it, user error, bad tool, or just cheap alloy bolts? Curious what you all torque your stem face plates to and whether you ever lube the threads.
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diana_king26d ago
Wow, so the bolt just decided to quit its job on your watch, huh? That is a classic torque wrench whodunit and honestly nobody ever wins the blame game on these. Here is the thing though, inch pounds is such a tiny amount that a click at 40 on a stem faceplate bolt is right at the edge of what those little M5 alloy bolts can take, especially if they are already stretched or have been tightened before. Greasing threads changes the friction a lot, so a dry bolt that felt like 40 actual got way more clamp force, that is probably what killed it. Your wrench could also be off by a few inch pounds and you would never know without checking it, most guys never do. Cheap alloy bolts are just plain weak too, so it is kind of all three, but if I had to bet money I would say the dry threads plus a tired bolt did you in. I run stem faceplates at 5 Nm and yes I put a touch of grease on the threads, and I still hold my breath every single time.
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