I've had this happen 3 times to me over the past few years...beginning to wonder if it's something I'm doing or if I'm just unlucky. 2 times it has been on bolts I did not install originally (one seatpost binder bolt, one front derailer cable binder bolt...different bikes); once on a bolt I did (front derailer cable binder bolt, yet a different bike, but suspicious!). Can overtightening cause it, only to reveal the damage once removed? Also in all cases the area in question has been very dirty/gritty which I thought was suspicious. Would attempting to clean before removal have prevented it? Anybody else experience this? Ideas?
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
One thought, how good are your allen wrenches? Cheap or badly worn hex keys can easily damage bolt heads since they don't fit properly and slip under load. Use relative new high quality wrenches and discard worn ones.
Also, are you using the proper metric wrenches? There are a lot of English (aka SAE) hex keys that are a "close but no cigar" fit in metric bolts and will damage the heads if used.
Yes, you are correct, over tightening can cause damage to the bolt threads. It is easier with small screws and screws with course threads. You won't notice the damage until you remove the bolt, or it breaks when tightening it. Using a torque wrench will help. Or just back off on the muscle.
When unscrewing something the thread may be dry or even rusty. If you feel resistance add a drop of oil and screw it back in again; this will take the oil into the threads. Back out again and if you feel resistance again repeat the process. Keep up this in and out process without applying too much pressure. You may only get it a fraction of a turn more each time but eventually it will unscrew all the way. If you just keep unscrewing a dry thread and it seizes it will just tear or strip the thread off; especially very small or fine threads.