Are you using the wrong sized spanner?
If your using metric then use an imperial one!!
I have a habit of using the wrong one and although it fits and seems to work. Once its tightened then it tends to slip and starts rounding the edges off.
I had rounded bolt (slightly) that needed removing. So I filed a couple of the spurs off and tapped (twatted) a socket (correct size) onto it and then I managed to get it off. I then twatted the bolt off the socket using a metal rod that fit into the square drive hole!!
The socket came out of it without any damage but thats probably down to luck!!
I wouldn't use an adjustable spanner as they are quite slack and hard to get fully tightened and stay tightened while turning!!
Not much use am I!!
