i blew the head gasket on my 72 norton fastback. i have been able to remove all but one of the bolts- the back one that is upside down at the rear of the engine. any ideas as to how to get it off- nothing fits- all the others are 9/16ths. any ideas are greatly appreciated. my bike is just sitting. stephen

It's 1/4" whitworth and you need a fairly small ring spanner (wrench) to get on to it. I've never tried using an alternative but I think it's one of those whit. sizes that doesn't have an equivalent in Unified or metric. 3/16" and 1/4" whitworth combinations as well as a small (3/8" drive) 1/4" whit. socket are really a minimum for working on the Commando Top end.