Its a M9 ribe bit unusual size that tend to break at the head. I've used several and the laser one is **** breaks and strips the head of the bolt. Very poor fit. The length is also critical or else you wont be able to do bolts 7,8,9,10 under caps one and six on the in and out camshafts.
Valve spring compressor adapter can't be bought anywhere I've found, custom machine M10 deep reach socket. Tool steel is hard as to machine.
A m10 ribe bit is also handy to recover striped heads!