I have a old Hifly board and with Akutec base. The tendon finally broke and are not available anymore so I have bought a Boge joint and will modify the base to make it work. The problem I am having is working out what the thread is in the Boge joint. Its says M8 but when I fit an M8 bolt is still feel loose....it still has some side to side play.

I have taken it to a bolt supplier and they could not figure it out. Does anyone know exactly what thread it is?