Bell housing to engine retaining bolts

I cannot get a spanner to fit correctly on the small bolts connecting the bell housing to the engine block.
The bolts have a major diameter of 6.8mm with a pitch of 1mm (about 25 or 26 to the inch). The spanner is somewhere between 11mm and 12 mm, the 12mm is lose and will not grip a tight bolt. The nearest I can find is a cycle thread but can it be that!!
Can anybody tell me the correct bolt and thread type for this application.