what size nuts?

i seem to have misplaced the nuts that attach the bellhousing to the flywheel housing and engine, does any one know what size these should be? (series 3 2 1/4 petrol engine.) it was a bit of a battle to get the flywheel housing to sit on without nipping the o-ring and i really dont want to have to take it off again to take it to the smiddy to get the size!

any help much appreciated.

They are the same as the nuts for the starter 3/8's UNF if anyone else needs to know