Wombat wrote:
Found out by trial and error with different taps - once I got the right one and cleaned the threads all was well

5/16" UNC is the one
GT what is the list you speak of?
Can I suggest not using taps for cleaning out threads, especially in soft castings like the gearbox. Get a selection of bolts, with all the different thead types and sizes. Clean the threads on the bolts with a wire wheel and check that they are good (not damaged or burred over). Then cut a cleaning slot lengthways along the threads with a triangular file, if you can, so the leading edge of the slot is purpendicular the the centre line of the bolt (like a tap). Usually one slot is enough. Longer bolts also make them comfortable to use.
These should clean the threads without cutting a new one (that the risk using taps for cleaning threads, as Harley found out
http://www.ausmini.com/forums/viewtopic.php?t=67354 ).
When using your thread cleaner, try to wind them by hand, although gentle use of a spanner or socket should be OK. In a little then out, clean, and go again. No cutting, only cleaning. In time they will knacker out, so make some new ones.
If your keen, you can make some nuts the same.
The list I speak of is several factory lists that I have. They are mostly complete, but like most factory literature, there are errors.