Morris Minor Owners Club

when threading a hole with for example an 8mm unc thread, what size should the hole be drilled? 8mm? 7mm? or 7.5mm?

7mm maximum 6.5mm for safety sake drills useually cut slightly oversize.better too small than too big

thats what i would, an 8mm hole for an 8mm thread would be pointless,

UNC is imperial so it won't be an 8mm OD thread. Do a Google search for "UNC tapping drill sizes" or buy a copy of Zeus Charts.
http://www.amazon.co.uk/Precision-Refer ... B0000CLZUO

Budgie wrote:7mm maximum 6.5mm for safety sake drills useually cut slightly oversize.better too small than too big

Drill the hole too small and there's a much higher risk of breaking the tap.

The way I find the right size is to try a drill in a nut of the right thread size. if the drill just slips through without waggling, then it is the right size (for tapping, not rolling) the thread.

6.5mm equates to 1/4" drill size use plenty of tapping grease and take your time slowly,slowly tappy thready.

1/3 turn, then back off enough to break the swarf coil that is forming, repeat ad infinitum, or you can end up with a mashed thread as the swarf gets caught up in the flutes!

Oh dear - I feel a disaster coming on..........

Haven't you got a fancy bit of Latin to cover that one, Roy?

For M8 you should be using a 6.8mm drill bit. Here's a useful link which shows all the information you require for both imperial and metric threads. Print it off and keep the copy in a safe place or hang it on the wall in your workshop.

Hope it helps
Regards
Declan

http://www.hahnreiter.de/docs/seite_1-6.pdf

Apparently - the Romans didn't DO disaster! Well done Declan!!