Every home workshop should have a copy of Drills Taps and Dies by Tubal Cain. This has an invaluable table of tapping sizes for all sorts of threads. TC recommends not using high levels of engagement unless thread strength is absolutely critical, for easier tapping and fewer breakages.

Having said that the lowest value of engagement he goes to is 65% for M8 for which the recommended drill size is 7.1 mm. For 70% engagement he gives 7 mm. If the linear relationship continues (it won't because of crest rounding) then 7.5mm would give ~40% which seems much too shallow. As you are making clamps maybe the best approach is to use 7 mm, take it easy and use lots of cutting fluid. Make sure the tap enters the hole straight, turn only a quarter of a turn at a time, reversing the tap so you feel the swarf clear, and take it easy and hope!

I really recommend getting a good set of proper HSS taps and drills – I bought a full metric set of high quality spiral flute taps with corresponding split point drills from Greenwood Tools at the last exhibition I attended and they are excellent.

A 7.1mm drill will give you 65% engagement, which is what I would use out of choice. However, 7.0mm is still only 73% engagement which is unlikely to give you too much trouble when tapping. Use plenty of tapping compound (or oil) and clear the thread of swarf occasionally by withdrawing the tap and you should be OK.

HTH,

Rod

Edit:  Cross post with John Haine – sorry to repeat his sound advice

Edited By Roderick Jenkins on 10/10/2016 20:59:01

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Don't.!!! (IMHO)

Buy good ones as singles when you need them.

In a set there will be some you use frequently and some you will never use at all.

Nick

How about a 9/32'' drill?

the Ig2 HSSE universal 8mm tap as sold by Axminster is about ?12.50 + works even in tough stainless – this type of tap is great for through holes and is my usual tap of choice for such – you need only the one tap – personally I wouldn’t buy carbon steel which tho can be very sharp when bought, its first encounter with steel will be its last useful contact. A good HSS 2nd tap will serve most uses tho not blind holes

The difference betwen 7.1 and 7mm is 0.1mm or 4 thou in Imperial ( yeah yeah .003937, I know) so one is looking at 2 thou a side, is it that critical?

Andrew is spot on regarding spiral flute machine taps – that pretty much all I use.

I still lack understanding why you want a bigger hole than standard 6.8 tapping drill unless it is metric fine or special material/application?

Mark

+1 on Roderick and others' comments: your 7mm drill is going to drill oversize so will be well close to a 7.1mm hole. Drill 7mm and proceed with caution as always with carbon steel "El-Snapo" taps.

Yes Rod,

I would be mightily surprised if one of my 6.5 mm drills came out anywhere near it's actual size.

Enter my el cheapo drill press into the equation and I will probably have an oval hole, anything between 6.7 & 7 mm. Give or take a tad.

After that, as Thor suggests, I usually go the serial tapping route, which is also quite forgiving.

It all depends, as usual, on what degree of precision is actually required for a given situation.

M8 is a rather meaty lump, after all

All the best,

Daniel

See what size hole your drill actually cuts. You could grind one lip a little longer to get a larger hole?
Depending how critical the work is I use a 7mm for M8.

So many answers, and yet the OP still hasn't told us whether his M8 is coarse, fine or some other pitch, and without that information any speculation on the correct tapping drill will remain just that.