what drill size for tapping holes?

[Tony oily bike]

You need the Zeus book, a must have 

http://www.amazon.co.uk/Precision-Reference-Drawing-Toolroom-Workshop/dp/B0000CLZUO

Never a truer word has been spoken, it truly is the engineers bible, not just for drilling and tapping but all sorts of formulae, tolerances etc etc.

Agree, the book for all that stuff.

Going off tack a wee bit, did you know we can thank good old Mr Whitworth for us all having standard threads (those with old motors will be familiar with whitworth theads), as it was he that decided that it was a good idea to standardise.

Before him, the idea of going to a store and buying a fastener to match one you had simply didn't exist, you had to make one that fitted what you had.

Some interesting reading:

http://en.wikipedia.org/wiki/British_Standard_Whitworth

http://en.wikipedia.org/wiki/Joseph_Whitworth

[Manky Monkey]

I find I can make any nut fit any bolt if I bash it hard enough wiv an 'ammer.

[triker_Chewie]

Drill the nut out and weld it
I knew a bloke who was laying a steel conduit for a petrol pump and couldn't get the die on the pipe (already in ground) so he used a holesaw to cut the threads out of the couple an araldited it in place. Ruff as guts

[Cabman77]

Mr Whitworth also made a very accurate rifle with a hexagonal bore/bullet. Queen Victoria tried it out at the great exhibition I believe and was very impressed. Idiots at the War Department went for the cheaper, less accurate and less reliable "Enfield" rifle.