"Cordless Crazy" wrote in message
...

If I want to form a M8 thread in a piece of sheet steel, what size hole
do I initially drill?

If the sheet metal is thin, you are better off using a clinch nut:

http://www.allproducts.com/fastener/...131163225.html

The ones in the top row are applied with a pop rivet gun, for which you need
one with an appropriate mandrel. The ones in the bottom row are inserted
using a press.

Alternatively, you can drill out a small hole and use a tapered drift to
enlarge it. That will throw up a heavy burr on the back of the sheet, which
increases the length of thread you can cut. You can also use a special punch
to press a hole of that form.

Colin Bignell