Hole size to be drilled for tapping

Hi,

What hole size should be drilled before tapping, is it half a size smaller, or a full size. For example, if I want a M8, would I drill a 7.5mm hole and then tap with my M8, or would I drill with a 7mm hole first? I have done a half size smaller and it worked well, but I've seen it described as needing to be a full size smaller.

Thanks

"Normal" practice, or rule of thumb, is to take the thread diameter minus the pitch and use that drill size, which will be very close.

eg.

12mm diameter, 1.75 pitch = 10.25mm drill.

12mm diameter, 1.0 pitch = 11.0 drill.

However, to make it easier, you will sometimes find the tap set has the drill sizes required or, sometimes, it will even be marked on the tap(s).

The last option is to just check on-line for a chart, such as this http://www.engineeringsupplies.co.uk/tapping-drill-sizes-i-15.html some you can download and print out and keep in a clear plastc bag (to keep it clean) somewhere handy for referring to. You may even be able to blow them up a bit to see better.

As for that particular tap, a 7mm drill should be OK, 7.5mm will be much to big.