Pilot Hole Dia. - Hardwood, Coach screws

[crinkles]

What's a good pilot hole size for an M8 coach screw into hardwood?

Screw length is 65mm. Shaft of the screw (inner diameter to threads) is 6mm.

Keeping in mind that I will most likely use a hand driven socket to get this thing in.

 

[JimPghPA]

Take a dial-caliber and measure the inner shaft diameter (below threads) (if you have to gring off threads to get measurement) and then measure the diameter of the thread. Chose something in-between, depending on how much bite you require, and how hard, and (easy to split)the wood is.

 

[crinkles]

great. thanks. I have a drill bit that is imperial size but just a hair under the shaft dia.

I'll test one on the shed and see how it goes. Found all me socket adaptors so can drill this puppy in.

 

[keithaikens]

Actualy the pitch of a metric screw taken off the magor dia is the ..tapping size,retired toolmaker

 

[crinkles]

measured the pitch at 4mm giving me a 4mm pilot/tapping hole.

In softwood this would seem OK, but even with my 6mm pilot hole I needed a lot of elbow grease to get the M8 into the hardwood.