Pilot hole size for Satnless Coach screws

[ETAF]

i'm putting some Coach screws into a 3x3 post and also a 45mm treated soft wood

so the M8 will go through the 45mm softwood - and then into a 3x3 post whats the best pilot hole for M8 in the 3x3 - 6mm or 5mm
and M12 will be going into the softwood 45 mm to mount heavy duty brackets A2 stainless - mixture of M12 and M10 - so again would a pilot hole for M10 - be 8mm or 7mm and M12 be 10mm or 8mm(dont have a 9mm bit - borsch set 5,5.5,6,7,8,10
i should i go down 1mm for those

 

[JohnD]

The pilot hole in the post can be of a size enabling the shank in the threaded part to pass through, tightly, while the thread bites into undrilled wood. As wood screws are tapered, I'd work on the first quarter or so of the screw. In softwood it can be even smaller. The pilot hole does not need to be as deep as the entire screw length, except in hardwood.

The pilot hole in the board should be larger, enabling the unthreaded shank of the shoulder of the screw to pass through tightly. You do not want the thread to cut into the board as this can cause it to be jacked away from the post.

You can get special drill bits that are sized to cut the two different pilot sizes, plus a countersink, but unless you are doing a lot of repetitive work in the same dimensions, such as screwing down high quality floorboards, they are not often needed.

Modern stainless screws are usually cheap Chinese material, deliberately made without nickel in the alloy to reduce the cost. This also makes them soft and weak, and they can easily be broken if you use normal force when tightening them, and you can strip the heads, so pilot holes are important. The manufacturers and the buyers have agreed to work to this specification as price is more important than strength. You can buy better quality if you are prepared to pay for it.

You may remember the days when brass screws were common, and we had to drive a steel screw in first to cut the thread.

 

[ETAF]

thanks for that - yes i will have a clearance hole on the main joining wood and penny washers - which had been in the same place for 6years and no rust - didnt even know that - just peeny washers from screwfix

OK 0 would have pilot holed the full length of the screw - so thanks so much for that

 

[morqthana]

Lubrication can be a good idea.

 

[ETAF]

i ahe done that in the past, My dad used use an old bar of soap and drag the screw across it , he was a carpenter

 

[morqthana]

Spray silicone polish is good, if you have some.

 

[ETAF]

no dont think i do have any , but will look