Pilot drill size

coffeehound

I have some large wood screws to fix into ceiling joists through plasterboard.  

The screws are 70 mm long (excluding the hex head), 8.7 mm overall thread diameter, and the solid core diameter is around 6.3 mm.  The things they are fastening are 6.5 mm thick metal plates.

Can you advise what pilot drill or drills I should use with these, pls?  

Also, should the pilot hole be the same length as the threaded portion, or two-thirds of the total length, etc?  Is there any worry with weakening the joists if drilled too deep?  Thanks

grumbler

IMO, there is no any real science behind this - just common sense.

Generally, if it's close to the middle of the joist, with good tools and some grease you can do this without any pilot hole. 4-5mm hole (and some grease or petrol jelly) will make your life much easier and you can always make the hole bigger if needed - no worries about weakening. The same about the depth/length - start with, say, 40-50mm.

Make sure that you use the right screwdriver bit - you'll possibly need No3 (not sure) and possibly Philips not pozi.

coffeehound

Thanks grumbler that's good to hear.  I hadn't considered using grease.  I've done metal work but never much with wood so common sense is in short supply..  Will try a 4 or 5 mm as you suggest.  The screws are so chunky they have plain hex bolt heads so will try a ratchet spanner

grumbler

With a hex head and a spanner 4mm will be fine.

coffeehound

4 mil it is then, cheers

stuart45

There are set pilot hole sizes for different screws sizes, which are different for hardwood and softwood. In theory the screw should go in easily and tighten up properly. In practice most chippies drive them in, as long as it does not split the timber. 

TELLIT01

stuart45 said:

There are set pilot hole sizes for different screws sizes, which are different for hardwood and softwood. In theory the screw should go in easily and tighten up properly. In practice most chippies drive them in, as long as it does not split the timber. 

Ah, the famous Birmingham Screwdriver. :-)