UKH

If I'm using 5.0 x 90 wood screws and I want to pre-drill a guide-hole thingy (apologies for technical jargon) what size drill bit should I use?

It goes against the grain to ask for help but it's a knotty problem and when I start on the kids' climbing frame tomorrow I want to spend as little time in the cold as possible.

Many thanks for your help.

Rule of thumb I use is for the pilot hole to be 2/3rds of the screw size.

So for 5mm screws, you want 3mm hole (or 3.5mm if you have half mm bits).

If you want to be really precise then get some callipers and make the hole a similar diameter as the shank, that way the wood can slip on the shank and clamp really tight. For this reason avoid fully threaded screws is screwing two large lumps together.

In soft wood however, pilots are somewhat optional for your screw size if just screwing lumber together.

Two screws driven without pilot holes are going to be stronger than one screw with a pilot hole all day long.

Using an impact driver (highly recommended) I can drive two screws in much quicker than I can pilot and drive one screw.

What I often do is start driving, if the two pieces of wood push apart a bit, back off the screw, put lots of weight behind the impact drill and screw it all the way in. Some may thrown at this technique but it works well.

Always use pilot holes on coach bolt or anything similar, and I find its good on ply wood etc, along with countersinking to avoiding damaging the wood.

Somewhere between 3 and 4mm. 

Have fun!

Ah, Monty, how much experience distilled in a single post! I'm very grateful for such a detailed reply and the time you took to write it.

Best wishes,

M

Thanks, Pete.

 I quite obviously had much less experience to distill into my post.

Maybe my (Makita 18v  brushless) impact driver is over powered because I think it's absolutely fantastic at *undoing* screws etc, but even at 5mm thickness screws it can clobber stuff if I'm not careful.

Make sure you have some good quality impact driver posi #2 bits (or whatever type/size you need for those screws) and really press hard at the end of you'll strip stuff with some impact drivers

Don't be too gung-ho

Or just distilled it to a higher proof.

Hold screw up to the light, hold drill bit in front - drill bit should cover the solid bit of the screw (or slightly less, if you're short on sizes), but not the threads.

As Monty said, you might need a clearance hole through the first piece if screwing 2 pieces together, or it won't pull up tight.

Having done lots of jobs using pilot holes for screws over the years  I now buy good quality screws with torx heads and lubricated double threads that don’t require a pilot hole They can be used quite close to edges without splitting the wood  

Using an impact driver I have never had a bit jump out of the torx head but have thrown loads of ruined Philips heads away.

Yes, I discovered those self piloting screws recently by accident. They are great, but you need the exact right bit for drill/driver. Some of them come with bit in the box. Completely obviates need for pilot in most cases, in softwood. Totally different scenario if you’re working with oak or sapele, though.

Well I would have taken your word about the oak but in summer I put four bird boxes up by screwing into oak trees and a beech tree with no problem. Whether cut or planed wood is different I’ve no idea.

Yes you do need the correct Torx bit but I have a couple of boxed Bosch bits which were only a few quid from Screwfix. My Philips heads have now gone the same way as the cross heads went which was towards the bin.

That’s interesting, but I suspect there’s a difference between screwing into a living oak tree and a piece of dry seaoned oak. 

+1 for Monty softwood

+1 for Last Boy Scout hardwood.

Also, not all screws are equal. Some or most are threaded the whole length, but for pulling together (imho) not thread for a last few cm works way better. 

Then you start a debate on counter sinking! 

Best practice, don't use the screws to pull the wood together, clamp it and then screw, even with smooth shank screws.

As to previous comments, ideally a clearance hole in the piece the screw is going through, and a pilot hole in piece being screwed to. Soft wood (not softwoods eg larch likes to split) is more forgiving.

As for oak, green oak is soft and easy to drill/screw into, seasoned oak can be like iron.

Oak will eat steel screws when green, hot dipped galvanised fixings are required for longevity. 

You can also get countersinks with a pilot hole drill bit incorporated from screwfix etc., this was super useful when i built my home wall.

3mm. 

The largest drill size that does not exceed the screw size minus the threads (easily done by eye) for hard wood. In soft wood or chip board you can go down a bit further in drill size.