I'm hanging a pair of gates using these hinges from Screwfix.
http://www.screwfix.com/prods/73148/Ironmongery/Gate-Fittings/Gate-Hinges/Gate-Hinge-Packs/Standard-Gate-Fitting-Kit-Spelter-Galvanised-460mm
The hinges have got a 20mm square where they go through the post.
I take it I need to drill a pilot hole rather than just bang them in like a nail.
If the hole's too big, they will work loose.
If the hole's too small, I might end up with the hinge wedged half way in the post with no way of getting it back out without damage.
I suppose I could try some trial holes on some scrap timber, but it would be easier if there's a professional gate fitter in the house who could tell me the correct size.

What size hole for a gate hinge ?

depends what size the fixings are?

Isn't it under compression? Therefore cut out a hole the same size or v slighty under?

First thing is you've bought a field gate fitting kit – is it for quite a big gate? If not you can use a much smaller kit.

The long strap hinge is for the top of the gate(pulling force) and the short hinge is for the bottom of the gate (pushing force). The hook to bolt (with thread on) is for the top of your gate post and the hook to drive is for the bottom of the post. Most people fit the top hook upside down (above the hinge)to stop people nicking the gate.

You'll need to drill a hole through your post for the top hook the same size as the thread or slightly smaller. Then drive it through (hit it with a hammer) and tighten the nut up. The the nut/ washer stops the hinge being pulled out.

For the hook to drive i would drill a small pilot hole to stop the wood spitting. Then belt it in with a nice lump hammer.

Have a look at these links: scroll down http://handbooks.btcv.org.uk/handbooks/content/section/3320
This is what it should look like when hung
http://www.gateinstaller.co.uk/wooden_gates/Large/WoodenGates9.jpg
or
http://www.beenthere-donethat.org.uk/images/footpathgate01.jpg (shows the top hinge upside to stop it being pinched)

Isn't it under compression? Therefore cut out a hole the same size or v slighty under?

I concur.

They're coachbolts. You need a hole that lets them be tapped gently through. So if they're 8mm bolts, use an 8mm drill.

As willyboy says. We use a 19mm bit through the post for the top hook to bolt; we also use the same bit as a pilot for the bottom hook to drive (the square is about 28mm across the diagonal so this is fine).

The bolts through the gates to which BigJohn refers are usually 10mm diam.

psling, that's the information I was after.
I'm making two pairs of wooden five bar gates.
I'm doing OK cutting the mortice & tenon joints, I've got a load of 10mm coach bolts for where the diagonals cross the rails and I'm dowelling the diagonals where they meet the rails and styles.
Drilling the holes in the posts to take the hinges was the only thing I wasn't sure about.
The top hinge is under tension and goes right through the post. The first half is 20mm square and the second half is threaded.
The bottom hinge is under compression and is also 20mm square, but just has a point on the end.
I guessed 19mm would be about the right size. If that's what you use, psling, that's what I'll try.

I haven't got a 19mm spade bit, so I used 18mm and a big hammer.
Trial assembly and hanging of the first gate. Looking OK so far, just needs the tops of the styles rounding over. Drilled for pegs and dowels, but I won't put them in until final assembly.

I bought the timber untreated, but paid treated price.
Once I've got everything made, I'll disassemble it all and take it back to go in their pressure tank. That way, it gets treated on the ends and holes as well.

When you get it treated you'll find the timber will swell up so you may have to leave it a while before you can re-assemble it and then you may get some shrinkage in the joints as it further dries out. Looks good 🙂