Drilling through a 60mm piece of oak to attach to a 70mm piece of oak. Screws are Coach Screws M8 x 100mm, zinc plated, very similar to this. Threaded segment is 75mm.

How wide and deep a pilot hole do I need?
Do I need a clearance hole? How deep?

slackalice or kayak are your boys for this, but my twopenny would be to drill the first hole to the unthreaded shank diameter and the second hole to the internal thread diameter.

Is this green or seasoned oak?
Clearance hole – yes – and basically ensure the bolts are
only cutting the thread in the hole like a tap.

Green will shrink/move a lot.
Seasoned can be more like steel than wood!

It’s seasoned oak. Around 11 years old.

How deep should the clearance hole go? As deep as the unthreaded part of the coach screw?

I would advise clearance all the way through the 60mm , otherwise there will be
no clamping force as the 60mm will effectively become a “nut”

Yeah – the screw should only be ‘screwing’ into the fixing piece of wood I would have thought.
So you want a clearance hole all the way through the 60mm piece so the screw just passes through that.
Obviously, this clearance hole will need to be larger than your unthreaded portion of screw.

Regarding the hole for the threaded portion you are actually screwing into. Erm. Not so sure. I have never used such a hard wood and that type of screw – bits of cheap pine and some wood screws are normally my limit.

I don’t know what you are specifically using this for, but have you considered Timberlok screws, which seem to be the fastener of choice for joining railway sleepers together? I only found them while thinking about a railway sleeper wall along the broundary of our front garden…..

http://www.railwaysleepers.com/railway-sleepers/railway-sleeper-fixings/timberlok-railway-sleeper-screws

https://www.fastenmaster.com/details/product/timberlok-heavy-duty-wood-screw.html

The pieces of wood I’m screwing together form a bench. I had to separate the bench to move it as it’s so heavy. The screws that were in there broke as I was undoing them, so I’m replacing them and reassembling the bench.

I’ve used these exact zinc coach screws on railway sleepers to make a raised flower bed. Not sure what state they’re in but then I don’t need to move the raised bed.

Yes – I’ve wrecked screws in oak before without large enough pilots. It is a hard, grippy wood.
I would go shank clearance for the 60 mm piece and thread minor diameter for the 70 mm piece, drilled full thread depth.

If you want to be able to undo it again, I’d go for bolts with nuts etc rather than screws.

OK, so what sort of bolts with nuts do you recommend I go for?

I may need to move it again, I don’t know.

The only way I can go for bolts with nuts is if the bolts are 130mm, the total depth of both pieces of wood.

You could countersink the bolt head and/or nut and use a shorter bolt?

I’d go with these:

http://www.orbitalfasteners.co.uk/en/products/m8x150-high-tensile-bolt-grade-8.8-bright-zinc-plated

And the just some standard M8 nuts (DIN 934, I think)
and some washers on either side too (DIN125)

Nuts:
http://www.orbitalfasteners.co.uk/en/products/m8-hexagon-mild-steel-full-nuts-bright-zinc-plated
Washers:
http://www.orbitalfasteners.co.uk/en/products/m8-mild-steel-form-a-flat-washer-bright-zinc-plated-din-125-1a-8-4x17x1-6mm-

You might also want some M8 half nuts to tidy up the protruding ends of the bolts (if they do protrude that is). These will also act as a lock nut:
http://www.orbitalfasteners.co.uk/en/products/m8-hexagon-lock-half-nut-mild-steel-bright-zinc-plated-grade-4-din-439

I’d go slightly smaller than the unthreaded shank diameter, compressing the wood strengthens the thread, but oak is going to be pretty solid, so not much under.

Proper beginner error. The 2nd piece of oak is 70mm wide, but 400mm deep. I’m screwing into the 70mm width, so the 100mm coach screws zinc plated seem fine for the job. Duh.

So the question comes back about how wide to make the pilot holes, and how deep to make the clearance holes??

I would go clearance all the way through the 60mm and pilot the 70mm with the size of the centre of the threaded section to a full depth that the screw will go into it.

make the clearance hole go all the way through the first piece of wood and a bigger than the the thread (rather than the dia of the unthreaded section). If the thread is able to engage with the wood you are screwing through it stops the two pieces of wood from being tightened together. The pilot hole in something as hard as oak wants to be as deep (or can be deeper) than the screw being driven into it, otherwise the screw is trying to split the wood apart but isn’t gripping any better for being tighter to drive in.

[edit – 3 hours go by then I’m too slow by 30 seconds]

Cheers Stoner 😀

As has been wisely suggested above, clearance hole through the 60mm bit, probably a mm in diameter bigger than the shank ( ie non threaded bit) of your chosen fastening, so M10 would equate to a clearance hole of 11 or 12mm. The pilot hole at least the depth of the amount of fastening that goes into the other bit of timber and for an M10, I would plump for about 6 or 7mm drill bit.

I would also suggest that you use stainless steel coach screws as even though your oak is 11 years old seasoned stuff, there will be some residual tannin, which will happily munch away at the zinc and steel, so for longevity and integrity, use stainless fastenings and washers beneath the hex head.

UK Stainless, based in Norwich are handy for small quantities and reasonably priced. The URL button still crashes my iPad, so here’s a copy n paste link that will hopefully take you straight to their A2 Stainless coach screw page:
http://www.ukstainless.co.uk/product.asp?idproduct=96

Alternatively, you could use some 6mm Timberlok screws – designed as a substitute for coach screws. I can vouch for the fact that they are seriously strong and suitable for fixing oak…
https://www.fastenmaster.com/tl_files/fastenmaster/product-info/timberlok/SellSheetUK/TimberLOK-SellSheet_UK.pdf

A little bit of vaseline in your piloted hole will ease your.. er.. screwing. As long as this piece of timber is hidden it shouldn’t be noticeable if it migrates a bit.