I need to fix some 12mm diam coach screws into wood.
Normal pilot hole rules may not apply to such screws .... these do not have a tapered shaft ... the main shaft is parallel (8.9mm)
Overall thread diam is 11.6mm
Initial thought are that I could use an 8.5mm pilot hole ..... don't want to put in too big a hole or it won't provide the fixing load .... and too small a hole will risk splitting of the wood.
Anybody any guide on this ?
As near as dammit to 3/8 of an inch. Use that as a pilot drill.
You could get away with that.
Dave
Nothing tabular but I fitted a decent sized vice to a ~40mm chipboard worktop with similar bolts, maybe 10mm. Obviously the material and depth will make a difference but I think you're not far away with your 8.5mm hole. I measured the shaft and drilled a size or two down then tried to screw in the bolt and increased the size as necessary. You'll soon feel if it's gonna get too tight (remember butter/candle wax on the thread). Admittedly, chipboard is fairly forgiving.
Driling a hole thats the inner diameter of the shank should work ok. If the bolt gets too tight on the way it you can always enlarge it very slightly.
NT
Assuming this is not hardwood you can go a little smaller than the core diameter but not by more than 1mm I would say so your 8.5mm looks fine. You will however need a clearance hole for any unthreaded length of screw under the head. If you mark your drill bits with masking tape as depth guides you can drill the pilot hole for the full length and then the clearance hole for the top part.
If I were you I'd test the fitment on a bit of scrap wood first and make sure that your chosen pilot hole size doesn't require too much force to fit a screw.
What kind of wood, and how close to the edge?
The hole (if required at all - you could for example drive those screws into a softwood joist without a hole and little risk of splitting), wants to be core size or just under. So 8.5 sounds fine in this case. The more forgiving the wood, the smaller the hole you can use.
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Going into 2" thick marine plywood ...... so reasonably tough wood.
Use Turbogold coach screws
That is not an option .... for 2 valid reasons ... they don't do 12mm and they don't do them in 40mm length. This is to go in existing fittings which are pre-drilled in steel plates for
1/2" coach screws .... and they can't be longer than 40mm or they would pierce through the 2" plywood.
update - I used 8mm pilot and it worked a treat, easy enough to start, and pulled up really tight ..... used Vaseline on threads as per normal practice into wood.
I prefer to use a vegetable based lubricant, such as olive oil, with coach screwws into wood; rather than a petroleum based product.
Poor advice all round really, There are PROPER charts for pilot hole in all sorts of woods, I suggest you look at them.
I'd suggest you look at the date of the post you've replied to.
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