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I am trying to install a safe using 8mm coach screws (legislation dictates this is the minimum required size) into Jarrah.

What size pilot holes should I drill? Is there a standard reference chart? I cannot find one.

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Less than thread diameter (8mm), minimum inner shaft diameter. The larger you go the easier it'll be to screw in but the less the threads will bite. Go as small as you can muster without splitting the wood.

For coach screws in particular, since you wanted a chart, this may give you confidence (this isn't metric but the concepts are the same):

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In particular note that for hardwoods in all cases the pilot hole diameter is roughly 5% larger than the nominal minor shaft diameter.

Note the depth as well, the pilot hole depth for coach screws should be as long as the straight portion of the shaft, not covering the tapered portion. According to whoever made that diagram, at least.

So if you want to go by that chart measure your inner diameter and choose the smallest bit you have that is larger than it, that'll get you about 5%. Of course this is still totally general.

  • If in doubt, I lean toward a thinner bit ratger than a thicker one. Any reasonable-size pilot hole reduces stress on the wood and reduces the risk of splitting, even if the screw is still having to forcevsome wood aside as it is driven. – keshlam Nov 16 '15 at 5:54
  • Thank you both for your very reasonable suggestions. I am looking more for a predictive table, or formula. I have previously heard that half the diameter plus 2mm for hardwood is a good starting size for a pilot hole, but I find this doesn't work very well (too small). – Vinciture Nov 16 '15 at 11:31
  • @Vinciture What Jason C has provided is the general guidelines on how to size the hole to the screw. I am not aware of any particular reference for this. I suspect a good one does not exist as there are too many variances in both wood species and screw types to make a comprehensive list. – Matt Nov 16 '15 at 11:52
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If you can't find an exact guide for the screw in question (see next paragraph about this) you can eyeball the bit size directly against the screw you're using. This is accurate enough most of the time. You want it to approximately match the core of the screw, and be less than the diameter of the threads.

There are actually published guides for screw size and the corresponding bit size, going back to well before metric fasteners were common. But any guide that gives just a single size for each screw gauge is not to be followed as gospel because pilot hole size should vary with wood hardness. Working in a hard wood you want a slightly wider hole than you would working in a softwood, or a soft hardwood.

The difficulty here is that it's very difficult to find incrementally different drill bits, at best you'll find 0.5mm increments in metric bits for sale retail and many sets offer full millimetre differences only. So very often you have to make do with as close as you can reasonably get.

So, where you're working with a very hard wood like jarrah and have drilled a hole that's a little narrower than ideal one thing that can help in physically driving the screws is to convert one or all of the screws so that they cut threads rather than form them using pressure, see previous Answer. It only takes a few moments to do and makes a huge difference.

In addition, you can also try lubricating the screw threads with oil, grease (e.g. Vaseline) soap or wax.

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I have used a product called Screw-mate from Stanley. It is a one piece bit and countersink for tapered wood screws. Just select the bit that matches your screw size and length and drill a hole that matches the screw. I have no idea what diameter hole is drilled, but it has always produced satisfactory results on the rare occasions that I have resorted to screws for joinery.

I did a little web search and found that they are now quite expensive - $20 or more per bit. Even though I bought my few screw-mates long, long ago, I cannot believe that I paid more than one dollar each in the 70's. (I can't believe I would have paid fifty cents each for them.)

  • DeWalt makes a set too, and there are others. I've used them too, they're nice. They don't chatter as easily as plain countersink bits do. – Jason C Nov 18 '15 at 15:31

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