View Full Version : what drill size for a given machine screw diameter?

11-04-09, 19:13
I want to tighten up a 'popped' rubbing strake with a machine screw into the grp hull. Given the screw is, say, 5mm; do I use a 5mm drill or 4 or 4.5? Many tks in advance.

11-04-09, 19:17
I would make a hole the same size as the solid part of the screw (don't know the tech name) and just let the thread bite into the GRP. I never measure it - just hold up a screw and bits until they look right.

11-04-09, 19:19
4mm is about right assuming that the set screw thread is 5mmx1mm

You will need a 5mm hole in the rubbing stake itself and the tapping size in the grp.

11-04-09, 19:22
Minor diameter

11-04-09, 19:24
THE correct drill size for tapping a M5 x0.8 thread is 4.2mm

I'd reckon in GRP you'd get away with 4mm provided you have a tapered tap

11-04-09, 20:10
Metric coarse sizes drill/bolt- tap size here


11-04-09, 20:22
Thinking about it I've recently drilled and tapped some holes in GRP.
I did use a 4mm drill for an M5 (coarse) thread. (still see traces of the grp on the drill bit)

Went gently, working back and forwards with the tap, and all satisfactory

11-04-09, 20:44
I don't know what a set screw is, but have a vague recolection that it's a parallel threaded rod; VicS recomends a 4.2 mm drill - can I buy one to that exact size? I don't have any taps to cut a thread so will go with the advice of 5mm thru' the wood and 4mm into the plastic. Thanks all - it's about what I thought but as I'm learning as I go, I didn't want to drop a boggle. And we all know what dropping a boggle means, don't we ? For gawds' sake, don't drop a boggle.

11-04-09, 21:43
Alternatively you might like to use a stainless self tapper which will give a better thread depth into a relatively soft material and save the cost of a tap.

11-04-09, 21:58
A set screw is like a bolt but is threaded all the way up to its head whereas a bolt has a plain unthreaded section below the head.

Countersunk and pan-head versions are known as machine screws.

Threaded rod is known as studding (not to be confused with the woodwork in a partition /forums/images/graemlins/confused.gif

Studs are ... er ... like double ended bolts. threaded both ends with a plain unthreaded section in the middle.

now stand by for the engineers to sort me out!

Yes of course you can buy a 4.2mm drill but you'll need a tool shop not a diy store to find one I expect.

12-04-09, 00:40
Buy a taper tap for your 5mm machine screws. I have used one a lot on various projects into f/g. I reckon that a telf tapping screw does more crushing cracking type damage to the f.g compared to a tap which will cut cleanly. I reckon a 4mm drill will be fine for f/g but you would need a 4.2 mm bit for tapping into stainless steel.
I use a battery drill to put the tap in. It is great with reversing power, use lowest speed possible. The drill enables you to hold the tap straight but be prepared to turn off quickly and often as you don't want to go too far in.

good luck olewill

12-04-09, 00:52
I bought a set of drill bits last year from Aldi very similar to the one they have this week on special it had a 4.2m bit in it.

12-04-09, 07:39
Can I ask why you want to use a machine screw in the first place? Something like M5 will probably self tap into a 4.2mm hole in GRP, but a wood or self-tapping screw would be a stronger fitment and is the more usual method of attaching a rubbing strake.

Or are you planning on bolting right through where the previous fastener has stripped? In that case 5mm will obviously give clearance, which would be the best option if you're planning on pulling down the strake with the fastener.

12-04-09, 20:05
replying to macd, I'm not planning to bolt right through, only to screw into the grp gunn'l just below the timber toe rail cum capping. I have been thinking a tapered screw - brass or s/s or copper, would force itself into the plastic with a destructive outward force, risking 'stripping' the thread and thus negating any holding power the screw would otherwise have.... thought a parallel threaded set screw would be stronger and give more pulling power to effect closure of wooden strake to plastic hull.

12-04-09, 23:19
I think I agree with you.

Self tappers are often used in GRP but they do expand the laminate and, unless you drill a clearance hole through it, fracture the gelcoat.
I only use they for minor uses, I find they need a generous sized pilot hole. I put them in with a dab of Araldite on the thread.

I don't think you'll get an ordinary M5 screw to tap its own thread, as suggested by malcd, at least not without drilling a hole more than 4.2 mm.

Tap a proper thread and use an M5 screw and I think you end up with a good job. But unless you can drill the holes all the way through you are also going to need a "plug" or "bottoming" tap to finish the thread off to the bottom of the hole.

12-04-09, 23:55
I experimented with a similar problem when I had to refit my genoa track onto the top of the toe rail.
Originally it had 2" size 12 s/s countersunk woodscrews as fitted in the early 70's.
I thought I could do better and tap new holes for machine screws. Wrong! They failed and stripped the threads on all the sizes of drill hole I tried for the tapping. I wouldn't hold out hope for helicoils either. I did think of using rawlbolts but was persuaded that they could have forced the toerail apart so i didn't go there.
My advice if you have the depth and soundness of GRP, is to use s/s woodscews. But, if they tighten sooner than nearly fully in, back off and open up the holes a little and DO NOT drive in the screws with a power drill as the speed heats up the GRP and seizes around the screw and then the screws wont go in or out... just snap.
I used an engineers screwdriver that has a place for a spanner so I could lean down with my weight and turn the scewdriver with the spanner.
I only took off my genoa track to replace the teak rail, and I reckon the tracks are probably down for more than the period of my lifetime with this method.
good luck!

13-04-09, 14:00
Usually agree with VicS, but later poster confirms what I suspect, that M5 thread simply isn't big enough to form a strong hold in GRP. Wood screws hold on my toe rail, and zillions of others, just dandy. As Vic cautions, screwing into GRP can distort it. The answer is a bigger pilot hole than you'd use for softer materials, such as wood. And, if screwing directly into GRP (rather than through a strake), countersink through the gel coat, otherwise it will crack.

And don't even think of using brass screws. They won't last and will be a bitch to get out.

13-04-09, 14:17
I don't think, despite what I said earlier, that I would have relied on tapped holes for a genoa track. Very definitely a case which requires through bolting IMHO.
I prefer to counter bore the hole through the gel coat when I do use self tappers rather than countersink.