Apologies for ole chesnut but now desparate

11 Jan 2009
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United Kingdom
Hi all

Couldn't find a specific forum for this. Apologies in advance as I know it's been done to death but this is a variation on a theme.

Am fitting vertical blinds over landing window and need to fit into recess. Drilled into top of window opening and predictably hit metal lintel after about 2". Already have plenty of 5*30mm self tapping screws so decided to use those. Used my 3mm titanium bit to drill pilot and it went through the metal like a knife through butter. Alas I couldn't get the screw to bite in the pilot hole despite much elbow grease so increased the size of the hole to 3.5 and then 4mm but still the screw wouldn't budge. I then used a 5mm HSS bit and guess what? The screw went straight through the hole and I can't remove it (its lodged there loose but I can't get it out)

I'm sure these self tapping screws are the right ones for metal/steel and I thought I was safe using a pilot hole smaller than the screw but this has not been the case.

What size hole would you recommend to get the 5*30mm screws to go into the lintel?

Also are you expected to use an industrial strength screw driver to get the screw to bite?

Many thanks

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Self tappers into a steel lintel.....
Self tappers are typically used in quite thin sheet, where they deform the hole slightly into a slight twist, and you'd probably manage with your 4mm bit. They aren't really expected to cut a thread through steel plate.
If you're fixing into say 3mm thick steel, you will need a much closer tolerance. Perhaps 4.5mm.

And yes, it will be quite tough, but it should work.
Thanks for this response.

As stated I tried 3,3.5,4 (not sure about 4.5) and then 5mm. You seem to go from one extreme to the other where the screws won't "self tap" to fitting snugly into a hole of the same size.

I also tried about 4 screw drivers and was applying so much pressure that the head of the screw was going each time.

Is it really that precise? Would you always use a 4.5 bit for a 5mm screw?

Is there a stronger alternative than self tapping screws for lintels which might work with a smaller bore hole?

Thanks again.

If you were tapping an M5 thread, you would predrill with a 4.2mm bit.
But you would then cut your thread with a tap, very hard and very sharp.
You couldn't push a self tapper through that.
Basically a self tapper is the wrong fixing, you should use a 4.2mm bit, a M5 tap, and 5mm bolts.
But failing that, you can get a compromise with a self tapper and a hole that's not too tight to snap the head off, and not too loose to grip.
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Drill a 4.2 mm hole (or 4.5 if you dont have 4.2) then cut a thread with a 5mm tap and fix blind with M5 screw

What do you mean by fix blind with M5 screw. I was OK until that point.
is my maths out here?

2" is 50mm ( ish ) so the 5x30 screws shouldn't even touch it...?

drill a 5.5mm hole through the lintel then fit a rawl plug into the hole and screw into that..
A rawlplug won't hold in a hole in a steel lintel. You would need to use a hollow fixing.

You've been given better advice by Sunray and Regsmyth.

If you need to remove the screw that's turning in the oversize hole, you could try using a pair of locking pliers to pull and simultaneously turn it. Also, the Veritas screw lifter has been designed for dealing with this sort of problem.

Sorry ColJack your maths are not out. I overstated the gap between the surface and the lintel. The 30mm (1.2") screw reaches it comfortably.

Must say I'm still concerned that I'll end up peppering the lintel with holes but not get any of my tap in screws to fill any of em!

How may folk use "taps" in these circumstances.

Never used one before but will purchase if necessary.



What do you mean by fix blind with M5 screw. I was OK until that point.

After using a tap you will have a thread in the lintel which will be the same as found in an M5 nut. The fixing will then be an M5 screw, for example and picked at random:
http://cpc.farnell.com/nettlefolds/1961050012071do/torx-xcrews-m5-x-12mm/dp/FN01351 or http://cpc.farnell.com/_/cp5m25/screw-csk-pozi-5m-x-25mm-100pk/dp/FNCP5M25
longer ones to 50mm available in the sheds.

examples of tap http://cpc.farnell.com/linear-tools/21-500-050/tap-set-5x0-8mm/dp/TL03459 and drill n tap http://cpc.farnell.com/_/20-365-050/drill-n-tap-m5/dp/TL08395

How may folk use "taps" in these circumstances.
Never used one before but will purchase if necessary.

Most of the expert tradesmen I know would not hesitate to do it this way.
If you do decide to buy a tap, go for a taper one or a second cut - a plug tap will be very difficult to start. (You may need an extension for the wrench too). Use a good squirt of WD40 in the hole! Use HSS taps - carbon steel ones are useless. ;)
if it's about an inch into the plaster, then a 5mm hole and a decent yellow wallplug will hold it as well as it would in any brick wall.. the "legs" of the wallplug will spread in the void of the lintel and hold it fast..

but as I said, use a decent plug with a bit of meat to it.. DO NOT USE TRI-PLUGS.. they are crap..

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