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How do I undo this screw?

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Gary Wooding22/06/2018 11:22:25
470 forum posts
94 photos

Some years ago I installed a small safe which I bolted to the floor with a couple of the supplied screws. SWMBO says she would like it moved to a more accessible place, but I can't undo the screws. The photo is the best I can do, but it does show the type of thing. It has two shallow, concentric, hex sockets. The large one is 6mm AF, and the small one is 4mm. They are both very shallow - too shallow for a normal Allen key to get a purchase. I must have used a special tool to insert them, but it's so long ago I just can't remember; and I certainly don't have it now.

I've tried using 6mm and 4mm keys, but they just cam out.

After a good look at the (rather poor) photo it appears to have a 3rd, even smaller, hex socket in the middle, but I can't measure it.

Any ideas folks?

safe bolt.jpg

larry phelan 122/06/2018 11:24:05
262 forum posts

Angle grinder ?

Mick B122/06/2018 11:35:39
758 forum posts
46 photos

How hard is the screw? Drill for the largest screw extractor that won't interfere with the thread?

Arc-weld on a bit of rod with a tommy-bar?

Fowlers Fury22/06/2018 11:36:27
257 forum posts
57 photos

Instead and despite the risks, have you not tried telling SWMBO it can't be done?

peak422/06/2018 11:40:59
602 forum posts
49 photos

If you have a welder, hold a, (non zinc plated), nut over the screw. Weld through the hole and unscrew with a spanner.

I've had more success by deeply countersinking the top face of the nut first, so it's easier to get the mig wire down to the bottom.


JasonB22/06/2018 11:46:09
13657 forum posts
1269 photos

Get a bit of silver steel and mill suitable hex sizes onto the end, harden and then use to fit the screw

duncan webster22/06/2018 11:47:48
1696 forum posts
10 photos

Are the heads hard? If not just drill them off, you've got a centre to start the drill. You'll need to sharpen the drill afterwards

David Standing 122/06/2018 11:49:19
1085 forum posts
43 photos

Use your 6mm hex key, but to stop it camming out, grind the end of the key flat, as if it is a decent key it will have a profiled end.

Gary Wooding22/06/2018 11:56:42
470 forum posts
94 photos

Unfortunately, there is no room in the small safe to wield a drill or an angle grinder. The best I can do is use a small Dremel type device, but I'd much prefer to get the special tool..

Trevorh22/06/2018 11:59:46
238 forum posts
41 photos

Why not contact the maker and ask for a replacement tool?

John C22/06/2018 12:12:56
239 forum posts
87 photos



It's 'Uni Screw'. I bought a box some time ago. The idea was that one bit would cover a range of screw sizes. Needless to say, the bit has long gone.........

I'm trying to upload a photo of the box.....just hang on.

I did a search on line and found a firm that makes the screws to order, but never got a new bit. I'll have a dig to see what I can find.



uni screw.jpg


Edited By John C on 22/06/2018 12:25:27

Edited By John C on 22/06/2018 12:29:39

John C22/06/2018 12:28:59
239 forum posts
87 photos

I'm struggling with the photo Gary - I'll get there eventually. Meantime, this may help:



Roger Baker 222/06/2018 12:35:41
12 forum posts

Hi Gary

You should get 5/16 hex bit with 6mm on the end, grind the 6mm end so that it is flat and force it into the bolt withsome type of lever to apply downforce. Use a suitable spanner to undo.



John C22/06/2018 12:45:15
239 forum posts
87 photos


I can get quite a good grip with a Torx wrench - might be worth a try,


Speedy Builder522/06/2018 12:50:05
1587 forum posts
108 photos

Sharp cold chisel to try and 'spin' them out.

Hopper22/06/2018 12:53:23
2956 forum posts
50 photos

A dab of valve grinding paste on the end of the allen key sometimes helps it grip.

Otherwise, Dremel tool is looking pretty good.

I think Uni-Screw was a turn-of-the-millenium idea that never really took off. Not much sign of Uni-Screw drivers or bits on the net today.

Gary Wooding22/06/2018 14:02:36
470 forum posts
94 photos

I think you're right John about it being a Uniscrew.

I managed to get a better photo, and also managed to get a reasonable impression on a bit of polymorph plastic - the is the other photo.. Now if I could get a bit....

safe bolt.jpgsafe bolt key.jpg

Brian Wood22/06/2018 15:16:01
1707 forum posts
35 photos

Hello Gary,

From the later picture and the impression it seems the fancy hex forms are contained within a raised section; does that leave you enough room to get a grip on the outside of it with a pair of Mole Grips?



Gary Wooding22/06/2018 15:23:12
470 forum posts
94 photos

Hi Brian,

No, I've tried that - the head is very shallow, as are all of nested sockets.

Jon22/06/2018 15:39:30
886 forum posts
43 photos

Should be easy on plenty of depth there just find a good fitting allen key.
No good if it has play in the hex, best found to date are the plastic covered Wera, fit hex better than anything else.

I use shallow head sockets every day, feed allen key in square press down whilst giving it sum.

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