bolts with screw-type slotted head

Why not just use coach bolts? There's no way anyone can turn those from the outside.

Do you mean "machine screw"? Having said that, the coach bolt idea suggested by Matty F is far more secure because the only way to undo them is from the inside. You could even use a dome headed bolt if the square shank on a coack bolt causes problems. One other possibility is a type of security screw I once saw where the sides of the slots sloped down to one end which effectively meant that they could be screwed in by the normal method but there was no purchase for unscrewing. There is a version of it here:

When I had a shed built a few years ago, they drilled a suitable hole through the middle of each hinge plate and put a coach bolt through from outside, so the hinges couldn't be removed from the outside.

Matty F wibbled on Thursday 17 June 2010 08:48

Aren't they called "pan head machine screws" or something like that?

Slotted pan head or round head machine screw? You might find them more difficult to get than crosshead types.

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Roofing bolts used with spring washers should be adequate:

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Roofing bolts used with spring washers should be adequate:

"Stove bolts" - usually with square nuts.

They're dirt cheap and everyone ought to stock up on a few mixed bagfuls (M5, M6, M8, M10) from Toolstation, along with mixed washers. Just the ticket when the handle falls off the dog, or the wheel's off your wheelbarrow.

For hinges, I generally prefer coach bolts as a minimum (many tee hinges have square holes) and use shear-off conical nuts to lock them, often with a spot of MIG weld too. As a quick hack for the same, rivet the ends over a little with a ball pein hammer. Be warned that many of these zinced stove bolts are also very lax tolerances and they might not remain tight in service.

They are called roofing bolts in toolstation. A cross head on the outside, square bolt on the inside. I'd lock with an extra nut on the inside. I was going to replace all the screws (6 per hinge) with M6 bolts but I could drill a couple of larger holes for larger bolts. Coach bolts would need a square hole, so no good there. Cheers, Simon.

I've been known to make square holes, either with a square file or by putting the hinge on a block of steel with a hole in it, and whacking an old coach bolt through it with a large hammer!