Nut and Bolt Puzzle




About: I studied Maths and Computing, worked in an Operation Research department, retrained as a civil engineer, worked on site for some major projects. I'm learning to be a structural engineer at the moment! I've ...

A workmate brought in a puzzle nut and bolt and asked me to undo it. A challenge to see how manly I was!?!

I could do the nut up, but not undo it!

It took me a few minutes to work it out, but I thought it was clever, and wanted one to take to the pub to confound my mates. So I set about making one!

My workmates brother works for BP and they had made some as promotional items, but these are brilliant as holiday gifts. Christmas seems to be a time when puzzles come out, whether they are from a cracker, a stocking filler or a handmade gift like this one! Alternatively you could just keep one in your pocket to confuse people!

This project will require some patience and time, as it takes quite a bit of trial and error to get it to all be accurate and 'tight'.

Date Made: Dec 2013
Approx Cost: £2
Approx Time: 1 to 2 hours
Difficulty: Medium


Step 1: What You Need


* Brass Bolt. Ideally M10 approx 50mm long. (You will lose 10mm in the stepping process)
* Brass Nut. To match bolt, but if you get hold of some manifold nuts they are deeper and so will conceal the secret of the puzzle better. Also I imagine a higher pitched bolt would work better as there would be greater mis-alignment in thread.
* Some Brass Washers. To match bolt. Number depends on how much fettling you end up doing!


* hacksaw (or dremel like I used)
* files

Note that I have used a much longer bolt in mine. I think this needs to be cut down (I will do this at some point!) as the longer the tail, the more chance someone will hold on to it and work out the secret.

Step 2: Cut Bolt

First, Cut the bolt in two. This needs to be done at a location that will be hidden by the nut.

I lined up three washers (as you can always take one or two away) and then a nut. I then lined up the cut with about 2/3rds of the way down the nut away from the bolt head.

Step 3: Cut & File Steps

The steps need to be formed in to each side of the bolt. These are 50/50 steps i.e. Cut in exactly half way in to the bolt.

They can be roughly cut and then filed (as you can see I did), or fully filed (which will take longer but less chance of cutting away too much).

To help keep these steps flat (perpendicular to the bolt), I used a sacrificial nut to screw on and file against to give me a flat surface. I also used this sacrificial nut to find the center line (from one point of the hex to the opposite point is the half way line).

The steps need to lock in to each other with the thread lining through so that when you put them together it's as if there is no cut at all. These need to be fairly accurate as it is these steps locking together that are the key to the puzzle.

As I said before, this project will require some patience and time, as it takes quite a bit of trial and error to get it to all be accurate and 'tight'. The more accurate it is the better it will lock and keep its secret (if its not accurate, the two sections of bolt will be loose and you will be able to tell that its not one piece).

Keep testing them and keep working away at them until you get them sitting together nicely with no gaps and continuous thread. Test this by putting them together and running the nut up and down, it should run freely.

I don't think there are any rules about how big the steps are, except they want to be big enough to latch and lock (minimum of two threads), but small enough to always be hidden by the nut. The bigger they are the stronger it will lock. On my next set (and I suggest on your first set!), I will make them deeper, as I don't think these are deep enough - it doesn't feel as strong as the BP set which has much deeper steps.

Step 4: Notch a Segment

Next, take the top (head) part of the bolt, turn it over and cut 'an hour' segment out of the higher part of the bolt. This needs to be on the side shown in the photo otherwise it won't work. Dont go quite to the center point - keep it just shy. Best thing to do is a little bit at a time. file a bit, see if it locks, file a bit more, see if it locks... If you go too far the nut and bolt will be loose and wont work.

Step 5: Put Together and Puzzle People

Finally put the two pieces together and screw on the nut. Use an appropriate number of washers on the top to space the nut. Put in enough washers so that the nut can't be done up too far so as to reveal the split. This should mean that the nut is always positioned over the cuts in the bolt.

Start the nut at the bottom of the cut, so that people can see that it can be done up, but not undone.

Ask your victim to remove the washer, or ask him to remove the nut, either way it will annoy them for a while!!

Step 6: 3D Model

I have made a 3D model to help people understand how it works, but could also be printed?

Be aware that this model is untested and hence classed as a prototype.

I have put the parts in 3 different colours, but obviously if you print it, it all needs to be the same colour so that people think the bolt is in one piece, not two!

Sets 1, 2 and 3 are all copies of each other, just positioned in different ways.

* Set 1 shows the 3 different parts separately.
* Set 2 shows the different parts but with the thread lining up (allowing the nut to do up).
* Set 3 shows the 'tail' rotated, which means the threads don't line up, hence lock.

(Credit due to Gustav Olsson who wrote the tinkercad script for creating M sized threads)

The Model can be found on tinkercad by searching for "Nut And Bolt Puzzle" or below:

Step 7: The Secret

The secret to this puzzle is that people instinctively hold the head of the bolt and the nut when trying to do up or undo the nut.

The notched segment means that when holding in this way, the nut will do up, but when trying to undo, the threads don't line up (due to the missing segment).

If you hold in reverse, with one hand on the nut, and one hand on the bottom/tail/thread of the bolt, then the nut will work in reverse too, with the nut undoing, but won't do up as the threads won't like up.

As I said before, I have used a much longer bolt in mine. I think this needs to be cut down (I will do this at some point!) as the longer the tail, the more chance someone will hold on to it and work out the secret.



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29 Discussions


Reply 5 years ago on Introduction

Ok, I made it last night, a rough and ready attempt from nuts and bolts I had lying around. I'll have to do a nicer one, maybe in stainless, maybe a bit bigger.

2013-12-14 11.25.11.jpg2013-12-14 11.27.38.jpg

Reply 5 years ago on Introduction

Yes, it's a thicker nut, about 5/16 instead of 1/4. I have no idea where it came from! The puzzle works with the ordinary nut but it needs an extra washer to make sure it hides the cuts. Also the thinner nut doesn't lock so well, you can force it to turn. The thread is 5/16 UNC.


5 years ago on Introduction

I turned to google and came up with this. It's a pdf that explains it.

2 replies

Reply 5 years ago on Introduction

I kept searching and found this too:
Which has a different method of fabrication i.e. Stepping the bolt before cutting it.


5 years ago on Introduction

Would it help to drill and inference fit a central stud up the middle, that fits into a hole on the other piece?

1 reply

Reply 5 years ago on Introduction

To be honest, I don't know! My guess would be that if you filed it accurately enough then no, but if you weren't that accurate then yes, as it would help keep everything 'tight'. If anyone gives it a go and it's an improvement then let us know!


Reply 5 years ago

You can use any bolt depending on the tools you have, but brass is softer than steel so it's easier to file, hence why I used it.


Reply 5 years ago on Introduction

I've just made one from a steel M6 bolt and a long nut, more like a threaded hex tube. Brilliant puzzle.


Reply 5 years ago on Introduction

The filing was not difficult but accuracy is needed.
Sorry,totally failed with the photos.


5 years ago on Step 6

So then, what is the secret to "undo" the "done-up" bolt, or do I get to build one and discover it can't be undone? (that actually sounds like a pretty good instructable).

1 reply

Reply 5 years ago on Introduction

Have a look under "Step 7: The Secret". The text explains it, but easiest way to understand is to watch the video.


5 years ago on Step 3

To be able to make longer slots, you could use an extension nut to cover them. These are much bigger than a normal nut, and give you more body to hide the slots under...