Introduction: Spark Plug Puzzle

About: My name is Troy. I'm a Mechatronics and Aerospace Engineer. I make things out of wood and electronics and spend time outdoors (especially SCUBA diving).

Puzzles are great! They make you think outside the box. I came across the idea of this puzzle online. Not knowing how it was put together or solved, I started thinking how I would make it. The original puzzle that I saw, only had one locking bar on each side. Thinking that I could make it better, I challenged myself to figure out a way to take this simple puzzle to the next level.

If you know how the original puzzle is constructed and solved, please let me know if their solution is any different than mine.

Step 1: Tools and Materials


  • Saw
  • Flat tooth saw blade/dado stack on a table saw or router (to make rabbets)
  • Drill
  • Drill bits of varying size
  • Countersink bit
  • Angle grinder or hacksaw
  • Sander


  • 2 - 1 1/2 inch x 3/4 inch x approx. 4 inch
  • 2 - 1 1/2 inch x 3/4 inch x approx. 3 inch
  • 1/4 inch x 1 inch long Steel Rod
  • 6mm x 3mm Neodymium Magnet
  • Spark Plug - Any inexpensive spark plug will work
  • Wire "bars" - I visited my local thrift store and picked up this dish dryer for a couple bucks. A cookie rack or other type of rack would work just as well.
  • Two part Epoxy
  • Lacquer

Step 2: Cut Rabbets

Cut your boards to length. The length of these boards will vary depending on the size of your spark plug. I cut mine to 3 5/8 inch and that allowed for enough space for my 3 1/4 inch spark plug to fit (with clearance hole made in the next step).

Cut rabbets in the ends of three of your pieces. I used a flat tooth table saw blade for this, but a router or dado stack would work just as well.

Rabbets should be exactly half the thickness of each board both in height and depth.

Step 3: Drill Bottom

Find the center of the bottom piece by drawing lines from corner to corner. Drill a hole just larger than the end of your spark plug deep enough for the spark plug to fit with all sides of the puzzle present. I used a countersink bit to open up the hole slightly to allow the spark plug to be easily removed from the puzzle.

Step 4: Cut "Bars"

I used an angle grinder to cut down my bars. In this step, only rough cut the length of the sides. Make sure that the height is correct. The width will be finalized in a later step.

Step 5: Drill Latch Rod Hole

This is probably the hardest step of the whole project. I had to drill this hole three times until I got it right. You want this hole to be at an angle so that the center of the hole passes through the location where your first bar will enter the block, but miss the second bar. You also need this hole to not come out the side lower than your rabbet will be.

Pre drill this hole with a smaller drill bit to ensure that your hole is located where it should be. This hole should be drilled 1/4 inch from the inside face. Final hole size should be 17/64 inch to allow the rod to slide freely. Drill as deep as possible without drilling through the other side.

Cut rabbets on each end to join the rest of the puzzle.

Step 6: Drill Opening for the Bars

This is another difficult step. You need to make an opening in the wood that appears from the outside as a single hole and on the inside has a sweeping opening that will allow the bars to swing. (if you can think of a better process to do this, please let me know!)

Using an undersized drill bit, drill the three holes almost all the way through. Adding a piece of scrap wood to increase your angle, drill another set of holes almost all the way through. Add a final piece of scrap wood and repeat. Be careful not to drill completely through!

Once you have cleared out this path with the undersized drill bit, repeat this process with the correct size drill bit that will allow for the size of your bars.

Step 7: Cut Bars to Length

Using an angle grinder or hacksaw, cut the bars to final width.

I cut the rear bars (that aren't removable) to 2 inches wide. The front bars will have different lengths on each side of the middle bar. When the bar is centered in the puzzle, the left hand side needs to just touch the latch in left side of the block. When the latch is out of the way, the right hand side needs to clear the right hand side of the block.

Step 8: Glue Magnet in Place

Use two part epoxy, to glue the magnet in place. Place the latch rod in the hole. Hold the magnet by attaching it to a thinner metal rod. I used an old bicycle spoke for this. Insert the magnet into the top of the hole. Make sure that it is parallel to the angled hole so that the latch rod connects squarely to the magnet. Leave the epoxy to dry with the holding rod in place to ensure it doesn't move.

Once dry, remove the holding holding rod and remove any excess epoxy sticking out of the hole.

Step 9: Glue Puzzle

Before gluing everything together, sand the interior surfaces. You will not have easy access to these surfaces once it's glued together.

Insert the rear bars and glue everything together verifying that the puzzle is square.

Step 10: Sand Flat

Sand the faces flat. Make sure not to sand too much as if you remove too much material you will reveal the latch rod hole.

Step 11: Sand Entire Puzzle

Sand the entire puzzle with increasing grit. This will make all the difference in the finish.

Step 12: Cover the Bars

Using masking/painters tape, cover the bars so they don't get coated in lacquer. This will keep the finish looking the same for the front removable bars and that back permanent bars.

Step 13: Sand Between Coats

Using high grit sandpaper or steel wool, sand between layers of lacquer. This will result in a smooth and high gloss finish.

Step 14: Remove Tape

Remove the tape and admire your workmanship.

Step 15: Assemble

Assemble your puzzle by inserting the bottom of the spark plug in the wood. Verify that the locking bar is not locked against the magnet (if it is, strike the puzzle against your hand to dislodge the lock). Insert the bars into the three holes on the left. Slide the bars into the holes on the right. Turn the puzzle over and listen for the 'click' indicating that the locking bar is back in place.

Step 16: How to Solve

To solve this simple puzzle, hold the puzzle in your right hand with your thumb applying pressure to the right. This will ensure that the bars are not in contact with the locking rod. Swing the puzzle down toward your left hand striking the left palm. Slide the bars to the left and pull toward you removing the bars. Remove the spark plug and replace the bars. Turn the puzzle upside down and listen for an audible 'click' as the locking rod is moved back in place.

To place the spark plug back in the puzzle, repeat the same steps to unlock the puzzle.

What other puzzles can you think of that will utilize this same locking technique? Could you implement it in a box or hiding spot? If you do, please share pictures of your projects as I'd love to see them!