Penny Stove - DIY

The following is some 'how to' notes for building an alcohol (meths) burning stove out of aluminium cans. Written from a New Zealand perspective and sufficient to boil water to feed two.

After a modest amount of experimentation, I made a stove that I have used for most of my tramping since 2016 or so - the only reason I don't still use the first one I built was that I had to go and fiddle with it.   The steps below are more complicated than you need for your first attempt so don’t be put off by the details; there are many, many ways to get a functioning stove and you will very quickly work out how you want to perfect your own design.

What is a penny stove?

A penny stove is a lightweight alcohol (methylated spirits or 'meths') burning stove that you can make yourself with three ingredients and a few basic tools. At its crudest; you cut the bottom off two soft drink cans, poke some holes in one of them and jam them together. The coin sits over the filler hole and acts as a regulator, i.e. as pressure rises in the stove it lifts the edge of the coin rather than blowing the stove apart. (I'm not convinced the coin is completely necessary but have assumed it is in the following)

Why bother? - pros and cons

Well, it is so simple and cheap the question should be; what possible reason is there not to give it a go? If this isn't enough for you, some other 'pros' are: it is ridiculously light, the fuel is less noxious then most other fuels and evaporates cleanly if spilled, it burns less fiercely and is less likely to take your eyebrows off, you only take as much fuel as you need (unlike butane), you can use the fuel as a disinfectant, and it's a great talking point in the hut at night. Did I mention cheap? $5 for a liter of meths at the supermarket and you only use a few capfuls each time.

There are some limitations of course. It's slower so not suitable if you are catering for a party (fine for two, I haven't tried it for three), there is a knack to using it, it's less robust, and doesn't have a volume control (although more on this later). I wouldn't use it on alpine trips and it's not as good in wind as other options. 

I use it routinely for light weight tramping or cycle trips when I am preparing dehy meals (i.e. just boiling water) for myself and maybe one other. 

Safety

If you need safety warnings spelled out for a project that involves knives, thin metal edges, flammable liquids and power drills then just approach this as a diverting read and please put the Stanley knife down. I will just make the point that sometimes the flame can be hard to see, so be sure the stove is out before pouring fuel anywhere near it (particularly if you have to run a second preheat; discussed further below).

There are safety considerations in operating the stove too of course.  Some of these are common for all stoves, some more relevant for the penny stove.  For example, carrying and handling flammable liquid, combustion by-products in enclosed spaces, heat shed downwards when the unit is in action, security of boiling liquids on whatever stand you bodge up. 

Operation

Summary: pour meths onto stove, place the penny, pour more meths, light.

I'll briefly outline how it operates which may help explain some of the design features (also see photos at end of article). The stove runs on methylated spirits (meths) which is basically ethanol with a few extras to make you sick if you are silly enough to drink it ('methylated spirits' refers to the now ceased practice of adding methanol to the ethanol (spirits) which had the undesirable characteristic of dissuading silly people from drinking it by poisoning them). There are various other products referred to on the internet, mostly American. I have never run a Penny on anything other than the meths you buy in the supermarket and would discourage experimentation unless you really know what you're doing or don't value your eyebrows.

Like many other stoves, the Penny requires a preheat phase before it settles down to business.  You first add meths to the body of the stove then place the coin over the fill holes and add more meths to create a pool on the top of the stove for the pre-heat. 

When lit, the pool of meths burns and heats the body of the stove (the theory is that the skirt of the stove conducts heat to the base), this heats the fuel in the stove which starts to evaporate increasing the pressure in the stove and forcing the evaporated spirits out through the jets to be ignited by the burning meths in the cup. When the jets get going, they maintain heat in the stove and the preheat stage is over.
When finished, a firm breath or two should blow the stove out, let it cool and tilt it to return unused fuel to your fuel bottle via one of the jet holes.  If you're nervous about blowing the stove out, you could devise a snuffer to slip over the top (e.g. a slightly bigger can base).  

Necessary extras

You will want a lightweight stand (trivet) for the billy and a wind shield.  Initially I bent a wire coat hanger for a stand and made a wind shield out of a tinfoil takeaway tray.  More recently I have made a more elegant alloy shield that doubles as the stand.  If you have a small billy, it gets a little tricky designing a trivet that fits inside the billy for transporting, is stable, and holds the billy at the right height above the flame.  My current stand, whilst elegant, falls down on the latter resulting in a sooty billy bottom which is a bit annoying. 

Ingredients

• Two identical cans (375 ish ml). Scrounge through the recycling bins at work and get as many as you can get your hands on.  Try to get ones that have no dents and no embossed numbers on the bottom. Ideally, one can will have the drinking hole cover still attached.  I've made mini stoves with small cans, but these were novelty value only. 

• A coin. Small and smooth edged. I used a NZ 10c piece but now use a US 1c for no other reason than it's what the stove is named after, and I had one. You want a smooth edge because the coin is supposed to seal the filler holes well enough to form a pool of meths in the cup for the pre-burn.

Tools

Useful

Someone else's book
Craft blade
Clamps
Ruler and/or calipers
Drill and 1.5mm bit

Maybe useful

Drawing pin or scriber
Sharp nosed pliers
Marker pen
Someone else's scissors
You can actually manage with just a craft blade and a drawing pin.

Instructions

Summary:  Cut the bottom off one can to make the base of the burner, cut a shorter amount off the bottom off the other can to make the burner top, drill jet and filler holes in the top, jam the top into the bottom, cut the top off one can to make a base for the stove.

These instructions should give you a burner that will boil enough water for a dehy meal for two. 
Making the stove bottom

Create a jig to scour a line around the can at a precise height.  

Insert the blade into the book and clamp firmly to the table so the blade is at the desired height from the table (33mm here).  If you want to be very accurate you might need to have a couple of goes to get the height right due to compression of the book. It doesn't matter that much though.

The next step is like cutting glass. Roll the upright can against the tip of the blade so it scores a line on the can (without breaking through). Be careful to keep the can base firmly on the table and at an effective angle to the blade. You will get better at this; you may need to go around the can a few times to get a complete line but it's generally better if you only need to go around once - if you wander off line it will be harder to split, and you may get sharp slivers. Try not to deform the can too much as this will cause the line to wander and may interrupt the scored line.

When the line is completed, very gently increase the pressure to work the point of the blade through the can at one point on the line - try not to dent the can or get a ragged edge.

This next bit is quite cool when you nail it. Use your thumb to press on the can beside the cut which will magically grow and neatly split the can around the line. If you haven't scored the line cleanly it may require a bit more work (and leave burrs) and the last wee bit can be a little tricky.

That's the stove bottom done

Making the stove top

Repeat the above steps for the other can but set the blade lower than for the stove bottom - I used 17mm for this one.

Making the base

The base is optional but you may find you need it for a better preheat. However, it is also useful for packing the stove: put the coin on the burner, reverse the base and place it over the burner and coin, put a rubber band (or wrap your cleaning cloth) around to hold it all in place. I use a cloth as it also protects the stove.  

A downside of using a base is that it lifts the height of the stove which means you need a higher (harder to pack and more unstable) stand for the billy.  On balance, I would experiment until you have a stove that doesn't need to be insulated from the ground in order to preheat enough.  Note that this puts your flames closer to the surface you are cooking on.  

Take one of the cans and repeat the above steps to remove the top of the can rather than the bottom - the height doesn't matter - I try to make it as short as possible, but the height of the blade will probably be dictated by the shape of the shoulder on the can.

This step will be harder because the can will distort without its bottom and the wall maybe a bit thicker at this point. You could of course just get a third can if your budget stretches that far. Rip the tear tab off (superfluous weight) and fold the hole cover back into place.

The base is upside down at front

Jets and filler holes

Mark where you want to put the jets. The pattern of holes is up to you, this version has 8 jets on the shoulder of the burner and three filler holes.  My current stove has 6 jets on the ridge at the base of the can and three filler holes.  You can also try putting jets in the cup part of the burner. I've found the inner ones light faster but that model didn't burn so cleanly. 

I mark the positions by eye but to be neater you can wrap a strip of paper around the can, mark it where it reaches once around. Take it off, measure it, divide by 6 (or 8), mark the paper at that spacing, wrap it around the can again and mark the rim of the can at those points.

Marked up for drilling

Use the drawing pin (or a scribe) to put a small dent at each mark so that the drill will catch.  

Carefully drill the holes in the can (and not your leg). Remove any burrs in or around the holes. I hold the can throughout this process, but you may prefer to be safer and clamp the can somehow without denting it.

With enough force you can punch the holes with a drawing pin. I did one with eight jets that worked OK.

 

The filler holes (or hole) need to be drilled in the bottom of the cup so that the coin will cover them. Mark the middle of the cup (a ball bearing can help find it) and three marks about equally spaced around it. Dent with the drawing pin and drill. I've seen variations in size and number of holes; three using the same bit as for the jets works for me (and that way I don’t have to change the drill bit).

Stretching (or crimping)

You need to fit the burner section with the holes inside the taller base section.  As the cans are the same size, they don't really want to fit inside each other. However, the aluminium will stretch enough provided you're not too rough and split it. 

Use the bottom of another can (preferably not one you have cut bits off - they tend to be sharpish) and push it into the bottom section of the stove. Roll it around firmly to stretch the top 5mm or so of the skirt. This should allow the top section to fit in neatly but don't start pushing it down yet.  It may take a few goes to get it in, if it isn't working you can try to stretch the bottom a little more or cut a piece off a discarded can to use as a shoehorn.

 

Top fitted into bottom - no force required

Some people crimp the cut edge of the top burner section to help fit it into the bottom - I no longer bother with this step having found it fiddly and prone to the folds causing lumps in the walls of the finished stove. The idea is to decrease the circumference of the burner section so that it just fits inside the bottom section - you can do this by running around the sharp edge judicially bending it inwards using needle nose pliers. 

Others have tried heating and cooling the respective sections to take advantage of thermal expansion - I found that the aluminium loses/gains heat so rapidly that any advantage pretty quickly vanishes leaving hot/cold fingers and a stubbornly unjoined burner. 

Another option is to drill holes a little way down from the top of the bottom section (5mm?) and use someone else's scissors to cut from the top of the skirt down to each hole (the holes stop the cuts extending). This should allow you to slide the burner into the base far enough to get started. This method can also sometimes create lumps in the sides of the finished stove though.

Assembly

Now you want to slide the top burner section down inside the bottom so that it forms a good seal and sits level. I use pistol grip clamps but they tend to be a bit jerky so you may prefer to try a G clamp and something to spread the load. You can also do without, e.g. tapping with a wooden spoon.
I get it started by clamping the book (to spread the load) onto the partially assembled stove and to the table - then gently tighten to ease the top gradually in 'til it's flush. Next I use two small pistol clamps to push it the rest of the way down -   You want to keep the load as even as possible - you can wreck it by pushing one side too far in.

It can be a bit tricky to know how far to push the burner in - when it starts to require more force it's time to ease up and just tweak so it is level (use a depth gauge if you are fussy). 

You should now be holding in your trembling hands a soon-to-be-functioning penny stove.

Stove and base

Stove, base and penny

When assembled it is possible that you will have leaks around the joint. Some people suggest a heat proof sealant, I have never bothered. The leaks point up towards the billy so the energy is not lost; I would only be bothered if it released so much pressure that the flames were too low or the preheat didn’t work, in which case I’d perform an interesting “test-to-destruction” experiment on it and build another.

Operation

Set the stove up on its base (if using one), pour some meths into the cup and let it drain into the base (you will get used to how much is needed).

Place the coin over the filler holes and pour some more meths into the cup.  
Check there is no meths on your fingers.
Light.

Put the shield up and billy in position to reflect the heat back into the burner.

The meths will burn down and drain under the coin - the flames may be hard to see in daylight and it can take a minute or two to pre-heat (no matter - it's still heating your water).

A flame should start forming above each burner hole.  If the meths in the cup burns out, make sure it really is out then add some more meths and repeat the pre-heat.  

Note:  relighting a stove that has been heated carries a risk that vapour in the stove ignites - I've found that the temperature of the meths quickly drops the temperature of the stove though.  However, if you find you have to repeat the reheat, you may not be getting enough meths in the pool above the penny.  See comments below for some solutions. 

As the stove gets up to heat you may hear the meths boiling and the flames will get stronger.

Just about at full noise

Whoosh!

Comments

You can (and indeed should) make several versions trying different hole positions, size of base, length of skirt, size and type of can etc. Some people put pink batts inside, I think to reduce risk of spillage and to wick the fuel; I don't bother. 
If you want to simmer (as opposed to bring to the boil) you can experiment with a simmer ring. This is just a strip of can that you slip inside the skirt once the stove is burning so as to extend the sides upwards. It restricts oxygen to the base of the flame meaning a cooler but still self-sustaining burn. Again, I don't bother as I generally just want to get water to the boil rather than keep it simmering.
Some stoves tend to go out in the transition from preheat to burn. Some possible solutions:

• Add more meths when you are initially pouring the pool above the penny

• Check the coin is sitting snugly (use a finger to scour the coin around on the aluminium to smooth the surfaces)

• Pre-heat with the wind shield in place and put the billy straight on top once lit

• Add more meths and repeat the preheat (see warning above about vapour in the stove)

• Hold a lighter against the side of the stove during preheat 

• Spill a little fuel around the stove before you light for the preheat (this idea offends my sensibilities somewhat and can alarm other hut users)

• I trialed a preheat dish under one stove

However, my current stove pre-heats fine so if you are having problems, I suggest making another stove with a different combination of skirt length and hole configuration.