Building A Steam Engine From A Lawn Mower

If you want to build a low-dollar steam engine, all you need is a 4-cycle lawn mower engine.
David Brezeal
March/April 1987

Convert a lawn mower engine to steam with this low-cost project.
Courtesy of Fotolia/Eva Schinschke
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Several years ago I decided to make a steam engine that could be made with simple tools. I thought I'd send it in so if someone else wanted a low dollar steam engine they could try my suggestions.

All you need is a 4-cycle lawn mower engine. It doesn't have to run — just be free — , a grinder, and someone with a welder (preferably wire feed). I got mine from the city dump one Sunday when no one was around.

The engine had low compression because of bad rings, but beggars can't be choosers, so I used it anyway. If you decide to make one, try to find an engine with good compression. I later found out that with bad rings the engine would run fine but the steam would slip past the rings and condense in the crankcase and push the oil out.

The carburetor and ignition can be removed and I took a chisel and cut the fins off the cylinder to help cut down on condensation. The spark plug stays to plug the hole and it would be a good idea to leave the recoil start or install a rope start pulley because when the intake valve opens it's running and it doesn't care what's in the way when it starts.

When I made mine I left the blade on, which I don't recommend, but I wanted to compare it to a gas job. So one day when Mom wasn't home to say 'no', I borrowed a garden hose for a steam line and tried it out. It seemed to work as good as a gas job plenty of power until the hose popped. It's amazing how fast neighbor kids can leave if they think there's going to be trouble!

The first thing to do is to take the camshaft out and have someone weld two lobes 180 degrees opposite the originals (figures A and B in the Image Gallery, figure A being the original cams). Now take the head off and decide which port is going to be the intake. The exhaust is usually threaded so it might be best to use that. I tapped the intake on mine but it doesn't matter.

Once you've decided which port to use decide whether you want it to run clockwise or counter clockwise and turn the engine in that direction until the piston comes up to about 1/8-inch before the top of the stroke. Now replace the camshaft with the original cam for your intake valve so it just starts to open the valve. If it doesn't drop in you may have to grind a little off the cam. The intake should close one thirde to halfway down.

The best way to grind the cams is to hold the ends of the shaft and rotate it into the grinding wheel. Try to grind only on the lobe of the cam.

This is your starting place so make a new timing mark. It may have to be moved later.

Rotate the engine until the piston is 1/16 and 1/8 inch before bottom dead center and grind the exhaust cam so the valve just starts to open. If it happens to be the original cam rotate it a little further and see where it closes. If it is between a quarter stroke and where the intake first opened, it's O.K. If it's a new cam grind it to the same height and so it closes between one quarter stroke and where the intake first opened.

The reason the exhaust opens so early is to relieve the pressure before the piston starts backup.

Now proceed until the piston is in the same place it was when the intake first opened and again grind the cam so the valve just starts to open, the same as the first time.

To get the cams to clear all the way around you may have to grind them narrower or you may have to move the timing mark as I said earlier, so don't grind too much until you're sure it'll clear.

The timing doesn't have to be exactly as I said to run. In fact, when I first put mine together it was timed to run in either direction by simply switching the steam and exhaust pipes around. It didn't run too good, though, so I changed it to the above.

It's also possible to use only the original 2 cams, but you need a very heavy flywheel and it doesn't have much power or a very steady speed.

One other place you might have trouble with steam getting into the crankcase is past the valve guides. If they're tight, leakage won't be too bad, but if it is bad, you might have to cut a groove around it to let the steam escape before the crankcase.

If what to use for a boiler is a problem, don't worry, as it'll run excellently on compressed air. In fact, if timed right, it'll almost run by blowing into it.

But now that we're on boilers, I have a few things I'd like to say. A boiler is a bomb in the hands of someone who doesn't know what they are doing. A boiler is constantly moving even if it's standing still. Every time the pressure raises the temperature rises and the metal expands, and when the pressure drops, temperature drops, and the metal shrinks just as if the boiler metal was being bent back and forth to a very small degree.

Boiler plate is a very soft high grade of steel. Good boiler plate up to 3/4-inch thick can be hammered over on its self without showing any signs of cracking or stress. An example of stress cracking is putting a rod or bearing vice and bending it back and forth until it breaks. The harder the steel the sooner it'll happen. That's why boilers should only be made of boiler plate. Also all A.S.M.E. boilers are stress relieved after being welded by placing them in an oven for several hours to relieve the stress of the welds.

A.S.M.E. boilers are hydrostatically tested at 150 percent of working pressure and no more. One inspector said that at the factory they'd have to scrap it if tested too high because of possible over-straining the boiler. I've heard several guys say they run their 100-pound boilers to 300 pounds to test them.

Stress cracks can be too small to see or hidden under grease or scale or some part of the engine.

Water used for a hydro should be between 70 degrees and 120 degrees. If too cold you might strain the boiler; if too hot you defeat the purpose.

I didn't write to talk about boilers, so I'll quit here. But boiler safety is a pet peave of mine. I saw a guy running an engine with 26 patches gobbed on and 13 were still leaking. When I saw that, I left. Apparently the guy didn't realize the responsibility he had to the public. Some people have no business running an engine and couldn't care less. Sooner or later one's going to pop and we're all going to pay.


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