Curtis Switch-breed

By Staff
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Vernon Achord's Curtis 'Switch-breed' compressor engine looks for all the world like an original stationary engine.
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Clearly visible VW-sourced sideshaft gears.
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Sideshaft, rocker arm and exhaust lock-out on the 'Switch-breed.'
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A sketch of Vernon's 'Switch-breed' highlight's the governor and exhaust lockout that he fabricated for the conversion.

My part of this story starts in June 2003 at the Hill Country
Antique Tractor and Engine Club Show in Stonewall, Texas. I was
talking to Randy Carll, who always has something interesting for
sale, and I told him I was looking for a small compressor from
which to build an engine. He pointed to a small, old air compressor
he was selling. It was stuck, but the price was right so I bought
it, along with a Monitor engine and several other items. He even
threw in another compressor so he wouldn’t have to haul it

After I got the compressor home I dismantled it, and fortunately
it didn’t take much to un stick the piston. With the engine
apart, I could take stock in what I had and make my plan of attack.
This compressor has a forged crank and rod, and a 2-1/8-inch bore
by 2-1/4-inch stroke. A dipper for lubrication is mounted on the
crankshaft, and the engine has a hollow base.

One problem with converting compressors to engines is that most
do not have any combustion chamber. They were designed to compress
air, not to fire a charge, so the piston comes very close to the
head on top-dead-center.

About the same time I was working on this project, GEM featured
another Curtis compressor engine in the September 2003 issue. The
fellow who converted that engine used a 1 -inch spacer between the
cylinder and the head to give the engine a combustion chamber. I
figured 1 had three choices: I could build another head with a
combustion chamber (it would also help with the valves and ports),
shorten the connecting rod or use a spacer.

Unfortunately, I don’t have the tools or the iron to build a
head. I liked the idea of shortening the rod, but there was nowhere
in the original head to  nowhere in the original head to put a
spark plug, so I ended up using a spacer. To use the original head,
I ground a new valve seat and moved one using my dremel tool. I
turned valve guides from rod stock and pressed them in the head,
and used valves from an old Clinton lawn mower engine.

I don’t own any sideshaft engines, so I figured this would
be the time to have one. For sideshaft gears, I used the crankshaft
and distributor gears from an air-cooled Volkswagon engine. These
gave me the 2-to-1 ratio I needed for the valve. I took a thick
flat washer, turned it down and pressed it inside the crank gear.
Then I drilled a 1/8-inch offset hole in it and the crankshaft, and
used a 1/8-inch dowel to keep it in time.

I made a bracket to mount the distributor gear and drilled the
distributor gear to accept a 5/16-inch rod, which runs vertically
up to the head to work the exhaust valve. At the head, I made
another bracket to support the camshaft and rocker arm. To make the
camshaft, I drilled a 5/16-inch hole through the center of a
3/4-inch piece of rod that is about an inch long. I drilled and
tapped it on the side to take a 1/4-inch set screw. I then welded a
heavy 2-inch OD washer to the top of the rod I had drilled out. To
make a cam lobe, I used some 1/4-inch thick flat iron. To find out
how long I needed the lobe, I turned the engine over, marked where
the valve should open and close, and marked this on the washer. I
cut a piece of the 1/4-inch flat iron to that length and ground my
ramps on it. I used a hammer to match it to the radius of the
washer and welded it to the washer. The rocker arm is made from
1/4-inch flat iron, and it has a small bearing (I think it came off
a router bit) that rides on the cam lobe.

I wanted this engine to be hit-and-miss, so I came up with a
simple idea for holding open the exhaust valve. The flywheel
already had some mounting holes near the hub, so I made some
flyweights and a sliding collar that slides over the crankshaft. I
then made a rod that pivots on the engine, with one end riding in
the sliding collar and the other running up to the rocker arm.

The way it works is this: When the flyweights pull apart, they
pull on the collar. The collar then moves the rod against the
rocker arm. As soon as the rocker arm pushes the valve down, the
rod slides over the top of the rocker arm and holds it down. When
the engine speed drops, the weights pull the rod away from the
rocker arm, allowing it to work again.

I made a mixer from a piece of 1/2-inch brass pipe. I made a
mixture screw from a brass screw and the seat from a 1/8-inch pipe
fitting with a 3/16-inch compression fitting on the other end. I
pressed a flat washer inside the pipe in front of the mixture screw
to give the mixer some restriction.

As I mentioned earlier, this compressor has a hollow base, which
I decided would make a good gas tank. I cut out a 3/8-inch piece of
steel and bolted it to the bottom of the base to seal off the
bottom. I drilled the base and tapped it for a fuel line, and I
also made my own check valve. Then I drilled the base and installed
a fuel filler made from a 1/2-inch pipe street elbow.

Ignition is a battery and buzz coil, with a swipe mounted on the
side of the engine contacting a nub on the sideshaft. For a
muffler, I used an old cast iron gas stove burner. It looks like it
was made just for this purpose, and it’s pretty quiet.

For the skids, I was just going to use some 2-inch-by-2-inch
pine and a board, plus a box to hide the battery and buzz coil.
After I got the 1-inch-by-10-inch board, I decided a trap door with
a compartment under it to hide the electronics would be a good
idea, so that is what I did.

After I got everything together, I put some gas in it and
cranked it up for its first run. However, it didn’t run very
long at first as it was starved for fuel. After a little
fine-tuning on my homemade check valve, however, it ran well, and
now it’s hitting and missing just as it is suppose to. One
problem with this engine is it has no counterweights, so it jumps a
lot – looks like I’ll be making some.

You may be asking about the name ‘Switch-breed.’ I
decided that compressor engines need a name to tell them apart from
engines that have a compressor built into them. We use half-breed
to talk about combustion engines that were originally steam
engines, so I decided to come up with a name for any internal
combustion engine that originally was used as something other than
an engine, be it a can opener or a compressor.

It’s a really neat feeling to see something run that never
did before. I enjoyed building this, and I look forward to seeing
more switch-breed engines.

Contact engine enthusiast Vernon Achord Jr. at: 14218 3rd
St., Santa Fe, TX 77517; e-mail:

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