Compressor Engine

By Staff
1 / 5
Looking at the finished product, it's easy to see why some old iron collectors mistake Robert Best's converted Au-To compressor for an original, vintage stationary engine.
2 / 5
Robert welded in horizontal intake and exhaust ports after fitting valves to the original suction and discharge pipes. The mixer (at right) is hand-made.
3 / 5
Robert's converted compressor features a vertical fly-ball governor. Note the linkage for exhaust lockout on overrun, plus the integrated spark-saver. Very nice work.
4 / 5
A distributor gear set from a Volkswagen drives the vertical flyball governor and camshaft. Robert also cut out material from the base of the cylinder to expose the crankshaft.
5 / 5
Looking very much like a factory-built engine, Robert's converted Au-To compressor engine is an attractive and certainly unique engine.

I wonder how many collectors have ever seen -much less owned –
an Au-To flywheel air compressor? Better yet, how many collectors
have seen an Au-To air compressor converted to an engine?

After seeing a few engines made from air compressors some 15
years ago, I decided to make my own. I attended numerous auctions
looking for a compressor that would make a good engine, and for $5
I picked up an Au-To compressor. The old compressor sat around my
shop for a while, and then I moved it to my storage building, where
I forgot about it until 2002.

Getting Started

Picking the project back up, and not sure exactly how to
proceed, I worked on my engine by trial and error. Using some
drawings of other engines as a guide, I finally adapted a concept I
thought would work.

I wanted the finished engine to have dual flywheels, but the
compressor was only fitted with a 14-inch flywheel on one side and
a ring gear of equal size on the other, which, I assume, was geared
to an engine that supplied power to the compressor. For the second
flywheel, I used one from a Cushman Binder engine. It’s a
little larger, but at some point I’ll turn it down on a lathe
to the same size as the original. The Cushman flywheel had a
tapered center hole, so I reamed it straight and installed a steel
bushing to size it to the Au-To’s 1-1/8-inch crankshaft.

I sourced a 2-to-1 distributor gear set from a Volkswagen, which
I decided would work well to drive the vertical flyball governor
and camshaft I planned on fabricating. Not surprisingly, the
Volkswagen distributor gear set also had to be sized to fit. Fixing
the larger drive gear was no problem, as all I had to do was
machine a bushing to reduce its inside diameter to 1-1/8-inch. I
knew I’d have to machine the smaller driven gear, but it was so
hard I had to anneal it before it could be machined. I did this by
heating it cherry red and slowly cooling it in a bucket of sand.
Then I turned it on a lathe to match bushings that I had

The compressor’s original crankcase was completely enclosed,
but I thought the finished engine would look nicer if I removed
some of the non-essential cast iron housing at the base so people
could see the crankshaft.

The compressor has a 3-inch bore and a 5-inch stroke, and at
top-dead-center there was only the slightest clearance between the
top of the piston and the head. But the Au-To also had a 1-inch
spacer housing the compressor’s check valves, and by removing 3
inches from the center of the spacer 1 opened up a nice area for
combustion. The piston originally had four rings, but I took out a
ring as I figured three would be sufficient. Sure enough, the
engine has more than enough compression.

Moving Ahead

The head ended up presenting the biggest headache, and at one
point I thought I might have to cast a new one to make the engine
work. The head was originally equipped with a -inch pipe outlet and
-inch pipe inlet that protruded about 1- inches above the top. I
ended up plugging these with cast iron pipe plugs, which I then
drilled out to act as valve guides. I fabricated the valves from
5/16-inch stainless steel rod that I welded to 1–inch-diameter
stainless steel heads. I then ground the valves to a 45-degree
angle so they would seat properly. I ground the valve seats into
the head, and I determined final valve length when I fabricated the
exhaust rocker arm.

The big problem came when I was trying to add material to make
ports for the exhaust and inlet. I had the valves set into the
vertical pipe, but I then needed to port the vertical pipes
horizontally, which required a creative solution.

I decided to weld in horizontal ports, but as many of you know
welding heavy cast iron can be problematic, to say the least. After
adding material to port the valves, I had small pin-holes in the
ports that I couldn’t close. To get around this I reamed the
ports out. I then pressed in a -inch piece of threaded pipe for the
exhaust, but I used smooth pipe on the inlet side so I could
install a slip joint for the carburetor. I fabricated the
carburetor from a piece of 1–inch brass with a venturi of
approximately -inch, which I bored for the needle valve and seat.
Next, I fabricated a 25-ounce-capacity brass fuel tank that I
installed directly below the fuel mixer seat. This gives me enough
gas to run the engine all day.

I then fabricated the exhaust rocker arm and cam, and with some
trial and error to get the right lift on the valve, it worked out
fine. I also fabricated brackets to support the cam gear and shaft.
The trick here was to attach the top bracket on the head before
finally drilling it for permanent location so I could adjust the
alignment of the shaft. It wouldn’t take too much misalignment
to wipe out the gears.

Ignition is by buzz coil with an adjustable wipe system mounted
on the cam. The wipe is adjustable to regulate timing. I also added
a spark saver since the engine coasts so long between ignition
cycles. I made the flyballs for my governor from 1 -1/4-inch brass.
I was anxious to fire it up, so before I made the fuel tank I shot
some fuel into the carburetor. The engine fired, but the fly-ball
shafts were so long the engine wouldn’t fire again so I
shortened the shaft lengths. I ended up changing the length of the
flyball shafts three times before I found the correct speed.
Springs can be added to the bottom of the flyballs to increase
engine speed, but to keep the engine running slow, I’ve left
the springs off. Now fully functional, this engine runs between 100
rpm and 150 rpm, and after it hits it coasts 12 to 15 revolutions
before firing again.

I also added a zerk fitting to the connecting rod so I can
grease the crankshaft through the opening in the base, and I added
grease cup fittings to the two main bearings. I put a drip oiler on
the cylinder wall to lubricate the piston.

I believe the air compressor was originally painted blue, so
that’s what I painted it, along with white pin striping. Since
the engine was my own design I decided to likewise make a truck of
my own design – oak trimmed in blue. I’ve shown this engine at
a number of shows, and it is quite an attention getter. Blue is an
unusual color for an engine, and when they look at the engine most
people can’t figure out what company made it.

I finished the engine in the spring of 2002 and first unveiled
it at the North Kansas City, Mo., Mo-Kan show in June 2002. So far,
not too many people have seen my Au-To compressor engine, but with
each and every engine show I attend, that’s changing.

Robert Best is a member of the Mo-Kan Antique Power Association
of Kansas City, Mo. Contact him at 3521 N.W. 60th Terrace, Kansas
City, MO 64157; e-mail:

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