Compressor Engine

Missouri Collector Breathes New Life into Au - To Air Compressor

Vintage stationary engine

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.

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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 available.

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: