The winter doldrums can set in pretty hard here in Ohio, so this winter I decided to keep busy making an engine from an old air compressor. The foundation for the project engine is a Curtis compressor with a 2-inch bore and 2-1/2-inch stroke, bought from fellow collector Gene Welch. The name of the engine, Curtis Crudo, derives from its compressor origin and the fact that all parts were crudely made from junk I had lying around my shop.
Creating the Crudo
The compressor didn't have a camshaft, of course, so I machined an old Briggs & Stratton camshaft gear to fit on the end of the crankshaft. This included shortening the camshaft and machining a bearing surface where the intake camshaft was. This left one camshaft to actuate the exhaust valve and also time the ignition.
I made a bracket from a piece of strap iron to hold the camshaft in place, and I used a camshaft follower from the same Briggs engine. The guide for the follower is a 1/2-inch bolt with a 1/4-inch hole drilled through the center. The bolt is long enough so the follower goes in half way from the bottom, and the push rod goes in half way from the top. The rocker arm is a 1/4-inch by 1-inch flat bar, and the pushrod and pivot are threaded clevises giving the necessary provision for valve lash adjustment.
A spacer between the cylinder head and the block makes up the combustion chamber. I obtained a 1-inch thick piece of steel, cut out a 4-inch circle (the diameter of the barrel) then cut a 1-1/2-inch hole in the center. I drilled the necessary holes to mount it to the barrel, plus I drilled and tapped a 14-millimeter hole for the spark plug (an Autolite 64). This arrangement gives about 60 psi compression. The cylinder head gave me the most problems. The intake valve, which is atmospheric, worked okay, but I had to put a 1/2-inch pipe and cap over the intake stem so fuel could be sucked from the port in the block. The original exhaust valve was set up to unseat and discharge air when the piston came up on compression. I reversed the valve for mechanical operation, and there was enough stock in the head to machine a seat in the proper place. I also relieved the compression spacer to give the valves proper clearance. Both the intake and the exhaust valves are from a small Briggs & Stratton.
The engine uses a buzz coil ignition. A plunger with a return spring (the plunger is grounded to the engine) rides on the cam lobe, and I mounted a set of automotive ignition points so the plunger strikes the metal arm on the points and not the insulated block. This completes the ground circuit to the buzz coil. The ground wire from the buzz coil is attached to the terminal on the ignition points, and timing is adjusted by moving the points assembly, which is mounted on a fiber plate.
The fuel tank is a brake fluid can, the carburetor is from a Briggs and the air cleaner is a chain saw oilcan with a foam filter. Crank pressure was originally vented around open main bearings, so I made seals for the crankshaft from felt and cupped washers and installed a 1/2-inch pipe in the block just below the cylinder to act as a breather.
It was a lot of fun making this engine, and it runs well. I'm planning on making a governor for it next winter, but in the meantime, if anyone is interested in more information, just drop me a line, I will reply.
Contact engine enthusiast John R. Heath at: 494 Township Road 232, Sullivan, OH 44880.