THE CURTIS CRUDO

By Staff
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Close up of the Curtis Crudo's valve train shows pivot and threaded clevises for valve adjustment.
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The completed Curtis Crudo engine project, looking very much like a factory offering.
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The completed Curtis Crudo engine project, looking very much like a factory offering.

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.

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