The Elbow Engine
This engine has been the most challenging build I have ever attempted. There is no room for error. Everything HAS to be perfectly square for it to turn, let alone run. I ran into a lot of problems along the way, but found a solution for each one of them.
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The picture at the top of this page shows the engine as it was first built. All the parts were made in three days. Put them together and the engine would not turn at all.
First step of the tuning up involved getting the upright support and the base square. To accomplish that I clamped the base directly to the table of the mill and lightly dusted the top face of the lower cylinder support to clean-up. Then in the same set up I took a long end mill and dusted across the the area of the upright cylinder support.
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The cylinder are identical. All the holes have to be straight and square.
For the pivot pins I used standard 10-32 socket head screws, and made thin wall bushings to fit the cylinders. The bushings are .005" longer then the cylinders to allow for free rotation.
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The pistons are where it really gets critical. They all must be identical. If they are not they will bind and the engine will not turn freely. The plans call for the pistons to be made form 1/4" drill rod. I made mine from plain 1018 cold roll steel.
To fit them I coated them with my favorite lapping compound, (Crest Toothpaste) and turned the engine by hand until that moved freely.
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After many evenings of tweaking, adjusting and polishing it finally came together.
Hooked the air to it and it
binds. 
Out of shear frustration
I grabbed a can of wheel bearing grease and smeared in on all the pistons.
It turned over very freely. Hooked the air to it again and it runs!
To be honest I'm glad this one is done. It has been the most frustrating model I have ever built.