Auto Enthusiast Carves Functional Two-Stroke Engine from Solid Metal

Camden Bowen has spent years chasing the perfect two-stroke engine built entirely from scratch. His earlier versions came from a 3D printer and then from parts picked up at the hardware store. Each one taught him something new about what works and what falls short under real fire. For his latest project he set a higher bar and machined the whole thing from billet aluminum on a basic mill and lathe.

He started with a clean design for a single-cylinder two-stroke engine. The crankcase was divided into two bolted sections, so everything fit together without the frustration of squeezing parts into a tight spot – just a single bolt and it all came together without too much trouble. That decision alone likely saved hours in the final assembly. Every key component began life as a length of aluminum bar or plate. Gary Bowen, also known as Bowen, carefully turned and machined them down with great patience, shaping the cylinder, crankcase, and mounting points until they all fit together like precision puzzle pieces.

However, the rotating assembly required a great deal of attention. He manufactured a crank pin and crank webbing from steel before riveting plates onto a solid shaft to form the crankshaft. The connecting rod was made from a half-inch-thick steel bar. He bored the holes with a 95mm offset and made the rod slightly longer than typical to compensate for side loads on the piston walls. Bowen didn’t bother with a complex rotary fixture on the mill table; instead, he held the rod in a vise with grippers and chopped the ends by hand. It was clean enough to make you forget that he had to improvise like that.

He then turned the flywheel from a 4 inch steel disk. One end of the crankshaft had a five-degree taper that corresponded to a matching bore in the flywheel. A keyway dealt with the ignition cam, and pinned joints kept everything straight. Manual threading on the ends was a nightmare; the dies would seize unevenly, leaving a few threads badly damaged. Bowen slammed the parts together, put a couple of tack welds where real welds would have made things too difficult to access later on, and called it good.

Camden made the piston by turning an aluminum blank to size and carving two ring grooves at the top. The wrist pin holes were reamed to provide a 12mm tight press fit. The cylinder was a bit of a challenge because he first tried to make it out of a square block cut from a cast block in a lathe but snapped an endmill, destroying the piece. So he converted to round stock, drilled the center bit straight, and pressed in a cast iron sleeve. The sleeve provided a firmer surface for the steel rings to ride on without the need for further lubrication.

After all of the machining was completed, assembly felt pretty straightforward. Bowen simply dropped the crankshaft into the divided crankcase, placed the connecting rod onto the pin, and lowered the piston into the sleeved cylinder. A few bolts held the parts together, and then he installed a 12 volt coil pack and built up a rudimentary pointed setup on a rotatable bracket so that the timing could be adjusted by hand; no electronics or sensors were required, just a basic spark at the proper time.

Pressure testing came first, as he tightened the cylinder and began cranking the piston all the way up to roughly 150 pounds per square inch on the gauge, which indicated that the rings and seals were working properly. After that, he mixed up a batch of gasoline with a little extra oil to be safe and put it into the tank before hand-spinning the flywheel to prime the engine’s crankcase.

On the first pull, the engine fired right away, starting smoothly and rapidly settling into a strong loud idle, but it was cut short due to the carbon monoxide, so Bowen weighed anchor and moved the whole thing outside to give it some more time to run. Later inspection of the tape revealed that the flywheel wobbled slightly, most likely due to one of those hand-cut threads, but the engine continued to run without missing a beat. Everything else checked out fine.
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