Mystery T-Head

Prototype or Production ? Ten Years After its Discovery This Engine's History Remains Unknown


| October/November 2002



Chain drive

Close up of chain drive, igniter actuating shaft and igniter trip mechanisms

It seems doubtful that anyone living can shed any light on the origins of Gene Stewart's mystery T-head engine. Whether an experimental prototype or an engine intended for production, its history has been lost with the passing of time. An innovative and finely crafted engine, its T-head layout points to engine design current in the early 1900s.

Engine Layout

In a T-head, the intake and exhaust valves are located on either side of the cylinder. In the early 1900s this was an innovative design, and it also led to a cross-flow cylinder; one in which intake occurs on one side of the combustion cylinder and exhaust on the other. Cross-flow heads are the norm now, but in the early 1900s a T-head layout was fairly advanced. A downside of the design, however, was heat dissipation; a problem Gene says this engines suffers due to the exhaust manifold being cast in unit with the main engine block. In fact, this engine gets hot enough that Gene has purposely fitted it with a small fuel tank so he won't run it for long stretches at a time.

Gene thinks this engine was designed for marine applications, and he points to its 'post' construction (a construction associated with marine engines of a century ago) to support this. In a post engine the lower half of the engine is open and a separate casting supports the crankshaft. This in turn is fixed to the upper case by means of posts. This simplified machining and construction, as an engine could be built up from smaller castings that were then individually machined instead of the labor of machining one large casting.

Ignition is by igniter, with one for each cylinder, and that presented a challenge of its own as Gene brought the T-head back to life. There was essentially nothing left of the igniters, but working with what little he had, and studying engines of similar design, Gene fabricated the igniters sparking the engine. Close examination shows that each trip on the igniter actuating shaft has a ramp acting to lift, or swing, each igniter. The ramp drops off abruptly to complete the process. Looking very much like an overhead camshaft, the igniter actuating shaft is chain-driven off the crankshaft and features what's essentially a sprag- clutch on the crank to release any tension on the igniter drive should the engine backfire or otherwise try to run backwards.

With atmospheric intakes valves the engine runs a single camshaft for the exhaust, somewhat unusual in that most T-head engines featured dual camshafts with positive actuation of both the intake and exhaust valves. That would make sense in an automotive application where engine load and speed can change rapidly. But in a stationary or marine application load and speed tend to be fairly constant, allowing a simpler (and cheaper) atmospheric intake design.

The engine features roller lifters, a design consider high performance even today. And those roller lifters are interesting, appearing to almost float in the air; closer inspections shows each lifter has a pivoting locating rod running to the other side of the engine to hold the lifter in place.