Some readers of this series of articles have reported to us an interpretation of our work that we did not intend to portray. The purpose of this article is to clarify what has led to this confusion.

The incorrect interpretation that exists is that we see oil flow to the rocker boxes as a mechanism for augmenting the existing air cooling of the cylinder head itself, thereby "helping" the cylinder head to cool the exhaust valve and guide. This is absolutely not our view and would be an unjustified one to adopt.

Heat input to the cylinder head from the combustion process is massive and only a very large volume of air flowing across the cylinder cooling fins will successfully cool the head. No amount of oil flowing to the rocker boxes will be able to augment this process, or at least no amount that is reasonably available from the engine's oil system. Any transfer of heat from the cylinder head by oil is only a very small percentage of the amount transferred by the flow of cooling air across the engine and is for all practical purposes insignificant.

The correct interpretation, which we had intended to convey, is that oil flow to the rocker boxes creates a second heat transfer path from the exhaust valve and guide and thus augments the cooling of these components. With sufficient oil flow across the hot valve stem and upper portion of the guide, both of which are exposed in the rocker box, heat is readily transferred from these components to the oil at the same time that it is passing through the cylinder head to be transferred to the air flowing through the cowling. It is this additional cooling provided by the oil flow that has been used successfully by Lycoming in the Mooney TLS per S.I. 1479. It is also this additional cooling by oil that accounts for why we have found far less exhaust valve and guide distress on the pilot side than on the co-pilot side of the parallel valve engines.

As you will read (or have read) in other articles in this series, the pilot side cylinders receive more oil flow than do the co-pilot side due to the design of the oil system in the Lycoming engine. As an explanation of the above concept, consider this simple analogy. If you heated the point of a safety pin until it was red hot, it would be a few moments before enough heat transferred out of the pin for it to be safe to touch. On the other hand, if you merely dunked the red hot pin into a single drop of water, the pin would cool almost immediately. It would do this because it has a low thermal mass and water rapidly transfers heat from the pin into itself. In like manner, the thermal mass of the exhaust valve is very small compared to that of the cylinder head. While air in tremendous volume is required to cool the cylinder head, and along with it the valve, only a relatively small amount of oil making direct contact with the very hot valve stem and valve guide will effectively cool those components.

The above clarification is important for two reasons: 1. Because of its design, the parallel valve cylinder is unable to transfer sufficient exhaust valve and guide heat to the atmosphere, despite normal cylinder head temperature, in some operating conditions. 2. Because of their design, the Lycoming mushroom style hydraulic lifters provide only a minuscule flow of oil to the rocker boxes of the cylinders. Significantly increasing this oil flow creates the second heat transfer path mentioned above. Our data shows that the addition of this path is successful in providing augmented exhaust valve and guide cooling that extends the longevity of those components which receive it.