OE Exhaust Valves, Sodium Options

[914werke]

I know Jake suggests tossing these, but unfortunantly Im rebuilding a set of heads. Id like to keep them stock, but Im more concerned about longevity.
Is there anything that can be done to improve thier function?

Cryo? (IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif)

[Bleyseng]

Good food for thought on how fixing one problem leads to other unforseen problems or Murphys's Law.


"The Problem of Sodium Filled Valves

As revealed above, sodium filled valves in the O-320 engine did not eliminate valve distress problems. To the contrary, our question is whether or not they either cause or increase these problems. Sodium filled valves are an extremely expensive component that we believe are greatly to blame for valve/guide distress incidents. They do not make heat magically disappear, as some would have you believe. All these valves do, at great monetary expense, is transfer heat from the valve head to the valve stem, or more generally, from the combustion chamber to the cylinder head via the valve guide. They merely move heat from point "a" to point "b." It still has to be eliminated from the cylinder head by either air cooling or oil cooling or both. The problem is that valve guides are wearing out prematurely and are doing so in spite of operators keeping CHT levels in the proper range. Excess heat is the primary cause of premature guide wear (in a properly assembled cylinder), as most engine shops will tell you. The problem with sodium filled valves is that they serve to import even more heat into the guide by transferring it up from the valve head. Lycoming's long history of valve/guide failure incidents in the parallel valve cylinders has shown that there is simply no way that the guide can shed all of its higher heat load via the cooling fins alone, and Lycoming's design provides for very little oil to aid in that process. The irony here is that Continental uses solid stem valves that dissipate most of their heat into the valve seat. Relatively little comes up the stem and into the valve guide and yet Continental has an abundance of oil in this area to aid in heat transfer. If sodium filled valves are needed at all, one wonders why Continental doesn't use them, since their barrel style hydraulic lifters provide substantial oil for additional cooling of the guide and valve. Lycoming's mushroom style lifters do not. We think that without any change in oil flow to the rocker boxes, Lycoming valve and guide longevity might well benefit from simply going to solid stem valves. Unfortunately, these are not available."

"One of the most vexing problems we had to consider is why some valve/guide failures result from excessive guide wear (the guide inside diameter increases) while others result from valve sticking, which is caused by a buildup of "cooked oil" (coking) on the valve stem. How could insufficient oil volume to the rocker boxes account for both of these seemingly opposite effects? The fact is that we cannot say for certain why one specific failure mode occurs versus another in any given engine. Both, however, are related to excessive amounts of heat in the valve/guide combination. Incidents of valve sticking appear to be greatly reduced now as compared to in the past. All but one of the incidents reported to us involved failures with either disintegrating valves in flight or with the valve's failure to seal during a compression check, caused by excessive guide wear. But we were able to find out how an increased flow of oil to the rocker boxes accounts for lower incidents of both excess wear and sticking.

The answer came in part from our testing and in part from a 50-year old Society of Automotive Engineers report. Our data was showing consistently that the odd numbered cylinders received considerably less oil to the rocker boxes than did those on the other side of the engine and that the odd side had much higher incidents of excessive valve guide wear. Lycoming has repeatedly stated that this distress is caused by excessive valve/guide temperature, although CHT levels in the affected aircraft were normal. We were simply finding that the additional oil to the rocker boxes evidently provides extra valve and guide cooling which is greater than what the cooling fins alone provide. And, not surprisingly, we found that cylinders with this additional oil (the even numbered ones) had generally longer valve and guide life than did those with a lesser amount of oil. But how did this relate to valve sticking problems?

The answer came from an extensive study done by the Society of Automotive Engineers nearly 50 years ago. As Robert V. Kerley of the Ethyl Corporation explained in a paper in SAE Quarterly Transactions, Vol. 1, No. 2 (April 1947): "Practice has indicated that sodium-cooled valves will tend to increase valve-sticking troubles unless lubrication is increased, preferably by an oil jet to the stem, or unless the stem is run dry. "Light or moderate lubrication normally causes coke formation resulting in sticking." (Our emphasis)."




(IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)