While the following applies to an engine long out of service, I think some principles may be of interest. Remember when reading, that these engines were always operated LOP, and had a 1,500 hour overhaul life if operated and maintained correctly.
I. The Cause and Correction of Exhaust Valve Seat Failures
A. This discussion will include basic principles of operation of valve and seat, types of failures, procedures at overhaul, changes in engine operation techniques necessary to obtain consistent cylinder reliability. This discussion will not include in detail abnormal causes of failure such as overspeeding or overboosting the engine beyond design limits and cracked or otherwise faulty parts. Temperature will be the key word throughout the discussion. The complete story of operation is presented in order that all personnel concerned can better understand all the overlapping phases of design, operation, type of failures, manufacturing, overhaul and operational technique necessary to insure reasonable cylinder life on any Reciprocating Aircraft engine.
B. The exhaust flame directly affects the valve, seat and cylinder; hence it is necessary to understand the effect of engine power, speed, and fuel air mixture on exhaust flame temperature before we can understand the complete operation of the valve seat. Starting at idle speed, the exhaust temperature rises rapidly with increased speeds until maximum cruise power is reached at which time the power enrichment valve starts to open which sharply reduces the actual flame temperature. Cylinder head temperature continues to increase at engine speeds above maximum cruise due to the increasing number of power impulses. Above maximum cruise the cylinder head has reached its basic maximum capacity to cool; and therefore, the amount of fuel is substantially increased as a combustion coolant to compensate for the cylinder cooling deficiency. Since use of added fuel over and beyond that normally used would reduce power excessively and since cylinders will operate satisfactorily under power at temperatures above those obtained at maximum cruise, a compromise between cylinder head temperature and power is used to give the best overall results.
C. The exhaust valve head is exposed to combustion temperatures during the power stroke and the entire head of the valve and part of the valve stem is exposed to exhaust temperature during the entire exhaust stroke; hence the valve is exposed either partially or completely to high temperature (1,800-2,600°F) for approximately 440° of crankshaft travel. Therefore, the valve is exposed either partially or completely to extreme heat approximately 37 seconds out of each minute. The valve dissipates this heat through the valve guide and through the seat where the two faces contact.
D. The ni-chrome steel exhaust valve seat used in Pratt & Whitney R-4360 engines is installed in the seat recess with a 0.0058-0.0073" pinch fit. This is accomplished by cutting the recess to a specific size, gaging and obtaining premeasured seat of a known size which will provide the required fit. The cylinder is then heated to around 500°F and chilling the valve seat to less than 10°F by use of dry ice. When the seat and seat recess are stabilized at room temperature, the seat is then between 0.0058-0.0073" tight. Ni-chrome R-4360 exhaust valve seat has a measured expansion rate of 0.0000045 per inch unit length for each degree Fahrenheit change. Aluminum has an average expansion rate of 0.000001234 per inch unit length for each degree Fahrenheit change. For example a 2.800" diameter valve seat and a 2.793" seat recess were elevated to 600°F; the 0.007" pinch fit is reduced to 0.0046" loose.
VALVE SEAT 0.0000045 X 2.800 X (600°-70°) = 0.00667
SEAT RECESS 0.000001234 X 2.793 X (600°-70°) = 0.0182
SEAT RECESS 2.793 + 0.0182 = 2.8112
VALVE SEAT 2.800 + 0.0066 = 2.8066
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VALVE SEAT LOOSE = 0.0046
When the cylinder is cold (70°F) and the engine is started the seat temperature rises first and to a considerable higher temperature than the seat recess. Hence, after ½ minute of operation, the seat temperature is about 600°F and the seat recess temperature is around 200°F, the pinch fit will have changed from 0.007" to 0.0099" pinch fit. Now the compressibility of aluminum is such that a maximum pinch fit of 0.007" is about all that will be retained, as the seat recess would be compressed approximately 0.0029" and would reduce the pinch fit under these temperatures of (600°F and 200°F) back to 0.007 inch. This, in turn, would reduce the pinch fit with a cold cylinder from the original 0.007" down to about 0.0049" pinch.
E. With the fundamentals now covered, it should be readily evident that the exhaust valve seat is retained in a hot engine by combustion heat saturated up by the valve seat itself and the valve. Take this heat away suddenly and you have a loose valve seat. The first several times a cylinder is exposed to this condition, it will probably do little damage because of a retaining lip designed into the seat specifically for this purpose. This locking device does not keep the seat tight; its only function is to retain a loose seat. Mechanical operation of the valve mechanism with a loose seat will eventually pound the seat into the cylinder head, upsetting valve clearance which in turn aggravates the pounding by the valve. This can then only result in the seat coming completely loose from the cylinder or total cylinder failure.
F. At this point it might be said “JUST FOLLOW THE NORMAL OPERATING INSTRUCTIONS IN THE AIRCRAFT OPERATING MANUAL” and you will have reasonable cylinder reliability. But some time must be spent discussing the specific condition known to induce cylinder problems:
1. Fast engine starts – The colder the weather the more detrimental – Keep engine RPM down until engine is warm.
2. Operation of engine less cowling – This should be avoided, except for short period oil leak check – RPM less than 1,200, CHT 160°C MAX, 2 minutes maximum.
3. In flight shutdowns – Not recommended, except in actual emergency.
4. In flight power reduction – Should be gradual – Never pull power off to idle when CHT exceeds 200°C, except in emergency.
5. Deterioration of the ignition system in service can be a major problem to cylinder life. Cylinders misfiring causes rapid cooling of the exhaust valve seat. This can result in a loose seat condition and will eventually cause seat failure. Malfunctioning engines should only be operated under emergency conditions.
6. Propeller thrust reversing – Should only be used when necessary – Causes high cylinder head temperatures and is always followed by low power (idle operation).
7. Engine shutdown – Must be below 180°C CHT – if this is impossible – Rotate engine several seconds with starter if possible.
8. Cooling Hood baffles – Must be latched and in good condition – always replace or repair defective hoods