I have been following the correspondence about the Lycoming exhaust valve breakage and wish to add the following information to the debate.

It is possible to calculate the rate of heat transfer from the cylinder head to the engine oil, if the following assumptions are made. An amount of 100ml of oil per cylinder is used for the minimum oil flow to the valve gear as suggested by Ian Findlay (AOPA May 1996). Lycoming data is used for the temperature of the oil reaching the cylinder head for cooling which is 82 deg. C (controlled by the oil temperature thermostat) and is assumed to reach the maximum and red line temperature of 118 deg. C as it flows over the head. It can then be calculated that this oil flow picks up head at the rate of 119 watts per cylinder. The method of calculation is available for readers who may be interested.

In a 180 horsepower four cylinder O-360 engine, the oil flow over the head and valves will collect only 476 watts of heat. At 75 percent power this engine puts 100,000 watts (100kW) into the propeller. It also passes approximately 117,000 watts into the cooling air and a similar amount out in the exhaust. This is the amount of heat absorbed by the oil as it passes over the whole cylinder head would make no significant contribution to the cooling of the exhaust valves as it is negligible when compared with the total heat output of the engine. Even the argument for a localized cooling effect is not sustainable as the 476 watts is minuscule when compared with the 117,000 watts dissipated into the cooling air. A similar situation holds true for other Lycoming engines.

It is interesting to note that the Lycoming 320/360/540-series of engines have been very successful when installed in Cessna or Piper airframes. In the case of the 320 engines have been very successful when installed in Cessna or Piper airframes [sic]. In the case of the 320 engines, some flying-schools operating on engine life extension 288 programs have engines running well beyond the normal 2000 hour TBOs without any problems with exhaust valves. Flying-school operations are not easy on engines and no top-overhaul is necessary on these programs.

If is necessary to look further at the possible causes of premature failure of exhaust valves in the Lycoming engines referred to by Ian Findlay. The exhaust valves essentially run dry in the guides and lubrication is not an issue. Some aircraft engines have no oil pressure feed to the valves, but rely for lubrications on splash from oil accumulated in the rocker covers.

When compared with the Cessna and Piper range, it is interesting to note how tightly cowled some other makes of aircraft appear. While these tightly cowled planes may maintain the Lycoming-designated air pressure differentials across the engine, this gives no information about the actual mass of cooling air flowing over critical parts of the engine, such as the cylinder head.

One problem with Lycoming engines has been exhaust valve sticking. To counter this problem some engine overhaul shops have reamed out the exhaust valve guides to give additional clearance to the valve stem. However, unless this is done carefully and the correct guide-to-stem clearances are maintained, then problems arise. It is essential that good contact is maintained between the valve stem and valve guide so that maximum heat transfer can take place to the cylinder head and ultimately to the cooling air. If the valve guides are over-enlarged, then rapid and premature wear will follow.

The information about the Mooney TLS (Lycoming Service Instruction 1479) and the associated oil cooling of the exhaust valve guides must be put into perspective and may not be relevant to the problems identified by Ian Findlay. Because the Mooney TLS cruises at altitudes where the air density is reduced, then the mass or weight of air flowing through the engine is significantly reduced, resulting in decreased cooling. This is of particular importance when combined with the long climb, at high power settings, to cruising altitude. Because the turbocharged engine is able to maintain its power output at altitude, this reduction in mass air is an important factor which must be taken into account when designing the cooling system. One solution is to rely on oil cooling in critical areas, such as the cylinder head. These problems are not usually encountered in normally aspirated engines operating below 10,000ft.

In relation to Lycoming Service Bulletin 388B, I believe that is was introduced to cover certain situations in engines installed in helicopters. These engines are more highly stressed, for example some of the turbocharged 360-series for helicopters have RPM rated as high as 3,200. These severe operating conditions result in significantly lower TBOs when compared with similar engines in fixed-wing aircraft. Again, it is unlikely that SB 388B is relevant to the problems described by Ian Findlay.

The issue of what constitutes the acceptable cylinder head temperature is worthy of further consideration. For example, the Lycoming Operator's manual for the 360-series engines quotes a maximum cylinder head temperature of either 475 deg. F or 500 deg. F, depending on the engine type. However, it qualifies this in all cases by stating, "For maximum service life of the engine maintain cylinder head temperature between 150 deg. F and 400 deg. F during continuous operation." There is a message here.

I suggest that this is not a general problem with exhaust valves in the Lycoming 320/360/540 range of engines. Careful reading of CASA's airworthiness Advisory Circulars - Summary of Defects does not support the existence of a particular problem with these exhaust valves. Finally, many operators of fleets of Cessnas or Pipers with these engines claim to have a satisfactory history. For example, one operator maintaining a fleet of about 25 Pipers could only recall one exhaust valve problem in 14 years, with his fleet often exceeding 3,000 hours per month. Another with a number of Cessna 172s claimed that they always reached the normal 2000 hour TBO without any problems with the exhaust valves.