Abnormal Valve Guide Wear in Lycoming Parallel-Valve Engines
are a set of articles by Bill Marvel, past president of the American Yankee
Association, and Bill Scott, an independent overhauler.
A general overview of the exhaust valve wear issue
Service Instruction 1479 and You S.I. 1479 -- Round 2
An open letter to TBO Advisor subscribers
Hydraulic lifters 101 and the rest of the [TBO] story... Clearing up some confusion
O-320H Cam spalling - suicide?
Lycoming exhaust valve breakage -- Intro
For over 30 years, Lycoming's parallel valve cylinders have experienced exhaust valve and guide problems that resulted in either valve sticking and/or advanced valve guide wear that led to compression loss or valve disintegration. Numerous Lycoming publications have addressed the problem, but no single solution (or cause) has ever been found. The burden has been shifted to aircraft owners via the infamous "wobble test" (Service Bulletin 388B) that is considered mandatory every 400 hours on every engine the company makes. This may soon change.
California based Grumman Tiger owner Bill Marvel and engine overhaul specialists Bill and Carol Scott of Precision Engine in Owensboro, Kentucky have made a discovery that is sure to turn heads.
During the past four and a half years, they have been investigating the Lycoming valve guide wear problem and have made several very interesting observations. They are convinced that the problem is closely related to the volume of oil that flows to the rocker box of each cylinder and thus to the amount of valve and guide heat that oil can remove. Specifically, they have found an inverse correlation between exhaust valve guide wear and the volume of oil delivered to the rocker boxes (the more the oil, the less the wear). Additionally, they have discovered that the number 1 and 3 cylinders (those on the copilot side) invariably receive less oil than do the even numbered cylinders, and have correspondingly higher incidents of valve guide distress. These findings led them to the most interesting investigation of all -- Lycoming's oil system design in general and their mushroom style hydraulic lifter design in particular.
The Scotts and Marvel researched the history of hydraulic lifters and learned that the Lycoming design is a variation of an automotive component first used in 1929. This automotive unit was initially developed for an in-line engine, which did not use either pushrods or rocker arms, components that are employed in present day aircraft powerplants. Lubrication was supplied to the valves in this engine by splash oil thrown off the crankshaft. When this automotive lifter design was later modified by Lycoming for use in their overhead valve engines, they did not provide any unrestricted oil flow path through or around the lifter to the rocker boxes, even though splash oil to the valves from the crankshaft was no longer available in their engine configuration. As a result, the only existing oil flow paths are past very tight clearances that allow only a tiny volume of oil to be provided for lubrication and cooling of the valves. This is the fundamental cause of the low oil flows to the rocker boxes that the Scotts and Marvel have measured. Interestingly, their efforts have revealed that Continental, in using this same type of automotive lifter for their aircraft engine, did incorporate an unrestricted flow path through the lifter that provides considerable oil to the rocker boxes.
Very significantly, Lycoming's own Service Instruction 1479, issued in January 1996, appears to substantially corroborate, however inadvertently, Marvel and the Scotts' findings. This S.I. modifies the cylinder heads of Mooney TLS engines to incorporate oil cooled exhaust guides to address a problem of prematurely worn exhaust valves and guides. Marvel and the Scotts believe this modification, or a similar one, should be incorporated on most Lycoming parallel valve engines.