S.I. 1479 -- Round 2
by Bill Marvel and Bill Scott

Readers of our major work on this web site, The Rest of the Story, should be well aware of our discovery of the large diameter oil return tubes and fittings on O-320H/O-360E engines. As we pointed out in that article, the reason for the larger tubes is that these engines are equipped with the barrel type of hydraulic lifter, which by its very design provides far more oil to the rocker box than does the mushroom type lifter that exists in most Lycoming engines. Accordingly, the larger lines are needed to return this greater oil volume to the engine sump. If they were not provided, oil would accumulate in the rocker boxes.

When Lycoming designed and built these engines, they clearly were aware of the greater oil volume flowing to the rocker boxes and of the corresponding necessity of providing adequate oil return capability from the rocker boxes to the sump. Lycoming's Service Instruction 1479, first published in January 1996 and mentioned in several of our articles on this site, provides for installing oil cooled exhaust valve guides in the Mooney TLS engine. These oil cooled guides are needed because of excessive exhaust guide wear and resulting compression test failures in the TLS despite its being operated at normal cylinder head temperature levels. In other words, excessive exhaust valve and exhaust valve guide temperatures occur in this engine despite its being operated within all designated limits. The Mooney TLS engine has the generic mushroom type of hydraulic lifters installed, and these, by their very design, cannot provide a significant volume of oil to the rocker boxes to aid in valve and guide cooling. Accordingly, the TLS engine is equipped with the smaller sized oil return lines that exist on most Lycoming engines.

When we first learned of S.I. 1479, both of us wondered how Lycoming planned to handle the additional oil volume going to the rocker boxes, given that the TLS engine had the smaller diameter oil return tubes installed. After all, Lycoming had designed the O-320H and O- 360E engines long ago and realized even then that the addition of more oil to the rocker boxes would require more oil return capacity. S.I. 1479 merely recreated the same problem in the TLS engine that they had solved once before in the O-320H and O-360E. Didn't they see this problem developing when they issued S.I. 1479? Evidently not. Our question was answered by the publication of a letter dated August 15, 1996 from Lycoming's Manager of Product Support, Edward D. Coudon, addressed to "All Textron Lycoming Distributors." A copy of that letter found its way into our hands, and we wish to share the text of it, without comment, with you: