Q: I have a Cessna 172N with the Lycoming O-320-H2AD 76 series engine, serial number L-3406-76. I’m trying to sell it, and I’m getting all kinds of questions regarding the “A” suffix, the “T” mod, and others that are way too far out there to even mention.
As an A&P/IA, I’m fairly familiar with the issues the “H” series had in the beginning of the series. I thought I had a pretty good grasp on the ins and outs of the engine’s history, but now I’m just as confused as the enquirers.
Many years ago while working at an FBO, I was told that all of the original engines that were installed in these particular airplanes were removed and sent back to the factory (or exchanged) for upgraded/modified engines. My engine was replaced on Dec. 22, 1977, 159 hours after the aircraft was built and received its Certificate of Airworthiness on June 15, 1977.
My question is this: How can I tell without looking at the dataplate if this engine has had the larger lifters, camshaft, etc., installed? S.I. 1406E doesn’t really tell me what I need to know. Is there another publication that explains this situation more clearly?
Chuck Luettgerodt, owner, Inland Aviation
A: Chuck, thank you for your question regarding the true configuration of your O-320-H2AD, Serial Number L-3406-76.
There has always been some confusion as to what configuration specific O-320-H2AD engines were, as time passed, since there were issues with some of early H2AD series.
In reviewing the text of the logbook you included with your query, I can tell you that your specific engine is the original configuration, which incorporated the smaller diameter tappets and the original width camshaft lobes.
As you mentioned, some of the early engines were replaced under warranty, which may have been the case with your original engine in December 1977. However, the big change came with the introduction of the large diameter tappets and the camshaft with wider lobes.
Since I was deeply involved with the “H” series engines from the beginning of their introduction in the Cessna 172, I researched some of my notes and found the following information: The introduction of the improved configuration utilizing the large diameter tappets began with the factory new engine serial number L-7976-76, which I believe was sometime in early 1980. The factory remanufactured “H” series engines incorporated that change with engines being remanufactured as of April 27, 1980. If my thinking is correct, these serial numbers and dates would indicate the time the “T” mod identification was included in the engine serial number suffix, i.e. L-XXXX-76T.
If you are absolutely certain the logbook you provided a copy of is the correct one for the specific engine on your aircraft, I’d say your engine does not incorporate the latest “H” series engine product improvements.
Please understand that there are still many original configuration “H” series engines still in service, which will attest to the fact that the series may not have deserved the somewhat bad reputation it was given early on.