Can you identify this engine?

QUESTION: A friend has an O-320 in his hangar. I have only the following data, and am wondering how or where I can use these data to determine exactly which engine this is. Data plate says: O-320, eng #2689-27. The case is cast with the number L-17001-27A. I tried using the Lycoming website, hoping for a serial number lookup table, but so far no luck.

Thanks in advance for any help!

San Diego

ANSWER: Thanks for your interesting question. I say interesting because of the two serial numbers you provided.

I can tell you that both of these numbers are potentially for the O-320 series, but we’ve got to go a little further to determine what you’ve really got here.

Since you didn’t mention whether there was a suffix following the 320, I’ll assume there was not, which tells me this is a very old engine. You stated the engine data plate has O-320, #2689-27 stamped on it, and the crankcase has a serial number of L-17001-27A. I assume the L-17001-27A was stamped on the top rear of the crankcase, on a flat machined boss on one half of the crankcase just forward of the crankcase/accessory housing parting surfaces. If this is the case, then it tells us that the engine is what is commonly known as a “”wide deck”” engine.

You can confirm this simply by looking at the cylinders. If they are retained by hex nuts, then we’ve got a “”wide deck”” configuration and I’d suspect that L-17001-27A is the correct engine serial number. However, if the cylinders are retained by barrel nuts that require an internal spline wrench to install or remove them, then we’ve got a “”narrow deck”” crankcase and the 2689-27 serial number may be correct.

Just off the top of my head, and please don’t hold me to this, I’d guess Lycoming began producing the wide deck configuration of the 320 series in the early 1960s and designated this change by adding the letter “”A”” at the end of the engine serial number suffix.

I’d be surprised if you find this engine is actually a narrow deck configuration. My suspicion is that at some point during this engine’s life, maybe during a field overhaul, the oil sump was replaced and the old engine data plate, which is affixed to the sump, was never removed. The oil sump on some O-320 series is the same part number for both the narrow and wide deck engine configurations and that’s why I believe you ended up with two serial numbers on this engine.

Since you didn’t mention anything about an engine logbook, I’d be curious to know what it gives us for a serial number and any other historical information about the engine.

I’m sorry I can’t be of more help in this case, but I hope the information I’ve provided will get you started in the right direction to determine what this engine really is — or, should I say, was!

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


  1. Matt Miller says

    I am looking at an O-320 engine that was overhauled a few years ago. The logbook states that it is “160” HP. The serial number in the log books is L-4898-27. The seller claims that it left the factory as a160 HP. There is no accompanying STC for a conversion. Did this engine start life as a 160 HP O-320, or was there some sort of conversion ( dome pistons?) without STC? Would you use this engine on a certified aircraft without the STC?

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