Confused about replacement parts?

Q, I have a Lycoming O-320 150 hp engine with no suffix in my PA-22 Tri-Pacer. In one of your articles you list the O-320-A1A and the O-320-A3A as the numbers to refer to for replacement parts. Lycoming does not list an O-320-A1A and shows the O-320-A3A fitting a Piper PA-23. Lycoming lists the O-320-E3D as the engine for my PA-22. I’m thinking of either replacing my engine or overhauling it, but I’m not quite sure what to look for in either case.


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A, Thanks for your question. Let’s see if I can get this all figured out for you. Yes, as my earlier article stated, the model you have in your aircraft, O-320 with no suffix, would use the replacement parts from the O-320-A1A and the O-320-A3A. This can become quite confusing, but let’s just leave it at this to keep things simple at this point. The difference between the O-320-A1A and the A3A is simply that the A3A utilizes 7/16-inch prop flange bolts.

From your inquiry, I’d guess you’re looking towards a complete engine replacement. My only concern with overhauling your engine is “”if”” the crankcase is cracked and possibly non-repairable, it may present a real challenge to locate another “”narrow deck”” crankcase, which your engine was built with originally. My guess would be for the last 40 or so years Lycoming has been using “”wide deck”” crankcases on the 320 series engines, which are beefier and use corresponding “”wide deck”” cylinders. If you were to purchase a factory engine now, you would get nothing other than a “”wide deck”” configuration, which is a direct replacement for your engine other than possibly trimming some of the engine baffling to accommodate the slightly larger cylinders in the flange area.

I’d like to set the record straight right now about what Lycoming lists as the engine for the Piper PA-22. I can assure it is not the O-320-E3D! This appears to be an error on behalf of Lycoming. (You don’t think things may have slipped a little since I retired from Lycoming do you?) The O-320-E3D is used in the 140 Piper Cherokee, but not in the PA-22. I did a quick check of the FAA Type Certificate Data Sheet for the Piper PA-22 and found the following engines are certified for use in that particular model: O-320-A1A, O-320-A1B, O-320-A2A or O-320-A2B in the 150 hp category; and the O-320-B2A or O-320-B2B in the 160 hp category. I’d recommend you check the FAA Aircraft Specification No. 1A6 to be certain exactly which engine model is approved for your specific aircraft serial number.

What’s the difference between these models? The O-320-A1B is the same as the A1A except it uses a different oil sump and carburetor. The O-320-A2A is the same as the O-320-A1A except it’s configured for a fixed pitch prop. The O-320-A2B is the same as the A2A except it uses a different oil sump and carburetor. Again, according to the FAA Type Certificate, any of these engines are approved for use on your aircraft, not to mention the 160 hp engines noted previously. If you’re thinking of stepping up to the 160 hp engine, I’d again be certain to check the FAA Type Certificate Data Sheet.

I realize this can sound confusing, but the main thing to keep in mind is the FAA Type Certificate provides the specific models approved. By following that you can’t get into any trouble.

If all of this causes you or your maintenance facility confusion, please don’t hesitate to contact Lycoming directly and request their assistance. Prior to placing your call be certain to have your current engine model and serial number.

I’m certain you can get this all worked out and continue to fly your beautiful PA-22 for many years to come.

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


  1. scott griesman says

    confusion X3 No one seems to know what the ma4 spa 10-3678-11 the 10-3678-12 and the 10-3678-32 differences are. They are all carburetors that were ,I believe correct for the pa-23-160.Marvel doesn’t list the -11 carburetor and I could not find out from them what the story was on the disappearance . Can you shed some light? I am trying to put an a3a on a citabria with a ma4spa 10-3678-11 and am looking forward to a cock fight with the faa thanks

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