Ask Paul: Why do my serial numbers differ?

A: I have a narrow deck configuration in my 1964 PA 30-270. The first entry in the aircraft logbook, made on Jan. 3, 1964, shows radio installation and LE serial #S or L 606-55 and the RE L 640-55. However the engine log books show LE L 938-55 and RE L 909-55 and the first oil change at 55 hours. Both are IO-320-BIA. I have not yet confirmed the numbers on the installed units — too many screws — and I don’t know the exact place of the info plate.Also, the N number was changed from N7373Y to N4009Y in 1966, but I have no idea why. Please help! I am baffled.

KARL ROESCH, Arlene, Montana

A: Karl, the difference in your logbooks is interesting and confusing, to say the least. The only thing I can think of is that the original engines were replaced at some point with the higher serial number engines. I am surprised, however, that this isn’t reflected in the aircraft log. I suspect that when the engines were replaced, the engine logbooks stayed with those engines and the replacement engines were provided with new logbooks. This is standard procedure and shouldn’t be anything to be concerned about.

To satisfy yourself about the engine serial numbers, I’d suggest you wait until your next regularly scheduled maintenance event when the cowling is removed and check the engine serial numbers at that time. The engine data plate is affixed to the side of the oil sump or you may also confirm the engine serial number from the top of the crankcase. Each engine serial number should be metal stamped on the machining boss on top of the engine just forward of the crankcase/accessory housing parting surface.

I wish I could explain the reason for the “N” number change, but there should be a history of the change somewhere in the records. If nothing else, I’d suggest asking your local FAA representative if he or she might have any suggestions how you could check the history of the “N” numbers. If you are an AOPA member, association officials also may give you some guidance.

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

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