The fine print: Your logbook has many of the answers you need

QUESTION: I own a 1968 Cherokee 140 with a 160 hp O-320-D2A engine that was installed prior to my purchase of the aircraft. According to the logbook endorsement and the tach installed in the aircraft, the redline on the engine was reduced when the new engine was hung. The new redline is 2,550 instead of 2,700.

This raises two questions: 1. Why did the redline need to be decreased with the upgraded horsepower engine? 2. Two years ago I replaced all four cylinders. I found a shop that had four serviceable cylinders that he rebuilt using my old pistons and intake valves while replacing the exhaust valves and rings. The other miscellaneous parts I am unsure about. When I got the overhauled cylinders from the engine shop, the owner told me he didn’t know why the redline was reduced in the beginning, but the new cylinders should take care of bringing it back to 2,700. Is this true?

I have owned the airplane for about eight years and the reason this is coming up now is because I need to purchase a new tach and I need to know where to have the limitations marked. On the tach issue, do you have a preference between the standard mechanical tach and the digital units?

Rick Rhine
Tempe, Ariz.

ANSWERS
Rick, I’ll take a stab at this one, but I’m going to approach it from a common sense perspective, which will get you started on the journey toward an “”official”” answer.

I have a feeling that the engine originally installed in your 1968 Cherokee was a Lycoming O-320-E2A, which was rated at 140 hp@2,450 rpm in the PA-28 140. However, this same engine was also rated at 150 hp@2,700 rpm and was installed in the PA-28 150. Both of these engines have the same compression ratio of 7.00:1 and the hp is determined by the rpm of the engine.

You stated that the present engine is a Lycoming O-320-D2A. This engine is rated at 160 hp@2,700 rpm and has a compression ratio of 8.50:1. I think you’ll find that the Cherokee 140/150 series were not certified for a 160 hp engine and therefore the engine currently installed in your aircraft could have been installed either under an FAA STC or a one-time field approval on an FAA Form 337. Since the tach was redlined at 2,550 rpm, my guess is that this would be about 148 hp and would stay within the original hp range on the Type Certificate and not require any further engineering data for a field approval. I’d suggest you do a close review of your logbook to learn how and by whom the engine conversion was accomplished.

In response to your second question, I can only ask a question in order to answer your question. Why did the shop owner think you could again use the 2,700 rpm rating if he used your old pistons, which I assume were 8.50:1 compression ratio? If you know the part number of the pistons he installed, that will make it easy to learn what compression ratio they are. Just as an aside, the part number for the 7.00:1 compression ratio piston is probably p/n 75413 and the high compression p/n 75089. If the high compression pistons were installed with the serviceable cylinders, then you should continue using the 2,550 rpm, which makes his comment untrue.

Finally, with regard to the tachometer question, I can’t see any reason why the continued use of a mechanical tach wouldn’t be OK. It is important to know, though, that they do require calibration. I’d even consider doing that to a new one right out of the box before you install it.

Rick, this was a great question and I’m certain there are many similar situations out there today. The main point that can be learned from this situation is the importance of reviewing the logbook to learn as many details about your aircraft as possible. This information becomes extremely valuable when you encounter an experience such as this. I guess you could say it falls into the category of “”read the fine print.””

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

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