Ask Paul: Narrow deck cylinders, the Twin Comanche and a Lycoming milestone

Geez, it’s really beginning to look like I’m losing it when it comes to misstating things in my columns. The most recent concerned the “Wide Deck” vs “Narrow Deck” cylinder column in my last column.

I thought I’d explained the difference between the two and used an example of the Piper Twin Comanche PA-30 as being the first to use nothing but the Wide Deck configuration. Boy was I wrong, and thanks to my “old” friend Charlie Melot, who happens to own PA-30 serial number 410 so he would know, I was informed of the error of my ways and, with his enlightenment, returned to the straight and narrow path.

Thank you Charlie! Other than the mention of the PA-30 never being produced with Narrow Deck cylinders, the rest of the information was correct, to my knowledge.

So there isn’t any confusion, the Piper Twin Comanche was built with engines using the Narrow Deck cylinders and, as Lycoming did a rolling production into the Wide Deck cylinders, they were eventually used on the PA-30.

Let me try to explain how I came to throw out the 1963 date about the cylinders. For you old-timers like Charlie and me, the the Twin Comanche sets an important milestone as far as Lycoming history is concerned and the impact this important change had on Lycoming engine TBO times.

What I’m referring to is the fact — accurate this time — that every Piper Twin Comanche had 1/2-inch exhaust valves installed. This product improvement was covered in Lycoming Service Instruction 1136 and is applicable to many other Narrow Deck cylinders on various engine models.

The benefit of this change not only increased the exhaust valve stem diameter but, more importantly, changed the exhaust valve guide material from bronze to Ni-Resist cast iron, which provided better heat resistance, resulting in better wear characteristics. It also greatly minimized and/or eliminated the exhaust valve guide bell-mouthing and misalignment of the valve to the valve seat experienced with the old 7/16-inch valve and bronze guide combination.

When Lycoming introduced the 1/2-inch exhaust valve and the Ni-Resist guide, it also increased the recommended TBO for many engine models, moving from the 1,200 hour TBO, which had been the norm for many years. There are still some TBO time limitations on certain engine models. Refer to the latest revision of Lycoming Service Instruction 1009 for specific details.

I hope I’ve redeemed myself, thanks to Charlie’s help, and will try to do my best not to let my sometimes feeble mind run off on a tangent in the future but, if it should, I’ll expect some of you to call me on it.

Following the completion of this column, I’m off to Fairbanks, Alaska, to participate in the annual Aviation North Expo, where I have the honor of presenting a couple of seminars at the ninth anniversary of this excellent event.

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

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