Ask Paul: Should I worry about my #2 cylinder?

Q: I had my Lycoming 0-360-A1A rebuilt at Victor Aviation in 2001 — new crank, new cam, new pistons with new Millennium cylinders. The engine, which now has about 400 hours on it, has digital cylinder head gauge probes in each cylinder. My question: On a warm summer day in Alaska (75°F), with a load in the plane on floats, during climb out, the #2 cylinder head temperature hits 400°F. In cruise, the temperature runs around 330°F. The other three cylinders remain below 380°F. durning climb out. I have exchanged cylinder probes and #2 remains high at climb out.

The Lasar mag system is on the engine. The #2 cylinder is getting enough air across the front of the cylinder. The engine is in a 1956 170-B (pictured).

PAUL YOUNG, via email

A: First of all, let me say your aircraft is a real beauty, which I’m certain you are proud of. Secondly, this is obviously an STC conversion with this Lycoming engine installed.

Getting back to the CHT temperature situation: I don’t believe there is a real problem, even though Paul does see some differences in temperatures on his digital CHT readings. On that warm summer day in Alaska, with a temperature of 75° and a loaded aircraft on floats, the #2 cylinder reaches 400°F on takeoff. I have no concern — at all — with a CHT like that since the maximum continuous CHT for the O-360 series Lycoming engines is 500°F. I’m assuming that we can have some confidence in the accuracy of the digital CHT system Paul has installed versus what we may have seen on the original analog type gauges.

Paul has done some fine troubleshooting by swapping the CHT probes and confirming that the #2 cylinder remains hotter, eliminating the question as to whether the probe may have been giving him a false reading. He has also confirmed the #2 cylinder is receiving adequate cooling air through the cowl inlet.

At this point, I do not see any reason to be concerned with the CHT readings you are seeing. One important point to remember here is the fact that we are dealing with a carbureted engine. We know that an engine utilizing a carburetor has poor distribution, at best. This is just a result of the manner in which the fuel air mixture is introduced into the combustion chamber. One of the factors contributing to this less-than-ideal distribution is the length of the intake pipe. Some are longer than others, which does have an effect on the mixture going to each cylinder. All and all, it still works out pretty darn good and the variances in the mixture and CHT readings are small.

The other temperatures Paul mentions are all where I’d like them and, even with this #2 cylinder being a bit different on takeoff on a warm day, I’d expect no problems resulting from this. A good service life should be expected, should nothing else change.

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


  1. Doug Rodrigues says

    At different power settings, a different cylinder may become the highest in temperature. If the baffling and seals are okay, then don’t worry about it. I got to do a lot of experimentation with my own engine conversion (STC791NM issued to me personally) and found that trying to achieve equal cylinder temperatures at different power settings was like chasing my tail around in circles. In fact, I believe that it is impossible to obtain equal cylinder temperatures in an air cooled engine no matter what you do.

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