This month Mike, Paul, and Colleen give an owner the ammo he needs to not overhaul his engine prematurely, help console an owner with super hot cylinders--maybe, console another with super cold cylinders, and give cover to a pilot who wants to refill his own oxygen. Send your comments and questions to [email protected] for a chance to get on the show. Full show notes below: Pat’s Diamond has one suspect cylinder. The mechanic said it has failed inspection, and needs to be replaced, but the engine is beyond TBO and they are refusing to fix it without an overhaul. The question hits right at the heart of some of Mike’s frustrations with shops and their lack of evidence-based practices. He suggests lapping the exhaust valve to bring the compression back up, and then keep flying it. Kirk wants to refill his own oxygen system, but it’s not covered specifically in preventative maintenance regulations. The hosts asks if you’re allowed to add oil, fuel, and air to the various systems. Of course you can. It’s servicing the airplane. Mike says there is no talk of servicing in the regulations, but of course we all do it legally. They view adding oxygen in the realm of servicing. Robbie doesn’t believe his CHT gauge in his Socata TB-10. The factory single probe gauge shows 500 degrees and rarely goes below 400. He’s wondering what he can do to make himself feel more comfortable that it’s a gauge problem. Colleen asks if he smells anything. She smelled burning oil when hers went beyond 420. The hosts agree that given his great borescope photos, and good oil analysis that he’s probably ok. They also suggest checking the engine grounding strap that goes from the crankcase to the firewall. It grounds the engine to the airframe, and without it the probe wire could be finding some interference. The paint would also change color at 500 degrees. Will is seeing exceptionally cold cylinder head temperatures on his Comanche. On a recent flight to New York, he saw temperatures in the high 140s during descent. He knows his probes are good, so he’s wondering if there’s a lower limit and if it's hurting the engine. After discussing a number of possible causes, they discovered that Will is going full rich on descent, which is absolutely contributing to the low temps. Mike said that while you may have to enrichen a bit in the descent, if you forget the engine will remind you by running a bit rough. That’s followed by a detailed discussion of leaning for the climb and descent. Carl flies a Cherokee 180 and is wondering if flying it on cold winter days with density altitudes below sea level could hurt the engine. The hosts agree that the takeoff mixture setting is so rich that there’s enough of a margin not to worry. Detonation could be a concern in certain experimental engines, but a normal certificated engine has a big detonation margin.