Steve Pierce
BENEFACTOR
Graham, TX
While doing the annual inspection on SJ's Super Cub I have run into a snag. SJ is kinda hard on stuff but the O-360-C1G that was installed new when the airplane was built has 1200 hrs on it and has performed flawlessly as long as I have been maintaining it. Normally I check compression, clean plugs, timing oil etc. However this year the normal 77 and 78/80 compressions were not to be on #3. Working at it I could get 70/80 but the disturbing thing was the rush of air I could hear rushing out of the exhaust pipe. Over the years I have had a few low cylinders that acted this way and they came back up after a short run. Unfortunately this one didn't come up after a short run, short flight, 45 minute flight or stacking the valve, it actually went down to 62/80 still pouring out the exhaust pipe. Borescope showed nothing abnormal but when I pulled out the old wobble test fixture it was obvious we had a worn exhaust valve guide. I then decided to check the other three cylinders and low and behold they were all worn, one not so bad but three of them were way out of tolerance. It used to be recommended to do the wobble test on Lycoming engines every 400 hours via Lycoming Service Bulletin 388C however when they came out with the new "HiChrome" guides in 1996 that inspection went to 1000 hrs per Service Instruction 1485A. SJ's cylinders do have the "HiChrome" guides as verified by the "C" stamped on the cylinder heads but why the abnormal wear? Lycoming didn't have an answer. They did want to know what the normal CHT's were which in this case is only known on one cylinder and rarely sees 370 but usually runs 325 or so which is well below the 435 degree redline for this engine. Some internet research shows that some turbo charged Mooney's have external oil lines feeding the exhaust valve guides and that Cessna moved the oil pressure pickup point (reads midway down the oil flow path instead of off the oil pump) and upped the redline on oil pressure on the new 172, 182 and 206. So is it CHT temps, lack of oil, or what. I am baffled at this point. I have all cylinders off and guides removed. Waiting on new guides to arrive so I can install and grind the valves and seats. I will definitely be doing the wobble test religiously and make notes of the clearances to get an idea of any wear patterns. Just wondering if anyone else has had issues or experiences with this problem?
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