Question for General Aviation News engine expert Paul McBride: I have a Mooney M20E with a Lycoming IO-360 engine. When purchased in 2015 the engine had 600 hours since the bottom end overhaul and 500 since the top end overhaul. The engine now has 1,100 and 1,000 respectively.
Since I bought the plane, the oil consumption has been consistently one quart every three hours. Higher than I wanted for sure, but I lived with it. Also, pressure tests have consistently been in the mid-70s.
I recently noticed a 100° drop in #2 EGT on the JPI engine monitor. After removing all the bottom caps, I found #2 was black and wet, #4 was black but not wet, and #1 and 3 looked normal. When my AI removed cylinder #2 we found a broken ring. We also decided to remove #4 because of its black plug and found nothing unusual.
By the way, #2 still showed a mid-70s pressure test even with a broken ring, proving that pressure/compression testing is not the best diagnostic tool.
We sent both cylinders and pistons to a reputable local cylinder shop for repair and inspection. They found the cylinder walls to be nickel-plated with no scarring and little or no wear without the need for honing. They installed new pistons and rings and reworked the valves.
Considering the engine timings and now having the pistons and rings no. 2 and no. 4 replaced and the valves rebuilt, I am very much inclined to perform the same maintenance on the nos. 1 and no. 3, with new pistons and rings, although not suspected. of any matter, but wanted to seek your wisdom on this first.
Also, I have received mixed information about the procedure for breaking in the new pistons and rings. Some A&Ps say they use regular Phillips XC 20/50, others call for mineral oil.
Your advice is greatly appreciated! Thanks in advance for your help!
Bart Chilcott
Paul’s answer: Bart, I’m not sure what caused the ring to break on your #2 cylinder, but it affected the combustion performance as noted by the lower EGT. I’m a little surprised you didn’t notice the oil turning black earlier than in the past, which should have been the result of the broken ring.
I agree with your decision to remove #4 cylinder for further inspection given the condition of the spark plug.
I’m a little confused by the cylinder shop’s decision not to refinish the cylinders regardless of what the crosshatch pattern looked like. Good shop practice would dictate honing the cylinders once removed, especially when new pistons and rings were installed.
Getting the rings to seat properly on cylinders #2 and #4 may require a longer run-in period. Be sure to break in with a straight weight mineral base oil until the oil consumption stabilizes.
Regarding proper engine break-in after installing new rings, the best information can be found in the Lycoming 1427C Service Manual. This post will advise the use of mineral base oil and also provide other valuable information for proper engine break-in.
If it is not suspected that cylinders no. 1 and no. 3 have any issues at this point, I think leaving them as they are would be fine. However, I might suggest that a close endoscope inspection might be a good idea.
