I’ve replaced the problem cylinders, should I replace the others? — General Aviation News

By Paul McBride · June 5, 2023 · 6 Comments

Question for Paul McBride, General Aviation News’ engines expert: I own a Mooney M20E with a Lycoming IO-360 engine. When purchased in 2015, the engine had 600 hours since bottom overhaul and 500 since top end overhaul. The engine now has 1,100 and 1,000 respectively.

Since I bought the plane, the oil consumption has consistently been one quart every three hours. Higher than I wanted for certain, but I lived with it. Additionally, the pressure tests have consistently been in the mid 70s.

Recently, I noticed a 100° drop in the #2 EGT on the JPI engine monitoring instrument. After removing all the lower plugs, I found #2 was black and wet, #4 was blackish but not wet, and 1 and 3 appeared normal. When my IA removed cylinder #2, we found a broken ring. We also decided to pull #4 due to its blackish plug and found nothing unusual.

Incidentally, #2 was still showing a mid 70s pressure test even with a broken ring — proving pressure/compression tests aren't 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 to no wear with no need for honing. They installed new pistons and rings and reworked the valves.

Given the engine times and now having #2 and #4 pistons and rings replaced and valves rebuilt, I’m leaning heavily toward performing the same maintenance on #1 and #3, with new pistons and rings, even though they aren't suspected of any issue, but I wanted to seek your wisdom on this first.

Also, I’ve gotten mixed information regarding break-in procedure for the new pistons and rings. Some A&Ps say use normal Phillips XC 20/50, others call for mineral oil.

Your advice is greatly appreciated! Thank you in advance for your help!

Bart Chilcott

Paul's Answer: Bart, I’m not certain what caused the ring to break in your #2 cylinder, but it did impact the combustion performance as observed by the lower EGT. I’m a bit surprised that you didn't notice the oil turning black sooner than in the past, which should have been a result of the broken ring.

I agree with your decision to remove the #4 cylinder for further inspection considering the condition of the spark plug.

I am somewhat confused by the decision of the cylinder shop not to hone the cylinders regardless of how the crosshatch pattern may have looked. Good shop practice would dictate honing the cylinders once removed, especially when new pistons and rings were installed.

Getting the rings to seat properly in cylinders #2 and #4 may require a longer break-in period. Be sure to do the break-in using a straight weight mineral base oil until the oil consumption stabilizes.

With regard to proper engine break-in following the installation of new rings, the best information can be found in Lycoming Service Instruction 1427C. That publication will advise the use of mineral base oil and will also provide other valuable information for proper engine break-in.

If the #1 and #3 cylinders are not suspected of any issues at this point, I believe leaving them as they are would be fine. However, I might suggest that a close borescope inspection may be a good idea.