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.
If it ain’t broke, don’t fix it. Leave the other cylinders alone.
First off it’s a leak test, not a compression test. Minimum leakage is determined after working the piston to it’s optimum static position, not reality. Any one compression ring being intact will give you acceptable readings. A compression test will tell you more. Differential in actual compression between cylinders, particularly with new vs older cylinders, will cause engine imbalance.
Installing rings new rings without at least running a glaze buster is a bad idea, as is using used rings, they will probably never fully seat.
When looking at factory tolerances on oil consumption just remember, if they posted what I consider acceptable they’d be re-ringing a lot of engines under warranty.
I run a fresh engine same as used, personally never had consumption problems after a rebuild,
I am not a professional, but hold a repairman certificate for experimental light sport. My experience is with lycoming, VW, Rotax, GM, and Chrysler engines. Having said that, I have never heard of replacing piston rings without, at least, honing the cylinders to facilitate break-in and proper sealing of the rings to cylinder walls. I have heard from more experienced mechanics that not doing a proper break-in can result in an engine with high oil consumption for the life of the engine, however I’m not sure that it is true.
I did assist in an overhaul of a lycoming 0-360 in my Mooney 30 years ago and experienced high oil consumption for 250 hours after the overhaul, but after switching from mineral oil to 20-50w the oil consumption was normal, which was somewhat confusing. Just my two cents.
Dee Waldron,
Excessive oil in the cylinder will hide a number of faults, including annealed or broken piston rings.
Compression checks can be misleading under the right circumstances. They are but one tool used in diagnostics…
Well, I’ve been doing this for 50 years now. From big radials all the way down to VW engines. There is more to a compression test then a number. This is a myth that has gone on among owner/pilots for far too long. So quoting numbers doesn’t mean anything. Yes, it’s one tool, but it tells SO MUCH if used and interpreted with experienced knowledge. My advise is to find a better shop with competent AMTs.
>>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.<<
I could NOT disagree more!