Does flight school’s 172 need new rings?

Q: Our flight school has a Cessna 172N with an O-320 H2AD. The engine was rebuilt 120 hours ago after cam/lifter failure. The engine uses one half quart of Aeroshell 100 Plus every hour! Oil analysis iron ppm is very low. Compression was 75-78 at 100 hour. Do you think that the rings did not seat? Do I pull the cylinders for new rings and hone?

Denney Marsh, Hemet-Ryan Flight School 

A: From Benton Visser: A few questions: How much oil do you put in it? The O-320 has a six-quart sump and if you put six quarts in it, it will blow out this much oil.

Also what kind of cylinders did you put on it? Who rebuilt it and what do they say? I would try running it at four to five quarts and run it hard (at least 75% power) for about 50 hours or so. If it does not change, you will probably need to pull the cylinders.

A: From Paul McBride: The O-320-H series only has a six-quart oil sump capacity and unlike other 320s, you need to keep the quantity close to that, which you’ll understand after reading the next sentence. Being connected to the highest efficient oil pump keeps lots of oil in circulation. The “H” never had a blowing overboard problem like the conventional 320, 360, and 540 series Lycomings when the full sump capacity was put in — you know, the old “blow the first quart out the breather” routine.

I’m leaning more to the lack of ring seating on this one, however it is still within the maximum allowable oil consumption for this engine, which is .72 quarts per hour, but this is at 2,700 rpm, which I doubt you are using.

Even at 2,450 rpm or 75%, you are limited to a maximum usage of .40 quarts per hour. The problem here, as you know, is it probably won’t get any better.

You may try to follow Ben’s recommendation of flying it hard for 50 hours (very difficult to do in a flight school) to see if you can bring the cylinders in. Failing that, as Ben mentioned, you are probably going to have to pull the cylinders, hone them, and install new rings and start over.

If the school is large enough where you can dictate this aircraft only be flown on long cross-countries where you can keep the power up, you might be able to get the rings seated, but it’s a question mark at this point.