The International Cessna 170 Association

Holey, Moley! If those stupid engineers at Ford, Chrysler, and GM only had a clue. Unfortunately, I've been assured that aviation fuel has been treated magnetically at the refinery so that that device is not effective on airplanes. Pity!

We made a ton of money running trucks on the Dyno for customers that had all kinds of filters, magnetics and other devices to enhance fuel mileage to show them how efficient the DEVICES were. Great snake oil advertisements can really lighten your pockets.

John

After trying to diagnose some intermittent engine roughness, I finally found the culprit. Long story short, I had an exhaust valve stick open and tried the rope trick to no avail. The valve was STUCK HARD! This led me to pull the offending cylinder and replace it with a new jug (old cylinder was out of spec). I was dumbfounded when I still encountered occasional engine roughness following replacement of the offending cylinder. I checked the plugs, did a borescope, checked compression, and everything checked out fine. While milder than the original problem, I would still occasionally lose a couple hundred RPM on initial climb along with some minor engine roughness that might last for 30-45 seconds. The problem always went away after leveling off and picking up some RPM. in an effort to track down what I was convinced was another sucky valve, I began removing exhaust valve springs. Everything looked great, all the valves spun freely in their guides. There was no way this problem could be a stuck valve. When I went to inspect the #1 exhaust valve spring, I immediately noticed a problem. It was about 1/2 inch shorter than the other valve springs on the engine. If a weak spring could cause valve issues, I have no doubt this bent deformed mess was causing my problem.

In my new to me c170B I had two stuck exhaust valves in the first 10 hours after the plane sat for 13 years. The first cylinder was changed, including the springs. When the second one happened I tested the springs on the removed cylinder. They looked ok but compressed at half the spec. So, we changed all springs and keepers on the other 5. There were two tight exhaust valves in the guides so they were cleaned. 30 hours later, so far so good. Fingers crossed.

This past week a friend flying his 170A experienced after lift off a small vibration lasting just a few seconds. It was unusual but he continued the flight. Then over the middle of Georgia strait, he lost 200 rpm. He turned back to the field, some of the rpm came back but he now knew something was wrong.
He checked his compression and found # 6 at zero, pulled the valve cover off and the exhaust spring was broken and spring seat beaten up. Pulled the cylinder, and the piston skirt at the crown was damaged. He checked the other cylinders and found another cylinder with a broken spring. He had replaced his tired engine with this 100 smoh engine but an engine that had sat for almost 10 years.
I think his experience is a testament to how tough the 0-300 is. Also I think the engine can hide a bit of degradation, as it’s unlikely he experienced two spring failures at the same time.

I have a Zip-Lok bag full of valve springs which failed to meet specification when I had some cylinder work done 15 years ago.

I sent two cyls down to JJ in Pleasanton, TX for inspection because they failed compression tests during Annual Inspection. JJ contacted me and told me the springs were lower than spec and recommended ovhl of the cyls. I replaced them with new Superior cyls instead, but had all four of the remaining cyls on the engine tested and found most of them under-specification. They were probably re-used during the last “overhaul” ..... which was many years earlier despite the engine having less than 300 hours on it. (The engine was rebuilt/repaired/overhauled by the restorer and placed in-storage for over a decade as the airframe was restored. If the springs were previously in-service and re-used...then sat for over a decade before the engine was placed back in-service.... it made sense those springs had outlived their design-life ....even if they’d met specification when the engine was “overhauled”.

As a reminder.... most “field overhauls” are not actually within the definition of an overhaul. They are in most cases “major repairs” because it’s very rare that such processes accomplish each-and-every-specificationl in the TCM Ovhl Manual.

I now have over 1000 hours on that engine, which was rebuilt/repaired back in the early ‘80s by the Emory School of Aeronautics in Florida. While the records lead one to believe it has 1000 hours “SMOH”.... and while the engine runs great and makes good power.... there’s no way that repair done in the early ‘80s was an overhaul despite impressive documentation.