Valve performance, O-360 parallel valve

Our local EAA chapter bought a boroscope for the tool crib, the $350 Vividia unit available at Spruce. For the first time, I've got very nice photo's of all the valves. Per the Mike Busch valve talk (EAA webinar), what we want to see is concentricity of heat signatures on the face of the valve. There is one exhaust valve that shows a less than ideal heat signature.


This cylinder had a high resistance spark plug showing some fouling. The valve face will be watched, but in the meantime, it would be nice to know the valve rotator is doing it's job.
Anyone have a favored method for confirming the valve rotators are working? Or perhaps I should be less concerned about that and just do the SB for valve stem clearance?

A friend showed me a severe failure, a O-360 parallel valve cylinder from a certified Piper. The exhaust valve head came off the stem and bounced around in the chamber. I had assumed these aviation engines are non-interference. Do I have that wrong? What happens that a valve head can break off? Interesting, NTSB SEA05FA097 discusses a failure mode when an exhaust valve becomes "over-open" due to the valve train continuing to operate, without the pushrod and rocker arm in proper alignment. Still, even when finding the valve stuck in an over-open position, they don't mention valve and piston interference.

nomocom said:

A friend showed me a severe failure, a O-360 parallel valve cylinder from a certified Piper. The exhaust valve head came off the stem and bounced around in the chamber. I had assumed these aviation engines are non-interference. Do I have that wrong? What happens that a valve head can break off? Interesting, NTSB SEA05FA097 discusses a failure mode when an exhaust valve becomes "over-open" due to the valve train continuing to operate, without the pushrod and rocker arm in proper alignment. Still, even when finding the valve stuck in an over-open position, they don't mention valve and piston interference.

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My first guess would be failed keepers that allowed the valve to fall into contact with the moving piston. The lycoming exhaust keepers are a poor design in my opinion and much less robust than traditional keepers, like the intake. I believe this is why lycomings typically swallow exh valves and not intake valves. Fatigued metal is also a possible culprit. I believe the valves are hollowed out up into the stem for the sodium fill. This leaves a LOT less metal to hold the main valve onto the stem then traditional valves. A worn guide will allow the valve face to hit the seat in one concentrated spot and this puts a lot of bending stress on the metal in the stem interface area due to the angle. The compromised valve design is much less tolerant of this abuse.

I am pretty sure the engine is not an interference design, at least on the parallel valve engines. Otherwise some percentage of incidences of stuck valves would result in bent or broken valves. If I remember correctly, Interference designs didn't really become an issue until designers started using angled valves; At least on low compression engines.

Just my thoughts and may not be accurate due to limited experience.

Larry

nomocom said:

Anyone have a favored method for confirming the valve rotators are working? Or perhaps I should be less concerned about that and just do the SB for valve stem clearance?

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While I haven't done it, this is what I would do. Pull the rocker and rotator cap. Then use a scribe to mark a light vertical line on the upper stem at the 12:00 position. Sand the line to eliminate any rough edge on the line and reassemble. Take it apart in a few hours and see if the line moved from 12:00.


You can also repeat the borescope in a few hours, paying attention to how you oriented the camera, and compare the pics to the one above. It has enough unique signature deposits to easily determine if it is rotating.

Larry

rotation?

"It has enough unique signature deposits to easily determine if it is rotating"

Good point Larry... I'll just plan on doing a boroscope again on that cylinder.

Thanks for your thoughts on rotation checks and valve failure modes. After thinking about it more, it occurred to me that I should go back and look at the cylinder again and see if the stem was still in the head, or if had been rattling around in the combustion chamber, getting chewed up and spat out. If it is stuck in the valve guide, the head end of the stem might give some clues as to what happened.

A valve head separation failure typically occurs at the base of the tulip. Typically, it's because the valve and stem got too hot and the head snapped off. With hydraulic lifters, it can creep out (stretch) and allow that to happen. Solid lifters just result in a badly burned valve and dead cylinder. It seems rare in our Lycs though, but possible.

Typically, if the valve is not rotating, the top of the stem will show concentric rings from rocker contact. With the stem cap it would be the same thing. If it is not rotating there is a wear line from the rocker arm.

Personally, that valve does not odd to me. The area of most concern typically is the outer edge. Are the leak down tests even? It would show there early.