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Exhaust valve life and rotators

nomocom

Well Known Member
A buddy noticed on his Continental engine the exhaust rotators weren't doing the expected job (much less than the 1 RPM expected). He replaced all six.

Is this a thing for those of us with Lycomings?

Anybody bothered to check for rotation?
 
To my knowledge, they are just caps, not rotators. Most rotation is done via geometery at the rocker / valve stem interface. The lycoming caps have a parallel bottom and top and therefoce can't produce any rotation in and of themselves.

Larry
 
To your life question, yes, rotators extend the life of exhaust valves. In general, exhaust valves pushed to the temperature limits (at the face) are the result of many design and operational factors. If they are continuously operating near the limits, rotators will lower the probability of burning, thereby extending the life. Think cruise (hrs) vs take off (3-10 min).

This is why marine conversions of auto engines might employ rotators and smaller valve diameters (among other changes) due to the high load factor. Think cruise (hrs) vs take off (3-10 min).

If rotation is needed, geometry is an unreliable method of achieving the objective as one will eventually stop rotating.
 
Continental and Lycoming have very different valve rotator designs. Here's a good short description of different design types:

https://marineengineersblogs.blogspot.com/2012/12/valve-rotators.html

Lycoming valves fall under the heading of "non positive type" and sub-heading "thimble valve rotator".

Per my 2017 call to Lycoming tech support, the "rotators" on Lycoming valves will *let* the valves rotate, but do not force them to do so. Per Lycoming tech support, it is fine if the valves rotate, but they don't have to.
 
rotator types, morning sickness, wobble test

Very interesting links, that explains the difference, thank you.

I can't help but wonder if the "morning sickness" that seems to be a Lycoming phenomenon, might have a correlation to the valves only being allowed to rotate rather than having a positive rotation design. Perhaps a valve that is being positively rotated will better keep clearance between the guide and stem? It would be good to know if there is any correlation between not rotating and failing a wobble test. One way to monitor rotation would be to see the valve in different positions, from one bororscope session to the next. Unfortunately, looking back through my last two sets of boroscope exhaust valve pictures, I don't see anything unique on the valve face that carried over from the earlier session to the latter.
 
... Unfortunately, looking back through my last two sets of boroscope exhaust valve pictures, I don't see anything unique on the valve face that carried over from the earlier session to the latter.
Fully symmetric valves are actually a good thing, and likely indicate that the valves are rotating as they should. :)
 
Symmetry and accident investigation

Fully symmetric valves are actually a good thing, and likely indicate that the valves are rotating as they should. :)

While heat signiture symmetry is a good thing, the valves often have spots a mm or two in size that are unique, however I don’t see these same spots at the next boroscope sesssion. I’ve seen pictures of non-Lycoming valves with serial or part numbers on the face and that would be very useful for tracking position.

Accident summary, non-rotation suggested as causation.
https://generalaviationnews.com/2016/07/06/exhaust-valve-failure-contributes-to-crash/
 
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