If there is a universally weak link in aircraft cylinders, it seems to be the exhaust valves. This shouldn’t come as a surprise as the exhaust valve functions in the most extreme environment inside your engine.
Continental and Lycoming each take a different design approach to coping with the heat and rotational needs of the valve. Continental engines utilize a solid exhaust valve that is designed to dissipate heat primarily through its contact with the valve seat. This is why the quality and amount of exhaust valve contact surface is so critical on Continental engines. Continental employs special “Rotocoils” on top of the exhaust valve springs that look like large washers, but have encapsulated springs that create rotation with every valve stroke.
Lycoming engines utilize sodium-filled exhaust valves that efficiently conduct the heat to the valve stem, where it is designed to dissipate primarily through the exhaust valve guide. This is why exhaust valve-to-guide clearances are so critical on Lycoming engines. To ensure rotation on Lycoming engines, the top of the valve stem is pressed off center by the rocker, creating an asymmetric “turning” force that rotates the valve with each stroke.
Having low compression with air leaking that can be heard at the exhaust pipe is often the first signal that there is a problem with an exhaust valve. However, with the aid of a borescope you can usually identify issues early on, before they require more serious intervention. You want to look for an uneven heat signature (coloration) on the face of the valve. The colors and buildup should appear circular and uniform to the valve. Any variation from a perfectly circular heat signature should be investigated further.
It is also important to ensure that the valves are rotating properly. You can test this with the rocker removed by carefully tapping the valve stem with a non-marring mallet to watch the valve rotation. If the valve has only a minor uneven heat signature and you don’t see rotation with the tap test, you should replace the Rotocoil, fly, and see if the heat signature returns to normal. If the valve and seat show signs of leakage, the next step is to lap the valve to remove the contaminants and restore the mating surfaces. The basic process is as follows:
NOTE: You should always replace the Rotocoils when working on valves on Continental engines. It’s very inexpensive insurance.
On Lycoming exhaust valves, the highest risk is having a valve stem to valve guide clearance that is either too loose or too tight. If it is too loose, the valve will wander in the guide and not seat consistently (causing accelerated wear of the stem, guide, and seat). If the valve stem is too tight in the guide (often from baked-on oil and carbon deposits), the valve can stick open. This is most commonly observed on the first flight of the day and is often referred to as Lycoming “morning sickness.”
A sticking valve can be very dangerous as it could allow the piston to strike the valve, destroying one or both of them. Several engine failures have been attributed to sticking valves. Fortunately, Lycoming has a solution in the form of Service Bulletin No. 388C, which is a critical procedure that should be performed at a minimum of every 400 hours (I would probably do it more frequently). It’s a simple procedure that doesn’t require much more than removing the valve covers and using a special tool to check the “wobble” of the valve stem in the guide.
If the valve clearance is too loose, the cylinder has to be removed to replace the guide. If it’s too tight, you can ream the guide without removing the cylinder. If your mechanic doesn’t have the special tool, buy it for them or get another mechanic. It’s that important.
Proactive inspection and maintenance of exhaust valves can dramatically reduce the likelihood of needing to remove a cylinder. Experts such as Mike Busch have written volumes about how to interpret borescope images of exhaust valves, and the key in all cases is to identify issues early on, when a minimal amount of intervention will have the maximum effect. If you borescope your cylinders annually and follow Service Bulletin No. 388C (for Lycomings), you should be able to catch most exhaust issues early enough to use Rotocoil replacement, valve lapping, or guide reaming to get your aircraft safely back in the air without cylinder removal. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.