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Sticky Situations: Causes and Cures for Stuck Valves

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June 2013

Sticking valves are a relatively common problem on aircraft piston engines. Lycoming Service Bulletin 388 addresses the need to regularly check clearance and provides a procedure to clean carbon accumulations to prevent problems.

Valve sticking is almost exclusively limited to the exhaust valves. Most issues with intake valves are usually associated with improper fit or machining during repairs or loose seats usually becoming apparent soon after the cylinder is put into service.

Most engines will give an important warning that valve stem clearance has been lost to carbon deposits, allowing for maintenance that can avoid the problem. The following will explain the most of the causes and what to watch for.


Many contributors factors can lead to the deposits that cause stuck exhaust valves. One very important thing operators can do is change their oil often at the regular intervals specified in the operators manual, thus removing suspended solids before they can accumulate in the guides.

Engines that use screen filters will benefit from changing to a full flow filter to remove more particulates from the oil.

Keeping cylinder temperatures in normal operating range with proper attention to air flow and baffle sealing will help by lowering guide and valve temperatures.

Proper air filtration can also help by keeping ingested solids to a minimum.

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