Valve Wobble Testing

Everything you ever wanted to know about the dreaded Valve Wobble Test... straight from Mother Lycoming.

The purpose of the Valve Wobble Test is to check for the proper clearance between the exhaust valve and the exhaust valve guide. Too little clearance, and the valve may stick in the guide and cause a loss of power or even a mechanical failure such as a broken valve or bent pushrod. Too much clearance leads to micro-fractures in the valve stem due to the valve seating off center on the tapered seat in the cylinder head, and wedging the head slightly crooked to match the seat taper. Repeated every combustion cycle at 1300-1400 degrees F, the valve stem eventually breaks and the head falls into the cylinder and is violently struck by the piston over and over. Tremendous damage results and the debris is often swept into other cylinders and causes more damage and power loss.

Needless to say, both possible failure modes, sticking valves from not enough clearance, or breaking valves from too much clearance are to be avoided.

The sticking valve is caused by a build up of deposits in the guide or on the valve stem. These are combustion deposits and cooked oil. Excessive CHT can encourage the oil that lubricates the guide to "coke" and turn to a baked on carbon layer. It is good practice to avoid CHTs over 400 degrees F. The stock baffling will do this if kept in good shape. After 30+ years, there is very little original baffle seal left in the fleet, and of that which has been replaced, it is quite common to see very poor installation, and often the wrong material used.

This is the Lycoming tool recommended by the SB-388C document... Lycoming-tool.JPG

A similar tool is available from Aircraft Spruce, but works slightly differently. The Lycoming tool depresses the exhaust valve spring when installed, the Aircraft Spruce one does not. The above tool is usually modified to use a dial indicator to get direct readings. As shown above, the small screw on the right is adjusted to be close to the valve stem extension and a feeler gauge is used to get readings.