A Simple Check Of Internal Engine Timing

The A&P mechanic is often called upon to troubleshoot an engine problem relating to low power. There are many causes for this kind of complaint. This article will discuss one possibility that should be considered if this problem occurs after an engine has been overhauled or disassembled for other reasons.

First, let us consider the symptoms. In case of an engine fitted with a fixed pitch propeller, the static RPM may be several hundred RPM below what is specified for this engine/airframe combination. For an engine with constant speed propeller which has the governor and propeller blade angle set properly, it is possible that both static RPM and/or performance may be low. The cause of these symptoms in an engine which has recently been disassembled may be the result of improper timing between the crankshaft and the camshaft. Misalignment by one or two gear teeth may have occurred during engine assembly.

If these symptoms exist and if improper timing is suspected, it is not necessary to disassemble the engine to check the internal engine timing between crankshaft and camshaft. The procedure for accomplishing this check will be detailed below for those A&P mechanics who have not been exposed to this method before.

First, insure that magneto and electrical switches are in the OFF position. Next, remove the cowling so that rocker box covers and spark plugs are accessible. Then rotate the engine so the piston in number one cylinder is positioned at top dead center on the compression stroke. The number one cylinder of Lycoming engines is the right front cylinder except for the 541 models which have number one cylinder at the left front position. For all Lycoming direct drive engine models, the top dead center position of number one piston can be verified by observing that the mark indicating the #1 TDC position on the rear side of the starter ring gear is exactly aligned with the split line of the crankcase at the top of the engine. As the last step of preparation, remove the rocker box cover from number two cylinder. 

Engine timing is checked by first observing the number two cylinder valve rocker arms. Both valves should be closed or nearly closed. The next step is to move the propeller slightly in one direction. Rocker arm motion should be seen as one valve starts to open. STOP. Now rotate the engine back to the original position with the #1 TDC mark again aligned with the split in the crankshaft halves. Both valves should again be closed or nearly closed. Now move the propeller slightly in the direction opposite from the first movement. Rocker arm motion should again be seen as the other valve starts to open. If the two valves started to open as described with only a small amount of engine movement in each direction, the engine timing is correct.

For some individuals it may be simpler to rock the propeller slightly with a back and forth motion while observing that first one valve and then the other will start to open. If movement in either direction exceeds twenty degrees of engine rotation before motion of the rocker arm occurs, the crankshaft to camshaft timing is not correct.

If the observed rocker arm movement indicates that internal engine timing is correct, then this is not the cause of the low power being investigated. On the other hand, if both rocker arms do not move from engine rotation within the parameters discussed earlier, the internal engine timing is not correct. This indicates a probable error during engine assembly and it can only be corrected by opening the engine and realigning the crankshaft and camshaft gears. Instructions for accomplishing this task will be found in the appropriate overhaul manual