Twelve years after Continental issued service bulletin SB03-3 directing maintenance technicians to use a borescope to inspect each cylinder every time a compression test is performed, its instructions are being routinely ignored—at a high cost to aircraft owners.
A compression test is one of the valuable tools available to a mechanic to diagnose cylinder health, yet it requires a degree of skill to perform accurately and even the best techs admit that they may not get the same results twice in a row. Above all, it is only one tool in the tech’s arsenal and should never be used by itself to make the decision to pull a cylinder off an engine. Too often low compression accompanied by a leaking exhaust valve has resulted in a yanked cylinder only to find that the valve and its seat are perfectly healthy.
Continental makes it clear, and we think it’s applicable for Lycomings, that a borescope must be used to check on exhaust valve condition. If it’s in good shape—AOPA’s poster below is a great reference for what it should look like—there is no reason to pull the cylinder.
If the borescope discloses signs of valve distress, pull the cylinder—if not, that borescope exam just saved you significant money.