About the article “Risky Business”: The author argues that “it's nearly impossible to install a cylinder properly when the engine is on the airplane,” that manufacturers' instructions are inadequate, and that it is pure “luck” if you do install a cylinder while the engine is in the airplane and it doesn't subsequently fail. These arguments are easily disproved by the hundreds of thousands of cylinders that have been successfully installed while the engine is in the airplane over a long period of time by mechanics who are following manufacturers’ instructions. “Luck” is not an appropriate word to describe such a long and successful history. Aircraft engines are generally tightly cowled with a number of connections for fuel, ignition, intake air, exhaust, et cetera, some of which must be removed to access a cylinder. However, experience shows that when manufacturers’ instructions are strictly adhered to, problems with cylinders do not occur. Before stating his opinions in a published article, it would have been nice if the author had actually read Continental’s service literature.
Nowhere in the article does the author exhibit any familiarity with the current revision of Continental Standard Practice Maintenance Manual M-O, which provides instructions for cylinder repairs. The author makes five points regarding obstacles to achieving proper fastener preload when performing cylinder installations in airplanes without one reference to this important Instruction for Continued Airworthiness.
Point number 1 is “The fasteners aren’t new.” The concern is that the cadmium plating on critical fasteners may be gone. However, in general the cadmium plating on cylinder studs, through bolts, and nuts will not be worn to the point where attaining the proper preload will be impaired by the infrequent occurrence of cylinder removal over the engine TBO. However, the referenced Instructions for Continued Airworthiness instructs maintenance personnel “Do not re-use worn, damaged or deformed fasteners. Do not replate cadmium plated fasteners or washers. If the cadmium plating has been removed, discard the item and replace it with a new part.” At TBO, all through bolts, cylinder deck studs, and nuts are to be replaced with new parts.
Point number 2, “Fasteners may not be adequately lubricated.” Both Continental and Lycoming require lubrication of these critical threaded fasteners and failure to follow that instruction will indeed result in inadequate preload. Continental warns “Torque values listed are for use with clean 50 weight aviation engine oil applied to the threads, unless otherwise specified.”
Point number 3, “The lubricant itself is rather poor.” The torque specifications for Continental critical fasteners were developed with the lubricant available to all aircraft engine mechanics, engine oil. This torque specification was developed by correlating bolt preload/stretch with applied torque using engine oil. Consistency of the torque/stretch correlation is the critical factor and is provided with the specified 50 weight aviation engine oil. Other thread lubricants are not approved because the needed correlation of preload/stretch to torque is not achieved and may result in less than desired load or an overload condition.
Point number 4, “Wrench access is limited.” Some removal of aircraft/engine components will be required to provide needed clearance during an on-aircraft cylinder replacement. Both the aircraft and engine applicable manuals should be used. The instructions in Continental's Standard Practices Manual call for the removal of components or systems that block free access for the stud nut wrenches and through bolt nut wrenches on both sides of the engine. The required torqueing of the opposite side nuts will also prevent inadequate through bolt preload. This so-called obstacle is a total red herring.
Point number 5 claims that the “Manufacturer instructions are incomplete.” An example cited is that ''there is nothing in the Continental guidance suggesting that the opposite-side nut should be removed and the nut and threads be lubricated, nor that the opposite-side threads be cleaned and the opposite-side nut be replaced with a new one.” This statement is incorrect. Once again reference to Continental's Maintenance Manual M-0 reveals an instruction to “Lubricate the cylinder base stud threads, through bolt threads and nut threads on both sides of the engine with clean, 50-weight aviation oil.” The emphasis in this statement is in the original document. M-0 also tells mechanics to “[i]nspect all threaded parts for nicks, damaged or deformed threads, faces or head.” and to replace any deficient parts with new parts. The requirement for torqueing the opposite-side through bolt nut is very important to assure proper through bolt preloading/stretch. “Failure to torque through-bolt nuts on both sides of the engine may result in a loss of main bearing crush, main bearing shift, crankshaft fracture, and engine failure.”
The arguments contained in the article contradict the published service literature and experience in the field. Owners and mechanics need not fear cylinder failure when the published instructions are strictly observed.
Michael E. Ward
Director, Certification & Airworthiness
Continental Aerospace Technologies
Mobile, Alabama
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