Aircraft Maintenance: Good cylinders gone bad

Installing most aircraft parts is a straightforward process: Remove the old part, put in the new one, sign off on the work, and you’re good to go with confidence. Pick up your bird with a new set of tires and you can head off flying with a smile, admiring your new treads at every preflight. Cylinder work, on the other hand, brings with it a far more complex process with higher stakes as well.

New cylinder inspection and setup

My approach to new cylinders can be summed up with the motto “respect the labor.” The amount of labor required to replace a cylinder is significant, usually involving the removal of all sorts of other items such as the cowling, baffling, exhaust, intake, fuel lines, and other parts. With so much labor invested in the cylinder installation, it pays to do everything you can to avoid having to redo the process.

Begin with a careful inspection of the new cylinder, including the paperwork, to ensure that it’s the correct part, the piston is the correct weight code (if you are matching cylinders), and there are no outstanding service bulletins or airworthiness directives that apply. Do a careful visual inspection of the cylinder and look for any defects or imperfections. Next, the cylinder should be turned upside down and checked for leaks, with mineral spirits or similar fluid to confirm that the valves seal perfectly in their seats. If the shop has the equipment, you can also check barrel dimensions, just to be sure. New or overhauled cylinders are generally well made, however it’s not unheard of to discover manufacturing issues after the cylinder has been installed and running for some time. Catching issues prior to installation protects both your engine and your wallet. Additionally, this extra set of checks will ensure that you respect the labor and have some great data from the time of installation, should an issue develop down the road.

The last part of the cylinder setup is one of the most important, yet one of the most neglected. New “loaded” cylinders consist of a cylinder with the valves and springs installed; a separate box containing the piston; and additional packages for the rings, seals, and any hardware. The rings must be test fitted and “gapped” to the cylinder prior to installation. This is a process by which the rings are positioned at an exact point in the cylinder and the gap at the open end of each ring is checked against the specs from the manufacturer. If the gap is too tight, it must be filed open using a special tool. If it is too loose, then either the ring or the cylinder requires replacement prior to installation. Unfortunately, many mechanics seem to skip this step and simply put the rings onto the piston, shove it into the cylinder, and bolt everything together. This is a recipe for disaster. If a ring gap is too tight, then it can bind up as the metals expand with heat and gouge the cylinder walls, causing excessive friction and heat. I’ve seen far too many brand-new cylinders that had to be removed after damage due to skipping this critical step.

One might ask why the manufacturers don’t gap rings themselves and deliver the cylinders with the pistons preinstalled. The answer is that many variables in the process make it much easier and cost effective to do during installation. There are also storage (ring-to-barrel corrosion) and shipping issues involved in having cylinders delivered with the pistons installed. In some cases, if a preloaded piston were to migrate too high in the cylinder barrel, past its normal travel, the rings could reach the barrel-to-head juncture, popping open a bit and making it impossible to remove. Such a cylinder becomes a very expensive desk ornament, another reason to load the piston just before installation on the engine.

Cylinder installation

Much has been written about proper cylinder installation, but here are some highlights:

• Follow the manufacturer’s instructions exactly, with no deviations.
• Always maintain a preload on the engine case through-bolts when cylinders are removed.
• Never move the propeller without proper tension on the case through bolts.
• Always use new cylinder base nuts, properly lubricated per the manufacturer’s instructions.
• Never use sealant, such as RTV, on the base of a cylinder.
• Use smooth torquing procedures for the cylinder base nuts (one person on each side of the engine for through bolt nuts).
• Take care to reinstall everything else (baffling, lines, and other components) exactly as it was prior to the work.

Cylinder break-in

This is another area that we can only scratch the surface of in this column. Suffice it to say that it is critical to follow the manufacturer’s break-in procedures exactly. If weather conditions would force a compromise, delay the break-in flight. Ensure that the fuel system has been set up properly to maintain cooling. Minimize ground run time and invest your time and attention to detail in the break-in process in the same way as during installation. Those first hours of break-in are critical to the future life of the cylinder. Finally, never rush the break-in process. Keep running on mineral oil and following the procedures until your cylinder head temperatures and oil consumption have stabilized in normal ranges. There are few drawbacks to breaking cylinders in for too long and many consequences for stopping the process too early. Abandoning the break-in process too early, not running the engine hard enough, allowing temperatures to get too high, or otherwise deviating from the process can lead to glazing in the cylinders, scuffing of the cylinder walls, excessive oil usage, or other issues that cannot be solved without cylinder removal.

A careful and disciplined approach to new cylinder installation is your best way to respect the labor and prevent having to go through the process again prematurely. And, don’t forget to monitor the cylinders regularly through routine borescope inspections. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.