Cylinder Replacements

April 2001

This month’s ShopTalk will discuss some guidelines on cylinder testing, repair and replacement. As you might guess, heat and friction are the main factors in cylinder life. The effect of high temperature is twofold. Lubricating oil is thinned, in extreme cases, burned away and metal hardness decreases. Both conditions increase the potential for wear.

Federal Aviation Regulations 43 appendix D outlines what is required when doing annual or 100-hour inspections on engines. Section d states, “Each person performing an annual or 100 hour inspection shall inspect (where applicable) components of the engine and nacelle group as follows: (3)…If there is weak cylinder compression, for improper internal condition and improper internal tolerances.” Nowhere in FAR 43 does it state how this compression test is to be done or what the permissible limits are for a particular engine.

To find the standard for compression testing, one must look at Advisory Circular 43.13-1B-2A paragraph 8-14. This paragraph describes the basic method of using a differential pressure compression tester to “check the compression of aircraft engines by measuring the leakage through the cylinders caused by worn or damaged components.” This section of AC 43.13 also establishes the standard for when a cylinder must be removed for repair. If the compression loss is 25% or more of the input pressure (80 PSI), then the cylinder may have to be removed for further investigation. This is referred to as “60 over 80”. I state, “may have to be removed” because there are many factors to take into consideration. When a cylinder is found to not meet the minimum standard of AC 43.13, you can run the engine again and/or you can stake the valve. This may help to increase the compression reading.

All said, so far, applies to all reciprocating engine types. Let’s get more specific. For normally aspirated engines larger than 100 to 125 HP, cylinders are built with a choke (slightly smaller diameter) at the top. In normal operation, the greater temperature at the top of the cylinder causes expansion and the cylinder becomes…what’s the word…cylindrical. Eventually, however, heat and friction take their toll, the top of the cylinder wears down and compression decreases.

As previously stated, this wear occurs faster in areas of high temperature. Especially high temperatures exist near the exhaust valve and this is where the most wear inside a cylinder occurs. The cross-section of the top 20-30% of the cylinder becomes egg-shaped. As this wear progresses, two detrimental conditions appear. First, ring gaps tend to migrate to the exhaust side of the piston. Second, the out-of -round section causes exhaust gas blow-by. Both conditions increase the surface temperature in this area. This increases the wear of the cylinder surface, at least until the cross-section is well out of tolerance. Typically, worn cylinder top dimensions will be .004” to .012” overall wear (decrease in the choke) with an additional .005” to .010” lost from the exhaust side.

Normally aspirated engines loose power, heat and cooling as altitude increases; it all tends to stay in balance. With turbocharging, the temperature effect is more pronounced due to larger, high-horsepower engines, high power settings, higher EGT (more turbo compression), and less efficient cooling as altitude increases.

New Textron Lycoming cylinders are nitrite-treated and have a harder surface than those of Teledyne Continental. On a properly maintained and well-flown turbo-powered aircraft, they will typically go to 70 to 80 percent of TBO before needing repair or replacement. With Continental, the numbers are about 50 to 60 percent. The remainder of this article is going to focus on Continental engines.

A&P mechanics are required to meet the FAA Administrators Minimum Standards and use approved or acceptable procedures when working on a normal or utility category certified airplane. These procedures are outlined in AC 43.13, previously mentioned. Teledyne Continental also has a standard procedure for testing compression in bulletin # M84-15. This bulletin is essentially a reprint of AC 43.13 standards but the TCM bulletin allows for no leakage at the valves. Because AC 43.13 defines the FAA standards and is not as restrictive as TCM bulletin M84-15, I tend to use AC 43.13. You, as the airplane owner, are also only required to meet the minimum standard set by the FAA. You may choose to follow TCM bulletin M84-15, but it is not necessary.

With a cylinder that does not meet minimum standard, the following questions will assist in determining how to rectify the situation.

  1. Is the cylinder in question installed on a high-time engine?
  2. Is the cylinder a high-time cylinder?
  3. Is the cylinder installed on a turbo-charged engine?
  4. Has the cylinder been worked on recently?
  5. What is an adjacent cylinder’s condition?
  6. Will the cylinder read 60 or better anywhere on the compression stroke?

On a high-time engine, if a cylinder is leaking past the rings, below 60 over 80, you must determine if the ring gaps have all lined up. If so, is the barrel out-of-round? The gaps will naturally line up at the out-of round part of the barrel.

By staggering the ring end gaps, you can get the compression above the minimum standard. However, the rings will realign after two or three hours of operation. The only correct solution to this common problem on turbo-charged Continental engines is to resize the cylinder barrel to new limits. This can be done by a number of ways. A new barrel can be purchased but this is a waste of money if it is installed on a high-time engine. Both Channel Type Chrome and CermiNil processes will re-plate the inside of the barrel and make it round again at a considerably less cost than new. The cylinder can also be bored oversize if there is an STC allowing this to be done, but this will throw the engine out of balance unless you bore all the cylinders the same oversize.

About the only time I recommend a new replacement cylinder is if the engine is low-time or is to be overhauled. High-time engines become more expensive to operate as items wear out or break. The failure rates for accessories increase as engines get near TBO, along with the cylinders. Why install a new cylinder on a high-time engine when you are only going to get 10 to 15% life out of the new parts. Most engine shops have serviceable used cylinder assemblies available for about half the price of a new one. Use one of these to get your engine to the next overhaul. Airplane owners often make the mistake on doing a top overhaul 100 to 200 hours short of TBO. This is a complete waste of money because the bottom of the engine won’t safely go much further than TBO on engines of 180 hp. or more.

Conversly, on a low-time engine, replacement parts should usually be new. Your goal should be to have all the parts last until TBO or better. Putting short life parts on a young engine is a foolish and frequently futile way to save a buck.

As an IA and A&P, my basic rule of thumb is, if the aircraft is operated under FAR part 91, I will usually allow 10% beyond TBO before I refuse to sign the annual. This is provided that all airworthiness directives are properly complied with and the aircraft is not flown more than 100 hours a year.

Let’s change our focus now to valves. If there is a valve leak, determine the faulty valve. The most common valve leak is due to the valve face and seat no longer matching. This can be caused by a weak valve spring; the valve will bounce instead of seating. Another cause is the valve guides can be worn, out-of-round due to a lack of lubrication; the valve seats in a different location each time it is activated. Worn out valve guides are a very critical situation; if not repaired can quickly lead to valve failure and possible catastrophic engine failure. To lubricate the guide and the valve stem, oil must flow through the lifter and the push rod. If the guide is worn out-of-round, suspect the lifter for that valve. This is especially true on exhaust valves and even more so on turbo-charged engines. So, if the guide of one of the exhaust valves is worn out of limits and worn out prematurely then always look at the lifter for that valve. Replace it if it is questionable at all. If you don’t examine the lifter carefully, and just replace the valve and/or guide for that particular cylinder, you may have temporarily solved the compression problem. However, you have not solved the valve guide wear problem; the condition will re-occur.

Most turbo-charged Continental engines use a special high flow lifter on the exhaust valves. Make sure that your engine has the correct lifters installed. Sometimes the parts book is not correct; you should call Teledyne Continental. Determine the exact part number of the lifter installed in your particular model engine. That’s the only way to be sure of using the correct part so your valve guides will not wear out prematurely.

Another common reason exhaust valves do not seat properly is improper engine operation. Excessive leaning is the common culprit. This generates a tremendous amount of heat on the exhaust valve itself and will cause the valve head to warp in relationship to the stem. Thus, it won’t seat properly. This serious situation can lead to a catastrophic engine failure if not quickly diagnosed correctly and repaired. Blisters or bulges on the exhaust system are a sure sign of excessive leaning.

Another cause of valve leakage is the easiest to correct, improper rocker arm-valve stem clearance. Both manufacturers refer to this as dry tappet clearance. If the adjustment is too small, the valve will not completely seat or it will seat late and open early. The latter case, on an exhaust valve, can cause excessive heating of the valve and seat. Follow the manufacturer’s recommendations for measuring and adjusting clearances.

A less common failure that can also lead to a catastrophic event is a crack in the cylinder head at a valve seat (usually the exhaust). If the seat insert loosens, it may misalign with the valve. A leak may also develop between the seat and the cylinder head. A failure of this sort can sometimes be repaired by specialty welding the crack and re-inserting the valve seat.

As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.