Opinion: Two out of three ain’t bad

After nearly four decades with my Cessna Turbo 310, I’ve learned to approach that date with a mixture of confidence, caution, apprehensiveness, and superstition. Because no matter how well your airplane has behaved during the past year, the annual is where reality shows up with a clipboard, checklist, flashlight, mirror, and borescope.

This one was my thirty-ninth annual ordeal since I acquired the airplane in 1987. By now, you’d think I’d have seen just about everything the airframe and two engines could throw at me.

You’d be wrong.

The usual suspects

As always, the process began by pulling the airplane out of the hangar and running the engines to get them good and hot for the compression test required by Part 43, Appendix D. The runups were uneventful, but I noticed one small anomaly: The right brake felt spongy and would not hold when I applied full throttle.

After shutting down and pushing the airplane back into my hangar, I confirmed that the right brake caliper was showing evidence of hydraulic fluid leakage. No big deal—replace the O-rings on the three brake pucks, replenish fluid, bleed the system. Routine maintenance, the equivalent of brushing and flossing. A minor squawk, nothing more.

Maybe—just maybe—this would be one of those rare “nothing to see here” annuals. I pulled the cowlings, removed the top spark plugs, and began the compression testing.

A tale of 12 cylinders

I started with the left engine. Five of the six cylinders produced healthy compressions in the 70s. Cylinder number two, however, came in at 46/80, with air audibly leaking from the tailpipe.

Now, 46/80 is technically airworthy—above my master orifice reading of 42—but it’s also the sort of number that motivates you to investigate further. It’s not a failure; it’s an early warning.

The borescope told the tale. The exhaust valve showed early heat distress—no obvious hot spot but uneven and asymmetrical. Exactly the kind of situation where minimally invasive intervention has a high probability of success.

On my clipboard, I jotted down: “LE #2: 46/80 exh (lap).”

Moving to the right engine, I found four cylinders in the 70s. Cylinder number three measured 33/80, again leaking out the exhaust. The borescope image looked very similar—early-stage distress, but nothing catastrophic.

Another note: “RE #3: 33/80 exh (lap).”

At this point, I was feeling cautiously optimistic. Two marginal cylinders, both looking like good candidates for in-place valve lapping. This was shaping up to be a textbook case for why we borescope before we reach for cylinder base wrenches.

The very last compression reading I took was right engine cylinder number six.

Cognitive dissonance

Something felt wrong immediately. I couldn’t find top dead center using the thumb method. That’s never a good sign.

When I hooked up the compression tester, the reading confirmed my concern: 0/80.

Zero? Zero? Really?

That’s not a number you interpret. That’s a number you react to.

In more than five decades of aircraft ownership, I’ve encountered a zero-compression cylinder exactly once before. In that case, the problem was obvious: a chunk of the exhaust valve had gone missing in action, leaving a gaping gap between the valve and the seat.

So, when I inserted the borescope into number six, I fully expected to see some similarly dramatic defect—a swallowed valve, a holed piston, or destructive detonation or preignition damage.

Instead, what I saw was…nothing.

The exhaust valve had an unusually whitish appearance, but no asymmetry and no obvious hot spot. When I opened the valve, the sealing surfaces of the valve and seat also looked whitish but undamaged. The piston crown looked fine, with normal combustion deposits but no evidence of corner melting or detonation signatures. The nickel-carbide cylinder barrel appeared pristine, with no signs of pitting, vertical scoring, or debonding. No head cracks. Nothing to see here, folks.

Disbelieving, I repositioned the piston to top dead center and repeated the compression test.

Still zero, zip, nada. I could hear air coming from the tailpipe, and I could hear air coming from the oil filler.

I was dealing with a classic diagnostic dilemma: When the test results and the visual inspection don’t agree, which one do you believe?

I wrote: “RE #6: 0/80 exh+rings (?).”

Zero Day

That night, cylinder number six kept me awake. Failures that explain themselves are easy. Failures that don’t are the ones that keep you up at night.

The next morning—“Zero Day”—I decided to tackle the worst cylinder first. I really hate pulling cylinders unnecessarily, especially when the borescope shows no obvious defects. Pulling a jug is expensive, invasive, risky, and often avoidable.

So, I decided to try lapping the exhaust valve, even though it didn’t appear burned. I figured the likelihood of success somewhere between slim and none but decided to try it anyway. If the valve wasn’t seating properly for some subtle reason—deposit buildup, sticking, minor deformation, or irregularity—lapping might restore the seal enough to tell me something useful.

After about two hours of careful work, I repeated the compression test.

The reading improved—to 10/80. Now no leakage was audible at the tailpipe, but lots was audible at the oil filler. The leakage was now all past the rings. Interesting information but not exactly cause to take a victory lap.

Gotta come off

The rest of the day was spent doing all that dreadful stuff: removing exhaust and induction components, pulling baffles, disconnecting fuel lines, and ultimately removing the cylinder and piston.

Once you’ve opened up a big hole in the side of the crankcase where the cylinder used to be, you’re pretty much compelled to stick your head in the hole and inspect the cam and lifters. Regulations do not require this, but ethics does.

The cam lobes and lifter faces looked fine. The lobes passed the fingernail test. Whew!

With the number six jug and piston on the bench, I did a thorough visual inspection. The barrel looked beautiful. The rings were free in their grooves. The piston showed no signs of damage. I saw absolutely nothing that explained the zero compression.

I packed up the cylinder and piston and shipped them to Western Skyways in Montrose, Colorado, for evaluation and repair. If there was something wrong, they’d find it.

Fortunately, I had a spare cylinder assembly on hand for exactly this sort of eventuality. So, on day three of my annual ordeal, I installed the replacement cylinder assembly, taking care to liberally lubricate the threads, install brand new hold-down nuts, and meticulously follow the torque specs and tightening sequence in the Continental manual. Induction, exhaust, and fuel system components were reassembled and torqued to spec. The dreaded dastardly deed was done.

Then I turned my attention back to the cylinders that still had a chance.

When minimally invasive works

Cylinder number three on the right engine had measured 33/80 hot and now read 21/80 cold. I removed the rocker cover, pulled the exhaust rocker and springs using the rope trick (see “On Course: Rope Trick,” May 2026 AOPA Pilot), and went to work lapping the valve.

This time, everything behaved exactly the way it should. The compression improved to 65/80 cold, with no audible leakage past the exhaust. That almost certainly meant it would test in the 70s when hot. I reassembled the valve train with a new rotocoil and moved on.

Next was cylinder number two on the left engine. Originally 46/80 hot, now about 40/80 cold. A bit of lapping brought it up to 70/80 cold—another outstanding result.

Both cylinders were salvaged without removal. This is exactly why minimally invasive techniques deserve a place in every mechanic’s toolkit.

But it’s also important to understand why they worked here. In both cases, the problem was confined to the valve interface and was caught early, before significant metal erosion or warping occurred. Once the sealing surfaces of the valve and seat were cleaned and restored by lapping with valve grinding compound, the cylinder performed normally.

Cylinder number six was a whole different kettle of fish.

The phone call

My cellphone rang. It was a technician from Western Skyways.

“I’m working on the cylinder you sent us,” he said, “and we’re finding quite a bit wrong. The cost of repair is getting close to an overhaul. Do you want us to go ahead and overhaul it?”

“Yes,” I said. “And please send me the work order—I’d like to see exactly what you found.”

Usually the borescope tells the story—but sometimes it doesn’t. Some defects simply don’t show up until the cylinder is disassembled, cleaned, measured and inspected in ways we can’t do in situ.

Lessons

This experience reinforced several important lessons.

First, borescopes are indispensable. Without them, we’d be making decisions blind, often removing cylinders unnecessarily.

Second, minimally invasive techniques like in-place valve lapping and solvent ring flushes can be remarkably effective. In this case, lapping saved two cylinders that might otherwise have been removed.

Third, diagnostics is about pattern recognition and judgment. The goal isn’t just to fix things—it’s to understand what you’re seeing and make the least invasive, most effective decision.

And finally, there are limits. Not every defect is obvious or even visible. Not every cylinder can be saved. Sometimes the only way to find the truth is to take things apart.

In the immortal words of Ralph Waldo Emerson and Sam Elliott, “sometimes you eat the bear, and sometimes the bear eats you.”

I guess two out of three ain’t bad.

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