Borescope Inspection: 2026 GA Owner’s Cylinder Guide

Last Updated: June 30, 2026 | By E3 Aviation Editorial Team

Your A&P pulled a spark plug and slid the camera into your number three cylinder. He clicked through six grainy photos. You nodded along because that is what you do at annuals. Then he wrote “cylinders normal” on the work order and you paid the bill. Here is the problem with that transaction. A borescope inspection is the single most useful diagnostic tool on your engine. And most owners cannot read the images.

This is the guide that fixes it. We’ll walk through what the inspection actually shows. What healthy and unhealthy cylinders look like. Which deposits and patterns matter. And why every serious shop now treats the borescope as the gold standard over a compression test.

By the end you’ll know what to ask for at your next annual. What to push back on. And how to spot a developing exhaust valve problem six months before it grounds your airplane.

What a Borescope Inspection Actually Is

A borescope inspection is a visual examination of the inside of each engine cylinder using a small camera. The probe goes in through the top spark plug hole. The technician removes one plug per cylinder and threads in the flexible probe. The camera then photographs the exhaust valve, intake valve, piston crown, and cylinder bore wall.

That’s the whole procedure. It takes an hour for a six-cylinder Continental. About forty-five minutes for a four-cylinder Lycoming. No teardown. No engine pull. No guesswork from oil analysis alone.

What makes it valuable is what it sees. A compression test pumps eighty PSI into a cylinder and measures the leak. It tells you that something is leaking. It does not tell you what, where, or how bad. A borescope inspection shows you the actual surface condition of the valves, the wall, and the piston dome. In high resolution. That’s the difference between a number on a worksheet and a photograph you can act on.

Here’s what most owners don’t realize. The FAA and the engine manufacturers now treat the borescope image set as the primary diagnostic. Leading maintenance analysis firms do too. The compression number is secondary. We’ll get to the regulatory side later in this guide. For now, just know that the borescope is no longer optional at any serious annual.

The Tools and Why Cheap Scopes Lie to You

You can buy a scope on Amazon for forty dollars. Don’t. The tools used for a real borescope inspection are different in three important ways.

The cheap units sold for plumbing or automotive work have a fixed-focus lens optimized for objects ten feet away. Inside a piston cylinder, the bore wall sits less than an inch from the camera. A plumbing scope returns a blurry blob. The very details a borescope should reveal are gone. Hairline cracks, the crosshatch microfinish on the wall, the bullseye pattern on a valve face — all of it lost.

An aviation-grade scope needs three things. A macro lens with a focal range somewhere between ten and thirty millimeters. A high-resolution sensor, at least 1080p on the chip itself. An articulating tip. So you can steer the camera up at the valves and down at the piston crown. Without removing and reinserting the probe.

The good news is the price has crashed. A smartphone-tethered probe with the right lens specs runs about three hundred dollars in 2026. The Vividia and Teslong aviation models in that price range produce images that rival shop scopes from a decade ago. Owners who run the inspection themselves between annuals recover the tool cost in one avoided valve job. More on that later.

One more rule. Never insert a borescope into a hot cylinder. Camera sensors start failing at around 140 degrees Fahrenheit. Plastic lens housings melt above that. Run up to operating temperature, shut down, then let the engine sit at least an hour before any inspection. If you can’t rest your bare hand on the cylinder head, the scope stays in the bag.

Reading Exhaust Valves: The Bullseye and the Burn

The exhaust valve is where most cylinders die first. It’s also where a borescope inspection earns its keep. Heat plus combustion deposits attack that valve face every flight. The pattern you see on the face tells you almost everything about how it’s wearing.

A healthy exhaust valve shows a symmetrical, concentric ring of light brown or tan deposits radiating out from the center. Mike Busch and the Savvy Aviation team call this the bullseye. It means the valve is rotating in its seat the way it should. Every revolution wipes deposits evenly around the circumference. Bullseye equals healthy. Bullseye equals fly it.

What you’re looking for as the problem signal is asymmetry. If one side of the valve face is darker, lighter, or greenish-tinted, the valve has stopped rotating. The rest of the face will look different. A non-rotating valve develops localized hot spots. The same patch of metal sees the same exhaust gas flow on every cycle. That patch overheats. Metal erodes. Eventually the valve burns through and you lose the cylinder in flight.

The Three Stages You Need to Know

The Savvy Aviation analyst team holds over 100,000 borescope image sets in its database. They classify exhaust valves into three buckets. Memorize these. They’re how every modern shop talks about valves.

Normal. Symmetrical bullseye. Tan, light brown, or even gray-white deposits, but evenly distributed. Fly it.

Early or mid-stage uneven heat pattern. A bell-shaped or asymmetric pattern starts to emerge on one side of the face. There may or may not be green coloration at the center of the bell. Without green, the valve can often be lapped at the next opportunity and the cylinder saved. With green showing through, the valve is past the lap-and-save window.

Advanced uneven heat pattern. Two distinct hot spots roughly 180 degrees apart. Usually with green discoloration at the edge of the valve where each hot spot lives. The classic image looks like two letter C shapes back to back. This valve has been run beyond redline CHT at some point. Pull the cylinder. Don’t fly it.

Honestly, this is where we’d push back on shops that gloss over a borescope inspection at annual. If your A&P hands you one grainy photo per cylinder and says “looks good,” push back. Ask to see the exhaust valve face on every cylinder. Well lit and in focus. You paid for this. Look at it.

Cylinder Walls: Crosshatch, Glaze, and Scoring

After the valves, the cylinder wall is the second thing the borescope should show you. Three patterns to know.

Healthy cylinder walls carry a visible crosshatch. That’s a grid of fine 45-degree honing marks left by the original cylinder finishing process. The crosshatch holds a thin layer of oil that lubricates the piston rings as they slide. As long as the crosshatch is visible across the whole bore, the wall is doing its job.

Glazed cylinders are the first failure mode. A glazed wall looks like a mirror. The crosshatch has been worn smooth. Usually that’s because the rings never seated properly during break-in. Or because the engine ran too lightly loaded for too long. Glazed cylinders burn oil. They show up as a slow but steady oil consumption climb on your engine monitor or oil tracking log. The fix is sometimes a re-hone in place. Sometimes a full cylinder replacement. Either way, you found it because the borescope caught the missing crosshatch.

Scoring is the failure mode you can’t ignore. Vertical scratches running parallel to piston travel mean something hard is dragging on the wall. Common causes are a broken piston ring or piston pin plug wear. A fragment of carbon caught between ring and wall is another. Light scoring sometimes polishes out. Deep scoring — a scratch you can feel with a fingernail through the spark plug hole — means a teardown. A borescope that catches scoring early can save the case half, the crankshaft, and forty thousand dollars.

The last wall finding is corrosion pitting. Rust appears as dark blotches or pinpricks on the cylinder wall. It happens most often in aircraft that sit. If you put the airplane down in November and pull it out in April, look hard at the lower bore. Your borescope inspection in May should specifically check for rust pitting on the lower portion of each cylinder. The lower bore sits below the piston at rest, holds less oil, and corrodes first. The fix at early stages is fly more and use a dehydrator. At advanced stages, it is new cylinders.

Intake Valves, Heads, and What Most Reports Skip

The exhaust valve gets all the press. But a thorough borescope inspection also covers the intake valve, the piston dome, and the combustion chamber roof. Most quick-shop reports skip these. They should not.

The intake valve runs cooler than the exhaust valve, so it almost never burns. What it does do is collect deposits. A heavy carbon buildup on the intake valve stem and head signals one of three things. The engine is running rich. The oil control rings are letting too much oil past. Or the induction system has a leak dripping fuel onto the cold valve at shutdown. The deposit color matters. Dry black carbon is normal. Wet, oily, gummy deposits are not.

The piston dome shows you combustion quality. You see it by looking down through the spark plug hole with the piston at bottom dead center. A healthy dome is light tan to gray-brown across the whole top surface. Sooty black deposits concentrated on one side mean an uneven fuel distribution problem. Often a clogged or out-of-spec injector. A wet, oily dome means oil is getting past the rings or down through the valve guides.

The combustion chamber roof and spark plug bosses tell you about detonation. A healthy roof has uniform color. Aluminum erosion or a sandblasted-looking surface around the spark plug boss is the classic detonation signature. Catch this and you have a chance to find the root cause before it eats the piston. Bad fuel. Leaned too aggressively. A failed CHT probe masking a hot cylinder.

The Continental SB03-3 Rule and Lycoming’s Position

The reason every modern shop now leans on the borescope is Teledyne Continental Service Bulletin SB03-3. Originally issued in 2003 and reaffirmed multiple times since. The bulletin directs that a borescope inspection of each cylinder be performed at every annual and every 100-hour. And any time a compression test is run. It applies to every TCM piston engine model in the field.

SB03-3 makes one other thing explicit that gets missed. The borescope, not the compression test, is the gold standard for assessing cylinder airworthiness. The compression number is a screening tool. The borescope image is the actual finding. If your Continental-powered airplane goes to annual without borescope images in the workpack, the inspection is incomplete. Under the manufacturer’s own bulletin.

Lycoming is less prescriptive in writing. But the practice is the same across every serious shop running Lycoming engines. The IO-360, O-360, IO-540, and IO-580 families all get the same treatment as their Continental counterparts. Same borescope discipline, same pass-fail framework. Lycoming Service Instruction 1191A covers exhaust valve inspection and shares the same bullseye-versus-asymmetric framework Continental uses.

Here’s what most owners get wrong. They assume the scope work is something the shop chooses to do. A discretionary add-on. For Continental engines, it is not discretionary. It is the manufacturer’s published procedure. Print SB03-3 and bring it to your next annual if you have to.

Doing Your Own Borescope Inspection

FAR Part 43 Appendix A lists the preventive maintenance items a certificated pilot can legally perform. On aircraft they own or operate. Spark plug removal and replacement is on the list. Once the plug is out, looking at the inside of the cylinder with a borescope is observation. It’s not a maintenance action that requires sign-off. You’re allowed to do this.

We’ll be straight with you. The value of owner-performed scope work isn’t that you replace your A&P’s annual look. It’s that you build a baseline between annuals and catch a problem at month four instead of month twelve.

The procedure is simple. Run the engine up to operating temperature on a normal flight. Land. Park. Let the engine cool fully — at least two hours, more in summer. Pull one top spark plug per cylinder. Crank the prop by hand to bring each piston down to roughly the middle of the bore. Thread the borescope probe in. Photograph the exhaust valve, the intake valve, the bore wall, and the piston crown. Save the images by date, cylinder number, and component.

The Savvy Aviation team publishes a free Cylinder Borescope Inspection Checklist. It walks you through the sequence one shot at a time. Even if you don’t subscribe to their analysis service, the checklist alone is worth printing. Keep it in your hangar.

Two safety rules. Never crank the prop on an engine where the magnetos are not confirmed grounded and confirmed dead. Even with the master off and the key out, a hot mag will fire. The prop swinging through is all it takes. People die from this every year. And don’t insert the scope at any cylinder that still has a plug on the bottom. Combustion-side gas paths and crank position matter.

The Trend File: Why One Image Set Is Almost Useless

This is the single most important habit to build. A single set of images at one point in time tells you almost nothing about the trajectory of your engine. A series of borescope inspections, captured the same way at three-to-six month intervals, tells you everything.

Here’s the framework every owner should run. Take your first set of borescope images at the next annual. Label every photo by date, tail number, engine serial, cylinder number, and component photographed. Drop the whole set into a labeled folder on your computer and back it up to cloud storage.

Take a second set six months later. Either at the mid-annual point in your owner-performed inspection or at the 100-hour. Compare side by side. Look for the things that move. A new deposit pattern on an exhaust valve. A darkening patch on a cylinder wall. A new score line on the piston dome. A healthy engine looks almost identical from one inspection to the next. Anything that changes meaningfully is your early warning.

Take a third set six months after that. Now you have a trend file. You know which cylinder is the strongest and which is the weakest. You know which exhaust valve is rotating normally and which is starting to drift. You know whether the wall finish is holding or wearing.

The shops and analysts who do this for a living built their entire business model on the trend file principle. Savvy Aviation in particular. You can do it yourself for nothing once you own a scope.

Frequently Asked Questions About Borescope Inspections

How much should a borescope inspection cost at annual?

For a four-cylinder engine, the borescope portion of the annual should add about one hour of shop labor. Roughly 90 to 150 dollars depending on shop rates. For a six-cylinder, plan on 90 minutes to two hours. If a shop is charging 400 dollars or more for the inspection alone, ask for a per-cylinder breakdown. Confirm they are saving and sharing the image set.

Can a good compression test rule out a bad valve?

No. This is the most common mistake owners and even some A&Ps make. A non-rotating valve can show a normal compression number right up until the day it burns through. That’s the trap. The asymmetric heat pattern is visible on the scope months before the compression number changes. The borescope inspection finds the problem early. The compression test confirms it after the damage is already done.

How often should I borescope my engine?

Minimum once a year at annual under Continental SB03-3. The better practice is two to three times a year. One at annual, one at the 100-hour or six-month point, and one any time something feels off. A CHT spike, a new vibration, a sudden oil consumption jump. The marginal cost of a self-performed inspection is essentially zero once you own the scope.

Our Take on Why Every GA Owner Needs This Skill

Reading a borescope inspection is the single highest-leverage maintenance skill a piston-aircraft owner can develop. Here’s the simple math.

A cylinder replacement on a Continental IO-470 or Lycoming O-360 runs roughly five to eight thousand dollars. That’s a 2026 figure. Parts, labor, and the catch-up annual that usually goes with it. A top overhaul on a six-cylinder is fifteen to thirty thousand. A full engine teardown after a swallowed valve runs upward of forty thousand. And that’s before the rental cost while your airplane sits.

Catching a developing exhaust valve problem at the early-stage uneven heat pattern phase usually means a successful valve lap. And another six hundred hours on that cylinder. Catching it three months later, after green appears at the center of the bell, means a cylinder replacement. Catching it six months later means a teardown.

Every owner who learns to read the images gives themselves a chance. To stay on the early end of that curve. Every owner who does not is at the mercy of whoever happens to look at the photos that day. We’d rather know what we’re looking at.

That’s the kind of owner-pilot the E3 Aviation community is built around. People who understand their airplanes well enough to know when something is off. Confident enough to push back on a sign-off that doesn’t match what they see. Join E3 Aviation if that sounds like the community you want around you.

Read Next

• Aircraft Cylinder Compression Test — the partner test to every borescope inspection, and how to read both together.
• Aircraft Spark Plug Maintenance — the plug comes out anyway, so know what it tells you.
• Annual Inspection: GA Owner-Operator Survival Guide — how the borescope step fits the rest of annual.
• Aircraft Engine Oil Change — pair an owner-performed scope check with the oil change for a full midpoint snapshot.
• Aircraft Propeller Overhaul Guide — the other major scheduled engine-area decision every owner faces.
• Aircraft Annual Inspection Cost — what the borescope should add to your annual line items.
• Unleaded Avgas Transition Guide — fuel changes ahead and how they’ll affect valve and deposit patterns.

External Authority References

• Teledyne Continental SB03-3 (PDF) — the manufacturer service bulletin that established the borescope as the gold standard.
• Savvy Aviation Cylinder Borescope Inspection Checklist (PDF) — the printable shot-by-shot checklist every owner should use.
• FAA AC 43.13-1B — acceptable methods, techniques, and practices for aircraft inspection and repair.
• 14 CFR Part 43 Appendix A — the preventive maintenance list defining what owner-pilots can legally do.
• Savvy Aviation Borescope Initiative — the public database of more than 100,000 borescope image sets used for trend analysis.