What does a borescope inspection show?
Updated · Wingman AeroCare, KFNL
Short answer: A borescope is a small camera on a flexible or articulating probe, inserted through a spark plug hole, that lets a mechanic look inside a cylinder without removing it. It shows the condition of the exhaust and intake valves, the cylinder walls, the piston crown, and the combustion chamber, revealing burned or burning exhaust valves, corrosion, scoring, and cracks that a compression test can miss. It is the best non-invasive way to decide whether a cylinder actually needs to come off.
What can you see inside a cylinder?
| Area | What a borescope can reveal |
|---|---|
| Exhaust valve face | Uneven heat patterns that indicate a valve is not seating evenly and may be beginning to burn |
| Valve seats | Erosion, recession, or poor sealing |
| Cylinder walls | Remaining honing cross-hatch, scoring, scuffing, glazing, rust, and corrosion pitting |
| Piston crown | Heavy carbon or lead deposits, erosion, and signs of detonation |
| Combustion chamber and head | Cracks, often near spark plug holes and valve seats |
| Foreign objects | Debris or damage from something entering the cylinder |
Why is the exhaust valve the most important thing to look at?
Exhaust valves run extremely hot. A valve that does not seat evenly develops a hot spot, and a valve with a hot spot can progress to burning, which can lead to a loss of compression or, in the worst case, a valve failure. On a borescope image, a healthy exhaust valve shows a fairly even color pattern around its face. A valve beginning to fail often shows an asymmetric or "pie slice" discolored area. Catching that early can sometimes mean a valve repair instead of a cylinder replacement, and it can prevent an in-flight failure.
A compression test can read normal while this is developing, which is why a borescope adds real value even to a cylinder that passed.
Is a borescope inspection required at an annual?
14 CFR Part 43 Appendix D does not specifically require a borescope inspection. It requires checking cylinder compression and internal engine condition. However, Continental's published cylinder evaluation guidance (Service Bulletin SB03-3) pairs the differential pressure test with a borescope inspection, and many shops and owners treat a borescope look at every cylinder during each annual as standard good practice.
When is a borescope inspection especially valuable?
- After a low compression reading, to find out why before deciding to pull a cylinder.
- On engines that sit a lot. Aircraft that fly rarely are prone to rust on cylinder walls and cam surfaces.
- On high-time engines approaching the manufacturer's recommended overhaul interval, to help decide whether to keep flying. See when to overhaul your engine.
- During a pre-purchase inspection, where it is one of the most useful engine tools available to a buyer.
- After an engine monitor shows an anomaly, such as a cylinder with unusual EGT or CHT behavior.
What else can a borescope look at?
The same tool is used beyond cylinders:
- Inside wings and tail structures for corrosion.
- Inside fuel tanks and around fuel bladders.
- Behind panels and into areas that are expensive to open.
- Into the crankcase on some engines, through openings such as the oil filler, to get a partial look at cam lobes and lifters. This view is limited and varies by engine model.
What does a borescope not show?
It cannot measure dimensions such as cylinder wall taper or valve guide clearance, cannot see most bearing surfaces, and cannot test function. It shows what is visible. Combine it with compression readings, oil analysis, oil filter inspection, and engine monitor data for a complete picture.
Why ask for the images?
Modern video borescopes record still images and video. Keeping them gives you a baseline to compare at the next annual, evidence if you sell the aircraft, and a clear way to see for yourself what the mechanic is describing.
How Wingman handles it
At Wingman AeroCare, findings go on your online estimate with photos and video, so borescope images of anything we flag come to you along with the recommendation and its label: Airworthy, Recommended, or Optional. We work on Lycoming and Continental engines and do cylinder and top-end work in-house. If you are buying, Phase 1 of our ClearBuy pre-purchase inspection covers the engine and records first, with a stop/go decision point before the airframe phase. See piston engine maintenance or contact us.