One of the most powerful diagnostic tools available to the aviation maintenance technician is the borescope — a precision optical instrument that allows internal inspection of engine cylinders, combustion chambers, and other enclosed spaces without removing the engine or tearing it apart. For piston aircraft engines, borescope inspections can reveal critical defects such as cylinder wall scoring, rust, pitting, carbon buildup, and valve damage long before those issues escalate into in-flight failures. Understanding how to perform a thorough, systematic borescope inspection is an essential skill for any AMT working on powerplant systems.
The borescope technique bridges the gap between a simple visual walkaround and a full engine teardown. It is neither a substitute for overhaul when overhaul is due, nor a replacement for proper operational monitoring — but it is an indispensable tool for condition-based maintenance decisions, pre-purchase inspections, post-incident evaluations, and routine annual or 100-hour inspection programs.
How Borescopes Work
A borescope is essentially a rigid or flexible tube with an illumination source and an optical or digital imaging system at its tip. Light is projected into the cylinder through fiber-optic bundles or LED sources, and the reflected image is transmitted back to the technician's eye or to a digital display screen. Modern video borescopes (also called videoscopes) capture images and video that can be stored, annotated, and compared across inspection intervals — an enormous advantage for tracking the progression of a defect over time.
The two primary configurations are the rigid borescope and the flexible borescope. Rigid borescopes deliver superior image clarity and are well suited for straight-line access paths, such as through a spark plug port. Flexible borescopes use articulating tips controlled by the technician and can navigate around obstructions, making them more versatile but sometimes slightly lower in resolution compared to rigid instruments of equivalent quality. For cylinder inspections on most horizontally opposed piston engines, a rigid borescope inserted through the spark plug hole is the standard approach.
Preparation and Safety
Before beginning any borescope inspection, safety and preparation are paramount. The engine must be completely shut down and cooled. Hot cylinder heads can cause severe burns and can also damage the borescope's optical components. Allow adequate cooling time — typically several hours after shutdown, especially after a long flight. The magneto ignition system must be confirmed OFF and the mixture pulled to idle cutoff to eliminate any possibility of inadvertent engine start during the procedure.
Remove the spark plugs from the cylinder being inspected, and it is common practice to remove all spark plugs on the engine, not just those on the cylinder under inspection. Removing all the plugs reduces cylinder compression, which makes it much easier to rotate the crankshaft by hand to reposition each piston for different viewing angles, and it also gives the borescope a clear port to enter while helping loose debris or dirt escape rather than settling deeper in the cylinder. Use a proper engine rotation tool on the propeller flange or propeller, always following aircraft-specific procedures, and always keep clear of the propeller arc.
Clean the area around each spark plug port before removal to prevent debris from falling into the cylinder. Compressed air can be used carefully to blow away loose contamination. Have the borescope, a light source, and recording equipment ready before the first plug comes out, minimizing the time cylinders are open to contamination.
Inspection Procedure
Insert the borescope through the spark plug port and position the piston as specified in the applicable engine manufacturer's borescope inspection procedure — commonly near bottom dead center (BDC) to expose more of the cylinder wall for viewing, though the exact piston position, and whether it changes for different viewing zones, varies by engine model and manufacturer guidance. Systematically rotate the borescope to scan the full 360-degree circumference of the cylinder bore. Move the borescope in and out along the cylinder axis to examine different zones — from the top of the bore near the combustion chamber to the lower barrel near the skirt. Then rotate the crankshaft to position the piston near top dead center (TDC) to examine the combustion chamber, the valve faces, the valve seats, and the top land area of the piston.
A structured inspection should cover the following areas in sequence: cylinder walls (for scoring, corrosion, and rust), piston crown (for burning, erosion, and carbon deposits), combustion chamber (for carbon buildup and hot spots), intake valve and seat (for burning, pitting, and carbon deposits), and exhaust valve and seat (for burning, recession, and guttering). The exhaust valve and seat deserve particularly close attention because they operate at the highest temperatures and are most vulnerable to heat damage.
What to Look For
The borescope reveals a range of conditions, each with distinct maintenance implications:
- Cylinder wall scoring: Vertical scratches or grooves in the bore indicate that metal-to-metal contact has occurred, often due to insufficient lubrication, foreign object damage, or overheating. Light scoring may be within serviceable limits defined by the engine manufacturer's overhaul manual; deep or widespread scoring typically requires cylinder removal and possible replacement.
- Rust and corrosion: Reddish-brown surface rust is common in engines that sit unused for extended periods. Light surface rust can sometimes be cleared by running the engine, but pitting — surface corrosion that has eaten into the base metal — is a more serious finding. Pitting in the cylinder bore can disrupt the oil film, accelerate wear, and potentially cause ring damage.
- Carbon deposits: A moderate, even layer of dry carbon on the piston crown is normal. Heavy, uneven, or wet (oily) carbon deposits suggest incomplete combustion, oil consumption issues, or improper mixture management. Carbon buildup on valve seats can prevent proper sealing, leading to compression loss and potential valve burning.
- Valve burning and guttering: A burned valve shows localized erosion or notching of the valve face, typically caused by improper seating, overheating, or operating lean of peak under conditions the engine was not designed for. Guttering is a severe form of valve burning where hot combustion gases have cut a channel through the valve face, allowing blowby. This finding requires immediate grounding and cylinder removal.
- Top land deposits and ring belt condition: The area immediately above the top compression ring is prone to carbon accumulation. Heavy, hard deposits can cause the top ring to stick in its groove, reducing compression and accelerating wear.
Correlating Findings with Other Data
Borescope findings should never be interpreted in isolation. Correlate what you see with compression test results, oil analysis data, engine logbook history, and the operator's reports of engine behavior. A cylinder showing pitting may also show reduced differential compression; a burned exhaust valve will often be flagged first by a low compression reading on a differential compression test. The combination of low compression plus confirmed valve burning on borescope is a clear, well-documented basis for a maintenance decision.
Record all findings with photographs or video captures from the videoscope. Mark which cylinder, which port (top or bottom spark plug), and the piston position (BDC or TDC) at the time of each image. Document measurements and descriptions in the aircraft maintenance record according to 14 CFR Part 43 requirements, including what was inspected, what was found, and what action was taken or recommended.
Key Numbers and Rules
- Serviceability limits for cylinder bore wear, scoring depth, and corrosion are specified by the engine manufacturer's overhaul or maintenance manual — not by general FAA standards. Always consult the applicable manual.
- Differential compression testing (typically performed with 80 psi input) should accompany borescope inspections; a reading below 60/80 psi is a commonly cited minimum serviceable value drawn from engine manufacturer guidance (such as Lycoming and Continental compression testing publications) rather than a fixed FAA standard, and the exact threshold and test method vary by manufacturer and master orifice tool used — always confirm against the applicable manufacturer limit.
- Borescope inspections are commonly performed at 100-hour intervals or annually as part of the annual inspection, and after any event that could indicate internal engine damage — such as propeller strike, overspeed, or oil pressure anomaly.
- Per 14 CFR Part 43, any maintenance performed, including inspections, must be recorded in the aircraft maintenance records with the date, description of work, and the signature and certificate number of the performing technician.
- Engine manufacturer overhaul manuals and service bulletins are the authoritative references for accept/reject criteria on all internal engine findings.
Common Test Traps
- Confusing scoring with normal wear: Normal cylinder wear produces a uniform, smooth surface. Scoring produces distinct vertical scratches. The FAA expects AMTs to recognize the difference and understand that any significant scoring requires manufacturer guidance before returning the engine to service.
- Assuming surface rust is always rejectable: Light surface rust from inactivity is not automatically cause for rejection. The key is whether underlying pitting is present. The inspection must distinguish surface discoloration from actual metal loss.
- Overlooking the exhaust valve: Test questions often focus on which component is most vulnerable to heat damage. The exhaust valve operates hotter than the intake valve and is the most common site of burning and guttering — always inspect it carefully.
- Skipping correlation with compression data: The FAA emphasizes that borescope findings must be evaluated in context. A borescope result alone is rarely the sole basis for a maintenance action; compression test data, oil analysis, and operational history all contribute to the decision.
- Improper documentation: Many AMT candidates underestimate the recordkeeping requirement. Under 14 CFR Part 43, both the inspection findings and any maintenance action (or decision to defer action) must be properly recorded. Failure to document is itself a regulatory violation.