What Is a Borescope Inspection? How Cylinder Inspection Works?

A borescope inspection uses a slim camera probe through a spark-plug hole to visually check an engine cylinder’s walls, piston, and valves without removing the cylinder.

A borescope reveals what a compression number cannot: the actual condition inside an engine cylinder. The probe slips through an existing opening — usually the spark-plug hole in an aircraft piston engine — and the camera sends back live images of the combustion chamber, piston crown, cylinder barrel, valves, and valve seats. Nothing is disassembled.

The tool exists because leak tests tell you there is a problem, not where it is. Pairing the visual exam with a differential-pressure compression test lets a mechanic locate the fault: a burned exhaust valve, a scored cylinder wall, or detonation damage on a piston top all look different through the lens. If you’re gathering equipment, this roundup of borescopes for cylinder inspection covers probe sizes and tip styles worth comparing.

How Does a Cylinder Borescope Inspection Work?

A cylinder inspection follows a fixed sequence that puts the piston in the right position for each view, because valve visibility depends entirely on where the piston sits. Lycoming’s official manual lays out the core procedure:

1. Secure the engine. Turn the ignition off, disconnect engine power, and let the engine cool fully.
2. Remove the upper spark plug from each cylinder to open probe access.
3. Set the piston at bottom dead center on the power stroke. The exhaust valve is open here.
4. Insert the borescope through the upper spark-plug hole and inspect the combustion chamber, piston top, cylinder interior, exhaust valve, and exhaust-valve seat.
5. Move the piston to bottom dead center at the end of the intake stroke. Inspect the intake valve and intake-valve seat through the same opening.
6. Log your findings and any corrective action in the engine logbook and inspection checklist.
7. Reinstall the spark plug to the torque and procedure in the engine maintenance manual.

Large reciprocating engines follow the same logic with one tweak: drop the piston to the very bottom of the cylinder so the widest possible stretch of cylinder wall is visible. A right-angle side-view optical tip suits general inspection, while a forward-facing tip gets closer detail on a specific spot. Field reports gathered by AOPA on top-overhaul work echo the value here — a borescope often catches valve distress before a compression test turns it into a removal.

What Do Inspectors Look For Through the Lens?

An inspector compares what the camera shows against the manufacturer’s limits, checking specific surfaces for specific defect patterns.

• Cylinder walls: a uniform honing cross-hatch, plus any corrosion, scoring, scratches, oil pooling, or loss of surface pattern.
• Piston crown: carbon deposits, erosion, detonation craters or pits, broken edges, and oil or water marks.
• Valves and seats: sealing-surface condition, cracks, burning, abnormal coloration, and uneven seating.
• Combustion chamber: rust, deposits, cracks, and uneven deposit distribution.

A borescope finding is not automatically a removal decision. Serviceability comes from the manufacturer’s inspection limits, applicable service bulletins, compression results, operating history, and any follow-up measurements.

When Is a Borescope Inspection Required?

Triggers vary by engine model, and the exact interval is set by that model’s manual — a 100-hour requirement in one Lycoming manual does not establish a universal rule for all engines. Common triggers include:

• Before cylinder removal
• Excessive or repeated high oil consumption
• Low cylinder compression
• Suspected valve sticking
• Suspected worn piston rings or cylinder barrel
• After an engine overspeed
• Scheduled 100-hour or 400-hour inspections, where the specific manual calls for them

The FAA adds an important caveat: a manufacturer’s service bulletin recommending a cylinder borescope does not, by itself, make the inspection universally mandatory. The legal obligation depends on applicable regulations, the current maintenance manual or Instructions for Continued Airworthiness, an Airworthiness Directive, or another FAA-approved method. FAA guidance on borescope inspections spells out that distinction.

Probe diameter, tip direction, access-hole size, piston position, inspection criteria, and recording requirements all have to match the engine maker’s current documentation. The cited procedures cover aircraft piston engines and large reciprocating or gas engines; they don’t set requirements for automotive, diesel, turbine, or industrial engines.

Common, avoidable mistakes: wrong piston position limiting the view, treating one compression number as a full diagnosis, skipping either the intake or exhaust valve, inserting a dirty or damaged optical tip, failing to label images by cylinder, energizing the engine with the probe in place, or applying generic defect thresholds instead of the manufacturer’s own tables.