Aircraft engines are packed tight. Wires, fuel lines, blades, seals, and housings all share very limited space. When you need to inspect inside, the diameter of your borescope probe can make the difference between a smooth check and a frustrating one.

Choosing the right probe size is not just about “small is better.” A thinner scope can go more places, but it may give up image quality, durability, or lighting power. A thicker one may offer a clearer picture but might not fit where you need it most. The goal is to match the diameter to the job, the engine type, and the access points available.

Here are the key factors to think about when selecting a borescope diameter for aircraft engine access.

Access Port Size Comes First

Start with the most basic limit. What is the size of the inspection port or opening you are working with?

Many aircraft engines have dedicated borescope ports. These ports are designed for internal inspection without major disassembly. The diameter of these ports often sets the maximum probe size you can use. If the port is 6 mm, a 6 mm probe may technically fit, but it can be tight. A slightly smaller probe, such as 4 mm or 5 mm, may pass through more easily and reduce the risk of rubbing against edges.

If there is no dedicated port and you are going through a spark plug hole, fuel nozzle opening, or other service point, measure carefully. Never assume. A probe that barely fits can scrape, snag, or damage sensitive parts near the entry.

How Deep You Need to Go

Diameter and length work together. In long, narrow paths, a slimmer probe usually travels more easily.

Inside a turbine or compressor section, you may need to pass through several stages to see the area of concern. A thicker probe can meet more resistance as it moves past blades and vanes. A thinner probe can snake through with less force. This lowers the chance of bumping into edges or getting stuck.

If the inspection area is close to the port, you may be able to use a slightly larger diameter without trouble. For deep internal views, especially in small engines or APUs, going thinner is often safer.

Bend Radius and Flexibility

Aircraft engines are not straight tunnels. You often have to curve around parts to see what you need.

Smaller diameter probes usually have a tighter bend radius. This means they can curve more sharply without damage. That can be very helpful when you need to look behind a blade, around a corner, or into a side cavity.

Larger diameter probes are often stiffer. They may give a steadier image in open areas, but they can struggle in tight turns. If your inspection path includes sharp angles, a thinner probe with good articulation at the tip will give you more control.

Tip Articulation Space

Articulation lets you steer the camera tip up, down, left, and right. This is critical for detailed engine inspections.

But articulation needs space to move. Even if a probe fits through the opening, the tip still needs room to bend once inside. In very tight spaces, a large diameter tip may not be able to fully articulate. That limits what you can see.

A smaller diameter probe usually has a smaller articulation section. This can move more freely in narrow cavities, letting you scan blade edges, seals, and combustion chamber walls more completely.

Image Quality Tradeoffs

It is easy to assume that thinner always means worse image quality, but that is not always true today. Still, physics plays a role.

Larger diameter probes can hold bigger camera sensors and lenses. This can mean sharper images, better depth of field, and stronger lighting. In wide spaces like the combustion section of a large turbofan, a 6 mm or 8 mm probe may provide a clearer overall view.

Very small probes, such as 2.8 mm or 3.9 mm, are great for access, but the image may be a bit less bright or detailed, especially in large cavities. You may need to move closer to the target area to get the same clarity.

Think about what matters more for your task. If you are looking for fine cracks on turbine blades, image detail is key. If you just need to confirm general condition in a tight path, access may be more important than perfect sharpness.

Lighting Power Inside the Engine

Lighting is a big deal in engine inspections. The deeper you go, the darker it gets.

Larger probes often support stronger LED lighting because they have more space for light fibers or built in LEDs. This helps illuminate larger areas like combustion liners or turbine cases.

Smaller probes can still provide good light, but the beam may be narrower or less intense. In very tight areas, this is often fine because the camera is close to the surface. In bigger open areas, you may notice more shadows.

If you often inspect wide internal spaces, a slightly larger diameter may give you better overall lighting performance.

Durability and Handling

Aircraft maintenance environments can be tough. Tools get moved, coiled, and carried between aircraft and hangars.

Thicker insertion tubes are usually more rugged. They can handle repeated use and minor bumps a bit better. Very thin probes are more delicate and need careful handling. Bending them too sharply or pulling them against sharp edges can cause damage.

If your team performs frequent inspections across different aircraft types, balance durability with access. A mid range diameter like 4 mm or 5.5 mm is often a practical compromise.

Risk of Foreign Object Damage

Any tool inserted into an engine carries some risk. A larger probe has more surface area and can more easily touch multiple parts at once.

In tight compressor sections or near delicate seals, a thinner probe reduces the chance of accidental contact. It is easier to guide and less likely to push against blades or liners with force.

No matter the diameter, slow and careful movement is key. But when space is limited, a smaller probe gives you more margin for safety.

Engine Type Matters

Not all aircraft engines are built the same. A large commercial turbofan offers more internal room than a small turboprop or APU.

For large engines, a 6 mm scope may work well for most routine inspections through standard ports. For smaller engines, that same probe might feel oversized. In those cases, 4 mm or even sub 4 mm probes are often better choices.

If you work across multiple engine models, having more than one diameter available can save time and reduce frustration.

Task Specific Needs

Think about what you are trying to find.

If you are searching for general wear, loose hardware, or obvious damage, access and coverage may matter more than ultra high detail. A slimmer probe that can reach more areas might be best.

If you are evaluating fine cracks, pitting, or heat distress, you may want the best image quality and lighting you can get. If the access allows it, a slightly larger diameter probe could give you more confidence in what you see.

Training and Technician Comfort

Do not overlook the human factor. Some technicians are more comfortable handling certain probe sizes.

Very thin probes can feel more flexible and harder to control in open spaces. Thicker probes can feel steadier but harder to steer in tight paths. Proper training with the chosen diameter helps technicians move smoothly and avoid sudden motions that could cause contact inside the engine.

Let your team practice on training aids or non critical inspections before using a new probe size on high value components.

Balancing It All

There is no single perfect diameter for every aircraft engine inspection. The right choice comes from balancing access, image quality, lighting, durability, and safety.

Start with the smallest access point you must pass through. Consider how far you need to travel and how sharply you need to turn. Weigh that against the level of detail required for the inspection. When in doubt, many maintenance teams prefer to go slightly smaller for safety and reach, as long as the image quality still meets their needs.

For expert help choosing the right probe size for your specific aircraft and inspection tasks, USA Borescopes is one of the best options you have. Reach out to them.

About the Author
This guest post was written by an aviation maintenance writer who covers practical inspection tools, smart borescope use, and safer workflows. They create clear, technician friendly content that helps maintenance teams improve accuracy, reduce risk, and make confident decisions when working inside aircraft engine systems.