How to Inspect Engine Cylinders With a Borescope: Step-by-Step

Owning the right borescope is only half the job — a good inspection also depends on technique. Rush it, and you can miss the exact defect you bought the tool to find. Here is a methodical, step-by-step approach to inspecting engine cylinders with a borescope, from prep through what to actually look for once you're inside.

Before You Start

• Let the engine cool. Never insert a probe into a hot engine — you risk damaging the probe's cable and jacket, and burning yourself in the process.
• Clean around the access point. Wipe down the area around the spark plug or glow plug hole before removing it, so dirt and debris don't fall into the cylinder when you open it.
• Have the right tool ready. Confirm your probe diameter clears the plug hole and, if you're inspecting cylinder walls specifically, that you have articulation or a mirror attachment — see our guide to choosing a borescope for engine inspection if you're still deciding.

Step 1: Remove the Spark Plug (or Glow Plug)

Use the correct socket size for your engine and remove the plug carefully, keeping track of which cylinder it came from if you plan to inspect more than one — labeling as you go saves confusion later. Set plugs aside in order rather than in a pile.

Step 2: Bring the Piston to a Known Position

Where possible, rotate the engine (by hand, using the crankshaft pulley bolt, or by bumping the starter briefly with plugs out) so the piston in the cylinder you're inspecting is at or near top dead center. This gives you a consistent, repeatable view of the piston crown and reduces the chance of the probe contacting a moving part. If you are not confident performing this safely on your specific engine, consult a repair manual for the correct procedure first.

Step 3: Insert the Probe Slowly

Feed the probe into the plug hole gently — do not force it. If you feel resistance, back off and adjust the angle rather than pushing through it. Watch the screen as you go rather than inserting blind; this lets you see the interior wall as you pass it and helps you understand the geometry of the space.

Step 4: Orient Yourself

Once inside, take a moment to identify what you're looking at before evaluating anything. In a typical cylinder view you should be able to make out:

• The piston crown (top surface of the piston)
• The cylinder wall surrounding it
• The valves, if the piston position allows a view of the cylinder head

If the image is dark or hard to interpret, adjust your probe's LED brightness before assuming something is wrong — overly bright LEDs can wash out close metal surfaces just as much as too little light can hide detail.

Step 5: Scan the Cylinder Wall Methodically

If you have an articulating probe, sweep the tip in a deliberate pattern — for example, working around the circumference in one direction — rather than jumping around randomly. If you're using a fixed tip or mirror attachment, rotate and slightly reposition the cable in stages, pausing to look at each new section before moving on.

The goal is coverage: you want to have looked at the full circumference of the wall, not just whatever happened to be facing the probe when you stopped.

Step 6: Check the Piston Crown and Valve Area

With the piston at or near top dead center, examine the crown surface and, if visible, the valve faces. This is where carbon buildup is most obvious and where you would typically spot a bent valve or obvious mechanical damage.

Step 7: Record What You See

If your borescope supports photo or video capture, use it — a saved image lets you compare cylinders side by side, review at higher zoom afterward, and document findings if you're reporting to a customer or keeping maintenance records. Note which cylinder each image corresponds to as you go; it is easy to lose track after the third or fourth cylinder.

Step 8: Withdraw the Probe Carefully

Pull the probe back out slowly and along the same general path you inserted it, rather than yanking it out at an angle — this reduces strain on the cable and tip.

Step 9: Reinstall the Plug and Repeat

Reinstall the spark or glow plug to the correct torque spec before moving to the next cylinder, and repeat the process. Inspecting all cylinders in one session, rather than just the one you suspect, gives you a baseline for comparison — a cylinder that looks abnormal is far easier to identify when you've seen what "normal" looks like on its neighbors.

Making Sense of What You Saw

Once you have a clear view and some saved images, the next step is interpreting them — distinguishing normal wear patterns from genuine problems like scoring, cracking, or excessive carbon buildup. We cover that in detail in how to read borescope images.

Next Steps

If you're still deciding on hardware, our guide to the best borescope for engine inspection covers what to look for, and articulating vs non-articulating borescopes explains whether you need a steerable tip for this kind of work.

Ralcam's automotive borescope cameras are built and sized for exactly this job, direct from our factory in Shenzhen. If you run into something unusual during an inspection and want a second opinion, feel free to send us a photo.