How to Look Inside an Engine Cylinder

The most practical way to look inside an engine cylinder is to insert a borescope (inspection camera) through the spark plug, glow plug, or injector opening on a cool, disabled engine, then rotate the crank by hand to inspect the piston crown, cylinder walls, and valves. This method provides a quick, non-invasive view to diagnose scoring, carbon buildup, detonation, or coolant intrusion without removing the cylinder head.

Why You Might Need to Inspect a Cylinder

Technicians and DIYers inspect cylinders to confirm suspected mechanical issues such as low compression, misfires, oil consumption, overheating damage, or coolant loss. A visual check can reveal crosshatch wear, ring or piston damage, valve issues, or cracks—evidence that guides whether to continue diagnostics, plan a repair, or avoid unnecessary teardown.

Tools and Preparation

You’ll need specific tools to safely access the cylinder and capture clear images. This preparation helps you avoid introducing debris or damaging threads and seals during inspection.

• Borescope/videoscope with 3.9–5.5 mm camera head (articulating or dual-lens models help see cylinder walls and valve faces)
• Socket set and torque wrench (for spark plugs, glow plugs, coils, and injectors as applicable)
• Compressed air or a hand blower and lint-free wipes to clear debris before removing plugs
• Dielectric grease for coil boots; new spark plug/injector/glow plug sealing washers or gaskets if required
• Battery terminal wrench; OBD-II scanner to clear codes after work
• Shop light and fender cover; safety glasses and gloves
• Optional side-view mirror tip for the borescope or a 90-degree camera adapter

Having the right scope and following a clean procedure reduces the chance of contamination and ensures you capture diagnostic images that are worth the effort.

Common Access Points by Engine Type

The safe entry point for your borescope depends on whether you have a gasoline or diesel engine and how the engine is configured.

• Gasoline engines (most cars): Spark plug hole is the standard access point for each cylinder.
• Gasoline direct-injection (GDI): Still use the spark plug hole; injector ports are not typically used for inspection.
• Diesel engines: Glow plug hole is ideal if accessible; otherwise, use the injector bore (requires injector removal and new sealing washer).
• Small engines/motorcycles: Spark plug hole or, if applicable, exhaust port (two-stroke) with the piston positioned appropriately.

Using the intended service access point helps maintain sealing integrity and minimizes the risk of thread damage or sealing failures on reassembly.

Step-by-Step Procedure

Follow these steps to perform a safe, effective cylinder inspection without removing the head.

1. Ensure the engine is cold. Disable auto start-stop and hybrid systems; disconnect the negative battery terminal.
2. Relieve fuel pressure if you’ll disturb injectors; otherwise, leave the fuel system intact for spark/glow-plug access.
3. Remove engine covers and ignition coils or glow plug connectors. Label connections if needed.
4. Blow debris from around the plug or glow plug with compressed air to prevent contamination.
5. Remove the spark plug or glow plug carefully. For diesels, follow manufacturer procedures to avoid snapping glow plugs (work only on a cold engine; use penetrating oil if needed).
6. Rotate the engine by hand using a socket on the crank pulley to move the piston a little below top dead center (TDC), creating lens clearance.
7. Insert the borescope gently. Start with low light to reduce glare; increase brightness as needed.
8. Inspect the piston crown for carbon buildup, pitting, melted areas, or foreign object damage. Capture photos/video.
9. Use an articulating or side-view lens to check cylinder walls for crosshatch, scoring, or rust around the full circumference. Rotate the crank slightly to see more area.
10. Tilt the scope upward to view the intake and exhaust valve faces and seats where possible.
11. Withdraw the scope, then reassemble: install plugs with proper torque, reconnect coils/sensors, and replace any seals/washers. Typical torques: M12 spark plug 10–15 Nm; M14 with gasket 18–25 Nm; glow plugs vary (often 10–20 Nm). Always confirm your vehicle’s spec.
12. Reconnect the battery, clear any diagnostic codes set during the procedure, and verify normal operation.

Working methodically minimizes risk and ensures you examine each cylinder consistently, capturing comparable evidence across the engine.

What to Look For During Inspection

Visual cues can confirm or rule out mechanical problems—and help you estimate the urgency of repairs.

• Crosshatch pattern: Light, even crosshatching suggests normal wear; a polished “glazed” surface can indicate blow-by or poor ring sealing.
• Vertical scoring: Deep lines suggest ring or debris damage; widespread scoring often aligns with low compression and oil consumption.
• Carbon on piston crown: Even, dry soot is common; heavy deposits, oil-wet carbon, or baked-on chunks point to oil control issues or rich running.
• Detonation/pre-ignition signs: Peppering (pitting), melted edges, or aluminum specks indicate knock or severe pre-ignition—requires immediate attention.
• Coolant intrusion: Steam-cleaned piston areas, sweet-smelling residue, or rust at the top ring area may indicate head gasket or casting cracks.
• Valve condition: Chips, tuliping, or heavy deposits on valve faces/seats can cause misfires or low compression.
• Rust or water marks: Suggest storage issues, coolant leaks, or water ingestion.
• Diesel specifics: Wet fuel sheen, cracked piston bowls, or glow plug tip damage require further investigation.

Documenting these findings with photos tied to cylinder numbers strengthens your diagnosis and aids repair decisions and warranty discussions.

Safety and Common Pitfalls

A few practical precautions prevent costly mistakes during inspection.

• Never crank the engine with the scope inserted; always rotate by hand.
• Avoid dropping adapters or debris into the cylinder—use clean tools and a parts tray.
• Use the correct-diameter camera head; forcing a large probe can score walls or damage threads.
• Mind torque specs and thread condition; avoid anti-seize on plated spark plugs unless the manufacturer specifies it.
• On diesels, do not muscle stuck glow plugs; use proper procedures or a specialist to prevent breakage in the head.
• Replace injector/glow plug sealing washers when removed to prevent compression and fuel leaks.
• Confirm the engine is cold; hot components can damage the scope and cause burns.

Attention to these details makes the inspection clean, safe, and unlikely to create new problems.

Alternatives and Complementary Diagnostics

Visual inspection pairs well with pressure-based tests to quantify cylinder sealing.

• Leak-down test: Identifies where compression is escaping (intake, exhaust, crankcase, or coolant) and correlates with what you see on camera.
• Compression test: Quick screening for cylinder balance, though less diagnostic than leak-down plus visual.
• Vacuum waveform analysis: Advanced method to infer valve and ring issues without disassembly.
• Cooling system pressure test: Helps confirm suspected coolant intrusion when you see steam-cleaned pistons.
• Head removal: The definitive but invasive option when cracks, severe scoring, or valve failure are suspected.

Combining test results with borescope images leads to higher-confidence decisions and better repair planning.

Choosing a Borescope Today

Modern borescopes are affordable and capable. For automotive work, a 3.9–5.5 mm camera with 1080p resolution, adjustable LED lighting, and either an articulating tip or a dual-lens/side-view adapter yields the best coverage. Standalone units are rugged and shop-friendly; USB/phone-based models are cheaper but ensure they’re oil-resistant and have good brightness control.

Summary

To look inside an engine cylinder, use a borescope through the spark plug, glow plug, or injector opening on a cold, disabled engine, rotating the crank by hand to survey the piston, walls, and valves. Prepare cleanly, choose an appropriate scope, follow careful removal and reinstallation steps with correct torque, and pair your visuals with leak-down or compression tests. The combination of clear images and quantified data provides a reliable basis for repair or further teardown decisions.

How to inspect cylinder walls?

Anything. So now let’s knock it. Down inch or so check it again. And again we’re good here. We don’t see any light anywhere between the ring and the cylinder wall. Knock it down a little more.

How to diagnose a bad cylinder?

Fortunately, they’re pretty easy to diagnose if you have access to the coils. If you’ve had enough sputtering to trigger a check engine light, an automotive diagnostic tool or scanner that plugs into the OBD2 port (for cars 1996+) can save you some trouble and tell you exactly which cylinder poses the problem.

What to look for when inspecting engine cylinders?

You should look for long gouges that align with the piston. The cylinder walls need to be very smooth and should have some hatch marks, which are light scrape lines at about 25 degrees off parallel with the waterline. Any lines, scrapes, or gouges that run up and down with the pistons are concerning.

How to check cylinders with a borescope?

And then I’ll take the prop. And turn it. Until I see the piston. Go all the way to the bottom. And so right there I have the piston all the way at the bottom. And I’m gonna rotate the scope. Around.