Compression Testers

About Compression Testers

Compression testers are essential diagnostic tools that enable mechanics, mobile engineers, and vehicle technicians to assess the health and performance of internal combustion engines. By measuring the compression pressure within each cylinder, these precision instruments help identify worn piston rings, damaged valves, blown head gaskets, and other mechanical issues that can affect engine performance and efficiency. Whether you're diagnosing a rough idle, loss of power, or excessive oil consumption, a reliable compression tester provides the critical data needed to pinpoint problems quickly and accurately.

Our comprehensive range of compression testers includes specialist kits for both petrol and diesel engines, from compact 6-piece starter kits ideal for mobile mechanics to extensive 37-piece master kits that cater for virtually every vehicle type. We stock professional-grade compression testing equipment from trusted brands including Sealey and Draper, offering solutions for everything from standard passenger cars to commercial vehicles and plant machinery. Many of our kits include multiple adaptors to suit different spark plug thread sizes and glow plug apertures, along with extended hoses for accessing hard-to-reach cylinders in modern engine bays.

Modern compression test kits have evolved to include digital gauges for precise readings, leakage detection capabilities, and Top Dead Centre (TDC) finding tools that are essential for timing belt replacement and engine rebuilding work. Whether you're running an independent garage, working as a mobile mechanic, or maintaining a fleet of commercial vehicles, investing in quality compression testing equipment pays dividends in accurate diagnostics and reduced comeback jobs.

Jargon Buster

• Compression Ratio: The ratio between the volume of the cylinder when the piston is at bottom dead centre compared to top dead centre. Higher compression generally means more power but requires more precise engine condition.
• PSI (Pounds per Square Inch): The unit of pressure measurement displayed on most compression gauges. Typical petrol engines read between 120-180 PSI, whilst diesel engines produce significantly higher readings of 275-400 PSI due to their compression ignition design.
• TDC (Top Dead Centre): The point at which the piston reaches its highest position in the cylinder. Accurate TDC location is crucial for timing belt fitting and valve timing procedures.
• Leakage Testing: An advanced diagnostic technique that introduces compressed air into the cylinder at TDC to identify where compression is being lost—whether through valves, piston rings, or head gaskets.
• Glow Plug Adaptor: A threaded connector that screws into the glow plug aperture on diesel engines, allowing the compression gauge to measure cylinder pressure. Different thread sizes are required for various vehicle manufacturers.
• Schrader Valve: A one-way valve mechanism (similar to tyre valves) that retains the maximum pressure reading on the gauge, allowing you to record the compression figure after cranking the engine.

Who Uses Compression Testers?

• Independent Garage Mechanics: Essential for general vehicle servicing and diagnostic work across all makes and models
• Mobile Mechanics: Compact kits provide portable diagnostic capability for on-site vehicle assessment
• Fleet Maintenance Technicians: Regular compression testing helps prevent breakdowns and schedule preventative maintenance efficiently
• MOT Testing Stations: Useful for investigating emissions failures and power-related issues
• Plant and Agricultural Engineers: Maintaining diesel engines in construction equipment, tractors, and machinery
• Marine Engineers: Testing compression in boat engines, both petrol and diesel variants
• Classic Car Restorers: Assessing engine condition before purchase and monitoring rebuilds
• Motorcycle Mechanics: Diagnosing performance issues in bike engines
• HGV and Commercial Vehicle Technicians: Specialist kits designed for larger diesel engines in trucks and coaches
• Advanced DIY Enthusiasts: Home mechanics maintaining their own vehicles and undertaking engine work

How to Choose the Right Compression Tester

Engine Type: The most fundamental consideration is whether you're working primarily on petrol engines, diesel engines, or both. Diesel engines generate significantly higher compression pressures and require gauges rated to at least 600 PSI, along with specific glow plug adaptors. Petrol engine testers typically measure up to 300 PSI and use spark plug thread adaptors. Master kits covering both engine types offer the best versatility for general garage work.

Gauge Type: Traditional analogue gauges remain popular for their reliability and no-battery operation, whilst digital compression testers offer easier reading, peak hold functions, and sometimes data logging capabilities. Digital gauges are particularly useful in poorly-lit engine bays or when taking multiple readings across a bank of cylinders.

Adaptor Selection: Check that the kit includes the correct thread sizes for the vehicles you work on most frequently. Common petrol engine threads include 10mm, 12mm, 14mm, and 18mm spark plug sizes, whilst diesel kits should cover both M8, M10, M12 and larger commercial vehicle glow plug threads. Extended adaptors are invaluable for recessed spark plugs in modern engines with deep plug wells.

Hose Length: A longer flexible hose (typically 300-500mm) makes testing easier on transverse-mounted engines and hard-to-reach cylinders. Some professional kits include extension hoses for particularly awkward applications.

Build Quality and Calibration: Professional-grade testers from established brands like Sealey and Draper offer better accuracy, durability, and calibration stability. Quality gauges maintain their accuracy over thousands of tests, whilst budget alternatives may drift and provide unreliable readings.

Additional Features: Consider whether you need leakage testing capability, which helps identify exactly where compression is being lost. TDC location tools are essential if you undertake timing belt work. Quick-release couplings speed up testing when working on multi-cylinder engines.

Popular Accessories

• Additional Thread Adaptors: Extra spark plug and glow plug adaptors for specialist or unusual vehicle applications
• Extension Hoses: Longer flexible hoses for deep-recessed or awkwardly-positioned cylinders
• Remote Starter Switches: Allow solo operation by cranking the engine whilst observing the gauge
• Compressed Air Adaptors: Convert compression testers for leakage testing procedures
• Protective Carrying Cases: Keep your compression testing kit organised and protected between jobs
• Thread Sealant Tape: PTFE tape ensures air-tight connections at adaptor threads
• Battery Chargers: Maintain battery voltage during extended cranking for consistent test results
• Work Lights: Rechargeable LED inspection lamps for viewing cylinder numbers and working in engine bays
• Engine Oil: A small amount introduced into weak cylinders helps differentiate between ring and valve issues

Safety Information

When performing compression tests, always ensure the vehicle is in neutral (manual) or park (automatic) with the handbrake firmly applied and wheels chocked. Disconnect the ignition system on petrol engines by removing the coil pack connections or using the ECU fuse to prevent sparking during cranking. On diesel engines, disable the fuel supply or use the engine stop control to prevent starting.

Wear appropriate PPE including safety glasses to protect against fuel spray or debris from spark plug holes. Be aware that diesel injection systems operate at extremely high pressures—never work on diesel fuel systems with the engine running. Ensure adequate ventilation when running engines in workshops to prevent carbon monoxide build-up, following HSE guidance on workplace exposure limits.

Check that batteries are in good condition before extended cranking, and allow the starter motor to cool between test cycles to prevent overheating. Never lean over the engine during cranking, and keep hands and tools clear of moving belts, pulleys, and fans. If working alone, consider using a remote starter switch rather than having an assistant operate the ignition key.

Frequently Asked Questions

What compression reading should I expect from a healthy engine?

Typical petrol engines should produce between 120-180 PSI, whilst diesel engines generate 275-400 PSI due to their higher compression ratios. More important than the absolute figure is that all cylinders read within 10% of each other—significant variation between cylinders indicates mechanical wear or damage. Always consult the manufacturer's specifications for your specific engine, as turbocharged and high-performance engines may have different normal ranges.

Can I use a petrol compression tester on a diesel engine?

No, petrol engine compression testers typically have maximum ranges of 250-300 PSI, which is insufficient for diesel engines that commonly produce 300-400 PSI or higher. Using an under-rated gauge on a diesel engine risks damaging the instrument and provides no useful reading. Always use a dedicated diesel compression tester with appropriate glow plug adaptors and a gauge rated to at least 600 PSI.

How do I know if low compression is due to piston rings or valves?

Perform a "wet test" by introducing approximately a tablespoon of engine oil into the suspect cylinder through the spark plug hole, then repeat the compression test. If the reading increases significantly (20% or more), the problem is likely worn piston rings, as the oil temporarily seals the ring-to-cylinder gap. If the reading remains similar, the issue is more likely valve or head gasket related, as oil cannot seal these areas.

Do I need to remove all spark plugs when testing compression?

Yes, removing all spark plugs is essential for accurate compression testing. This eliminates the resistance of compressing air in the other cylinders, allowing the starter motor to crank the engine at a consistent speed. Testing with only one plug removed produces artificially low readings and puts unnecessary strain on the starter motor and battery.

How often should compression testing be performed?

Compression testing isn't a routine maintenance item but rather a diagnostic procedure performed when engine problems arise. Common indicators include rough idling, loss of power, poor fuel economy, excessive oil consumption, or failed emissions tests. For high-mileage vehicles or those used in demanding applications, annual compression testing can help identify developing issues before they lead to breakdowns.