Compression testing is one of the most useful ways to gauge a used engine’s internal health. Understanding the numbers helps you interpret what a seller tells you.
What Is Engine Compression Testing?
A compression test measures the pressure each cylinder builds during the compression stroke. Healthy, even readings across cylinders suggest good sealing from the pistons, rings and valves; low or uneven readings can indicate wear or damage.
What the Readings Tell You
Even Readings
Consistent compression across all cylinders is a good sign of internal health.
Low Readings
Low compression in one or more cylinders can point to worn rings, valve issues or head gasket problems.
Uneven Readings
Large differences between cylinders warrant further investigation.
Why It Matters When Buying
- It’s an objective health check beyond visual inspection.
- It can reveal issues before you buy and install.
- Combined with mileage and history, it builds confidence.
Frequently Asked Questions
What’s a good compression figure?
It varies by engine design; evenness across cylinders is often more telling than the absolute number.
Can every engine be compression tested?
Not always before removal, but many can. Ask the seller what testing was done.
Does a good test guarantee a good engine?
It’s a strong indicator, not a guarantee — combine it with other checks.
Conclusion
Compression testing gives you an objective window into an engine’s condition. Ask sellers whether it was performed and what the readings showed, especially for higher-value units.
Want an engine with documented condition? Request a quote and ask about testing. See Engine Sourcing.