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FAQs

Petrol engine: warm engine, all spark plugs out, throttle held wide open, ignition disabled, screw the compression-tester adapter into the test cylinder, crank the engine through 4-6 compression strokes, read peak pressure. Diesel engine: warm engine, all injectors or glow plugs out, throttle held wide open, fuel cut-off engaged, screw the compression-tester adapter into the test cylinder, crank through 4-6 strokes, read peak pressure. Repeat for each cylinder and compare readings.

Petrol engines run compression in the 120-180 PSI (8-12 bar) range — gauge scale 0-300 PSI is typical. Diesel engines run much higher compression (300-600 PSI / 20-40 bar typical, with some up to 700 PSI/48 bar) — diesel testers have gauge scales 0-1,000 PSI. Diesel testers also fit the injector or glow-plug hole (different thread sizes and adapter requirements) rather than the spark-plug hole. A petrol tester cannot read diesel compression accurately.

Three points: (1) absolute pressure should be within the manufacturer's specified range — listed in the workshop manual; (2) all cylinders should read similar, with no more than ~10% variation between highest and lowest; (3) a wet test (squirt of oil into the suspect cylinder, retest) helps differentiate ring wear (compression rises after wet test) from valve seat damage (compression unchanged after wet test). Use compression results alongside leakdown testing for a complete diagnosis.

Yes. Adapter sets for older engines (M10, M12, M14, M18, M22 spark-plug threads) and specialised diesel injector and glow-plug threads are available — including some less-common automotive and agricultural thread sizes. Search by the thread size needed, or WhatsApp 07754288583 with the engine make, model, and a clear photograph of the existing adapter or injector for cross-reference.

Every product is backed by our Fit Guarantee. If a part is incompatible with your machine, you can return it hassle-free. For damaged-on-arrival items, report within 24 hours with photographs to [email protected] — we will arrange collection. For other returns, email us first to obtain a Return Merchandise Number before sending. Returns are accepted within 30 days of purchase, in original packaging, on UK orders. A restocking fee may apply on returns where the buyer has changed their mind. Refunds are issued to the original payment method once the return is inspected.

Delivery time depends on the product, your destination, and availability — the estimated time for any item appears on the product page, and "Check where we deliver" confirms coverage for your postcode before you order. Next-day delivery is available on most products; the option appears at checkout when applicable. We deliver across the UK, Europe, the Middle East, and Africa. Use the country selector at checkout to confirm coverage and delivery options for your destination.

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Compression Testers

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Within Electrical Repairs, Compression Testers covers the engine-diagnostic tools used to measure cylinder compression pressure and to identify worn rings, valve seal failures, head-gasket leaks, and other engine-condition issues. Compression testing is a fundamental engine-diagnosis step — a routine that confirms whether the engine's combustion chambers are sealing correctly across all cylinders.

Parts for Machines supplies compression testers for petrol and diesel engines across the automotive, agricultural, plant, and commercial-vehicle ranges. Petrol compression testers screw into the spark-plug hole and read peak compression pressure; diesel compression testers screw into the injector or glow-plug hole and read the (much higher) diesel compression pressure on a separate gauge scale.

Within Compression Testers you will find Petrol Engine Compression Testers (typical scale 0-300 PSI / 0-21 bar, with screw-in adapters for the common spark-plug thread sizes), Diesel Engine Compression Testers (typical scale 0-1,000 PSI / 0-70 bar, with adapter sets for the common diesel injector and glow-plug threads), and Combination Kits with both petrol and diesel adapters and gauges.

Product pages list the gauge scale (in PSI, bar, or both), the gauge accuracy (typically ±2-3% of full scale on trade-grade testers), the adapter set included (the spark-plug, injector, or glow-plug thread sizes covered — common sizes M10, M12, M14, M18 with extension hoses for engines with deep plug wells), the hose length (300mm-600mm typical), and the gauge-and-hose connection. Compression testing is a comparison test as much as an absolute test — readings should be similar across all cylinders within a few PSI or bar of each other. Significant variation (more than 10% between cylinders) indicates a problem in the low-reading cylinder. Trade pricing on multi-tool and workshop-kit orders is available; contact the team via WhatsApp 07754288583.

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FAQs

Petrol engine: warm engine, all spark plugs out, throttle held wide open, ignition disabled, screw the compression-tester adapter into the test cylinder, crank the engine through 4-6 compression strokes, read peak pressure. Diesel engine: warm engine, all injectors or glow plugs out, throttle held wide open, fuel cut-off engaged, screw the compression-tester adapter into the test cylinder, crank through 4-6 strokes, read peak pressure. Repeat for each cylinder and compare readings.

Petrol engines run compression in the 120-180 PSI (8-12 bar) range — gauge scale 0-300 PSI is typical. Diesel engines run much higher compression (300-600 PSI / 20-40 bar typical, with some up to 700 PSI/48 bar) — diesel testers have gauge scales 0-1,000 PSI. Diesel testers also fit the injector or glow-plug hole (different thread sizes and adapter requirements) rather than the spark-plug hole. A petrol tester cannot read diesel compression accurately.

Three points: (1) absolute pressure should be within the manufacturer's specified range — listed in the workshop manual; (2) all cylinders should read similar, with no more than ~10% variation between highest and lowest; (3) a wet test (squirt of oil into the suspect cylinder, retest) helps differentiate ring wear (compression rises after wet test) from valve seat damage (compression unchanged after wet test). Use compression results alongside leakdown testing for a complete diagnosis.

Yes. Adapter sets for older engines (M10, M12, M14, M18, M22 spark-plug threads) and specialised diesel injector and glow-plug threads are available — including some less-common automotive and agricultural thread sizes. Search by the thread size needed, or WhatsApp 07754288583 with the engine make, model, and a clear photograph of the existing adapter or injector for cross-reference.

Every product is backed by our Fit Guarantee. If a part is incompatible with your machine, you can return it hassle-free. For damaged-on-arrival items, report within 24 hours with photographs to [email protected] — we will arrange collection. For other returns, email us first to obtain a Return Merchandise Number before sending. Returns are accepted within 30 days of purchase, in original packaging, on UK orders. A restocking fee may apply on returns where the buyer has changed their mind. Refunds are issued to the original payment method once the return is inspected.

Delivery time depends on the product, your destination, and availability — the estimated time for any item appears on the product page, and "Check where we deliver" confirms coverage for your postcode before you order. Next-day delivery is available on most products; the option appears at checkout when applicable. We deliver across the UK, Europe, the Middle East, and Africa. Use the country selector at checkout to confirm coverage and delivery options for your destination.

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