Aprilia RS 125 compression testing RS125

Aprilia RS125 Compression Testing & Engine Health Guide

Compression is the ultimate indicator of the health of a 2-stroke top-end. This guide explains how to accurately measure the sealing of your piston rings and cylinder.

• The Methodology: * Engine Temp: Perform the test on a warm engine (not cold or scalding) for the most accurate reading of ring expansion.

  • Throttle Position: You must hold the throttle wide open while cranking the engine to allow maximum air intake.

  • The Spark Plug: Remove the plug, screw in the tester, and crank the engine until the needle stops rising (usually 5–8 seconds).

• Reading the Results (Rotax 122/123):

  • Healthy: 120–140 PSI. This indicates a strong top-end ready for high-RPM use.

  • Worn: 100–110 PSI. The bike will still run but will feel "flat" and have trouble starting when hot. A piston and ring change is recommended.

  • Critical/Failure: Below 90 PSI. The engine is at high risk of a "blow-by" seizure. Do not ride until inspected.

• The "Teaspoon of Oil" Trick: If compression is low, add a teaspoon of 2-stroke oil into the spark plug hole and re-test. If PSI jumps significantly, your piston rings are worn; if it stays the same, the leak is likely a head gasket or a cracked piston.

Aprilia RS 125 compression testing is important on a 2 stroke and give an indication of piston / rings wear and general engine wear and tear .

Here are some good compression testers see here on Ebay uk 

or Here on Amazon uk


Here is a video of a compression test and below some typical results and there general meanings.

To do a compression test properly -

Warm engine to operating temperature

Remove spark plug

Screw in compression tester

Open throttle wide open

Engine kill switch off

Hold starter button for a few seconds until maximum pressure is achieved



As a guideline:

80 psi - should be struggling to start no long left till failure

100 psi - very low change rings /  barrel
120 psi - rings wearing

150 psi - rings in as new condition for stock machines

180 psi - rings in as good condition for tuned machines




Find your Aprilia rs125 workshop manual here 

Aprilia rs125 troubleshooting

Aprilia rs125 will not start 

see more Aprilia RS 125 posts here

BUY the ultimate Aprilia RS 125 workshop manual bundle HERE