Quit guessing if your saw is performing at it"s peak level. This simple tool makes testing your saws compression a snap. Tester has both imperial and metric scale that goes up to 300 psi. Has a side release valve for multiple tests also. Head accepts 14mm and 18mm plug holes, so it will work on most all chainsaws and other small gas engines made today. It"s check valve is in the end of the hose and the release valve is on the gauge. Most chainsaws need a minimum of 110 psi to function properly. Though a chainsaw may start at less than a 110 psi reading, it will not perform at the optimum level. Testing Procedures: 1. Run the engine for about 15 minutes until it reaches normal operating temperature. 2. Stop engine. Disconnect all spark plug wires one at a time and label for proper reconnection. 3. Loosen all spark plugs about one turn, then use an air hose or tire pump to blow all the dirt out of the spark plugs and gaskets. Place them on a clean, flat surface in the order in which they were removed. This will help to correlate any compression problems with the conditions of the plug from the particular cylinder involved. 4. Remove the Air Filter and block open the carburetor throttle plates to maximum. Remove the high tension lead from the center of the distributor and ground it. To disable electronic ignition system, disconnect the electronic ignition module or remove the primary battery terminal from the distributor cap. (On Ford V-8 and V-6, disconnect the primary lead from the distributor cap.) 5. Screw the spark plug adapter hose into the spark plug hole. Hand tighten only - DO NOT USE A WRENCH. 6. Now crank the engine for at least 4 compression strokes or until pressure stops rising on the tester. 7. Record the compression reading and repeat the test on all remaining cylinders. (Step No. 5 and 6). Test Results: 1. On a normal cylinder, the needle should advance on each stroke until it reaches a peak. All cylinders should test within the engine manufacturer