G4101 - Gunson Hi-Gauge Compression Tester G4101. The Gunson Hi-Gauge Compression Tester is designed to record the pressure created by the piston in each cylinder. This information can then be used to find faults in the valves, pistons, piston rings, cylinder bores or head gasket. A good tool to have in your garage for routine servicing as well as diagnosing engine problems. Additional Information, calibrated in BAR and PSI (300psi Max), suitable for petrol engines with 14/18mm spark plug. Instructions included. Fits most petrol engines in standard form, and additional adapters are available for vehicles with different size spark plugs. See the following products: 10 mm - G4055A, 12 mm - G4055B , 14 mm - G4055C Tapered seat, 14 mm - G4055D long Reach, 18 mm - G4055E. Also available with the following adaptors: 12 mm colortune plug - G4171.