This kit converts the thread on Colortune and Hi Gauge Compression Tester to fit most popular sizes of spark plugs fitted to most modern and classic cars, motorcycles, garden machinery and boats. The kit includes a 14mm to 12mm thread adaptor and a gas seal washer. It is suitable for use with Gunson G4101 and extends the use of your compression tester, making it ideal for classic motorcycles and two-stroke engines. Other adaptors are available separately, including 10mm (G4055A), 14mm tapered seat (G4055C), 14mm long reach (G4055D), and 18mm (G4055E).