3 sizes of Compression Tester adaptor for specific automotive applications: 10mm, 12mm and 18mm. They convert the thread on Colortune and Hi Gauge Comp and extends the use of your compression tester. - 10mm Thread size: Suitable for most modern and classic cars. - 12mm Thread size: Ideal for Two Stroke motorcycles and classic motorcycles - 18mm Thread size: for 18 mm plug holes and Ford OHC engines See the How do I adjust the carb on my vehicle ? article in the Technical Articles section