This kit converts the thread on Colortune & Hi Gauge Compression Tester to fit most popular sizes of spark plugs fitted to most modern & classic cars, motorcycles, garden machinery & boats.14mm to 18mm thread size Compression tester adaptor For use on 14mm colortune & Hi Gauge Suitable for most modern & classic cars as well as Available in additional sizes 10mm, 12mm, 14mm or 18m