Compression testing on engines can prove difficult if the spark plug sockets are located deep in the engine compartment. Laser Tools compression tester adaptor (part 4332) is designed to fit 12mm plug sockets and is extra long, to get into difficult to reach spots. This extra long Compression Test Adaptor is designed to fit 12mm Spark Plugs. This tool is now essential on BMW E90 335i with N54 engine 2006, BMW Mini Mercedes-Benz M156 (AMG engine manufactured from 2006 onwards) Also used on Peugeot 207/307 and Renault 1.2 16valve.