Compression tester fittings

[Chillywilly]

Hi, wanting to do a compression test on my 150, fittings on the tester are 14mm and 18mm - don't fit into the plug ports. Where can I find the correct size of the ports on this engine. Thanks in advance.

 

[DangarStu]

What type of spark plug does it take? If you look up the specs for the plug you'll get the thread diameter.

Stuart