efp Posted May 15, 2016 Share Posted May 15, 2016 Another small, possibly inane, question from me, but as always the patient wise input from you guys is always much appreciated.Was just about to pull the plug (sorry..) on ordering an engine compression tester from Amazon, but I noticed that all the plughole adaptors are metric fittings. I have no idea what the spark plug threads are, but given that its a 50 yr old car on which almost everything else is UNF, Im suspecting am M14 or whatever adaptor may bugger up my head threads? Or by fitting badly give inaccurate results? What do you guys use? Quote Link to comment Share on other sites More sharing options...
Nick Jones Posted May 15, 2016 Share Posted May 15, 2016 According to Machinery's Handbook (24th ed pg 1650), there are four standard SAE spark plug threads, all metric: M18 x 1.5; M14 x 1.25; M12 x 1.25; and M10 x 1.0.You want M14 x 1.25Nick Quote Link to comment Share on other sites More sharing options...
efp Posted May 15, 2016 Author Share Posted May 15, 2016 Thanks Nick. Metric even in the 1960's? Surprising. Cheers Quote Link to comment Share on other sites More sharing options...
Hogie Posted May 16, 2016 Share Posted May 16, 2016 The first spark plugs of various shapes and sizes came from Europe. It may have happened that 'Lodge' followed suit to have a bigger market.Roger Quote Link to comment Share on other sites More sharing options...
efp Posted May 16, 2016 Author Share Posted May 16, 2016 Learn something new every day!cheers chapsJohn Quote Link to comment Share on other sites More sharing options...
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