Spark plugs

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Original specifications usually came from the makers catalogues or from ones recommended from experience. Get in touch if you can fill in gaps or heads not listed here. Be aware that the wrong grade plug can be awkward to suss out. Too hot a grade will burn and eventually not fire, the insulator will be a grayey white colour. Too cold will foul and misfire, similar to an oily engine though with a dry black appearance rather than wet and oily. These can look very similar to a weak or rich mixture—so you start fiddling with the carb as well just to compound the problem!

 

Some useful websites that could help if you need it;

· The Green Spark Plug company specialise in old an obsolete plugs for older vehicles.

· For a good read about plugs and racing plugs in particular try this old KLG article.

· A good plug comparison table.

·            All you need to know about plugs on this site

·            Cross references between manufacturers (seems to be more for currently available ones though)

·               Champion plugs

· Reading plug colours

As a general guide the following table appeared in the February 1960 750 Motor Club Bulletin. It suggested that the plug grades were transferable across different heads, even to similar side valve Ford heads. If nothing else it gives something to start from. Modern equivalents will have a wider heat range so it shouldn’t be as much of a juggling act as it was then.