BTH Components Ltd.

We have had good results with NGK plugs and some suggested types for popular machines are listed below.  The NGK plugs have a good heat rangeso the suggested grade will usually perform well for most normal riding.  If you have a modified motor or  you are competition riding the suggested type can be a good starting point to find the best grade of plug.  Use the gap recommended in your machines handbook. 
If you don't know  the recommended gap, 0.020" (0.5 mm) is a good starting point.  A plug gap of 0.018"-0.020" was recommended for most bikes that originally used a magneto.
A large gap is not necessary unless you have plug fouling problems and will just work the ignition system harder than necessary. 
Some machines had long production runs with early models using iron heads and later ones aluminium.  As a general rule iron heads used short reach plugs and aluminium heads long reach.  Check you have the correct reach plug before fitting.

N.B.  A local dyno expert who has tuned many race winning vintage machines has found that a plug gap as low as 0.007" (seven thousandths of an inch) does not reduce the power output registered on the dynamometer. 

A tuned engine, e.g. raised compression, may need a harder than standard plug.

A general guide to plug manufacturers equivalents.

A good article on spark plugs and heat ranges can be found on the NGK website at the following link:

http://www.ngksparkplugs.com/techinfo/spark_plugs/techtips.asp?nav=31000&country=US

Information on what the letters and numbers mean on an NGK plug here. (Acrobat pdf file)

Telephone: 01234 880605

Email: bthcomponents@gmail.com 

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