Home Shop Ignition Spark Plugs Zündkerzen #2: Modelle 1909-1926
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Zündkerzen #2: Modelle 1909-1926

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Spark Plug Heat Ranges

Harley spark plug heat ranges are marked by numbers, from 2 to 12. No. 2 is the hottest plug, no. 6 would then be a relatively cool plug. The coolest plug used by Harley-Davidson is marked 10R12A. Double numbers characterize multi-range plugs.

With older motors, it was not uncommon, that you achieved an optimum with plugs of two different heat ranges. Unlike the well known prejudice implied, you had to choose cooler plugs for the front and hotter plugs for the rear cylinder. On later Harley engine designs, heat distribution is balanced to such an extent that you should not have to rely on those tricks anymore.

Stroker owners should refer to OEM instructions for their specific type of motor, unless the manufacturer recommends entirely different plugs.
Spark plug by Competition Distributing; fits all models 1909-1926; thread: 7/8”-18; thread length: 5/8 ”; wrench size: 15/16 ”; OEM labelling: 2; replaces OEM 37-09; USA
W&W# Picture net price* Available from warehouse Germany?
12-002 Zündkerzen #2: Modelle 1909-1926 manuf. labelling: W14 $29.03 each
in stock
* United States of America: Tax free shipping. Please check your local import fees.


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