Home Shop Ignition Spark Plugs Zündkerzen #2: IOE, 45”, V Models and Knuckleheads
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Zündkerzen #2: IOE, 45”, V Models and Knuckleheads

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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; fits IOE models 1927-1929, Singles 1926-1934, 45 cui/750 cc Flathead →1955 (exc. K-models), V models and Knucklehead; thread: M18 x 1.5; thread length: 1/2 ”; wrench size: 7/8 ”; OEM labelling: 2; replaces OEM 32304-09, and 37-09B
W&W# Picture net price* Available from warehouse Germany?
12-028 Zündkerzen #2: IOE, 45”, V Models and Knuckleheads by Champion; manuf. labelling: UD16 $5.94 each
in stock
* United States of America: Tax free shipping. Please check your local import fees.


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