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Home » Featured, Technology

Harley-Davidson Dual 10mm Spark Plug Head

The right way to convert a cylinder heads to twin plug!

The jig that is generally available to Harley-Davidson workshops and tuners, used to convert Harley-Davidson heads to a twin spark plug set up does not provide the best solution. Whilst this promises a performance increase, the end result is not the best regarding plug position in relation to the flame front. The additional plug gets positioned half way down the combustion chamber and is very inefficient.

See images below.

The Cylinder Head Shop Solution.

At CHS, we provide a much more efficient solution. We engineer a 10mm long reach spark plug in a much more effective location, relative to the flame front and can flow the head to suit.

Our ‘Gold Service’ includes welding up the original 14 mm plug hole and fitting a 10mm long reach plug in its place to match. So you only need one type of plug and plug spanner.

See pictures below.


Welding new plug location top to create required 10 mm long reach depth costs £45.00 plus £130.00 = £175.00 plus VAT.

Cost to mill top plug sealing area, drill, tap and fettle to suit plug. Weld up existing plug costs an extra £20.00.

Results of the Old Jig Method

Results of the CHS Method

The Right Way to Twin Plug Your Harley-Davidson

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