Shovelhead and Ironhead Harleys benefit greatly from adding an additional spark plug. Due to the necessity of a rather huge dome piston due to the hemi head design and the placement of a single spark plug all the way off to one side. The dome of the piston tends to obscure the flame front. That makes for very inefficient combustion.

With dual spark plugs your motor will be more resistant to spark knock, start easier and will pick up a little power as well. It is very hard to deny the benefit of adding an extra spark plug to these hemi heads.

The rocker boxes will need to be split or notched like the ones in the photo to clear the new plug and wire. We can notch or split your rocker boxes to accommodate the new spark plug. All notching must be done with the corresponding head in house at the time of service. Some shovel Rocker boxes need a lot of hand work even when notched. Prices reflect the extra time needed to make them work with dual plug heads.