Champion RN12YC (Shovelhead 1975→, Evo Big Twin 1984→)

The standard spark plug for most Harleys — all 1200 and 1340 models from 1975 onwards, plus most aftermarket motors — uses the longer 14mm thread, except for Sportsters.

Because there are still plenty of custom Shovelhead heads out there, make sure to double-check if a short-thread plug might actually be required.

• Spark plug by Champion
  
• fits Shovelhead 1975→, Evo Big Twin 1984→
  
• also for: S&S engines and cylinder heads
  
• steel / ceramic
  
• thread: M14 x 1.25
  
• thread length: 3/4 ”
  
• wrench size: 13/16 ”
  
• manufacturer-code: RN12YC
  
• replaces Harley-Davidson part number HD 32312-77, 32311-83, and 32342-04
  
• gross weight: 70 g

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.