The Harley-Davidson Fat Bob 114 uses a 6R12 spark plug. A worn plug causes hard starting, rough idle, and poor fuel economy — and it’s one of the cheapest parts on the bike to replace.
Inspect the plug every service and replace it at the factory interval or sooner if the electrodes look rounded or the insulator is heavily discolored.
Harley-Davidson Fat Bob 114 quick service specs
| Item | Specification |
|---|---|
| Engine oil | 20W-50 |
| Oil capacity | 4.7 liters |
| Oil change interval | 5000 miles |
| Chain slack | See user’s manual |
| Spark plug | 6R12 |
| Tire pressure (F/R) | 36 PSI front / 40 PSI rear |
| Battery | 12V 17.5Ah 310CCA AGM |
How to replace the spark plug, step by step
- Make sure the engine is cold. Pull the spark plug cap straight off — don’t yank the wire.
- Blow or brush away dirt from around the plug so nothing falls into the cylinder.
- Remove the plug with a spark plug socket. Note how tight it was on the way out.
- Check the new 6R12’s gap against the spec in your owner’s manual and adjust carefully if needed.
- Thread the new plug in by hand to avoid cross-threading, then torque to spec — snug, not gorilla-tight.
- Push the cap firmly back on until it clicks, then start the bike and confirm a smooth idle.
Common mistakes to avoid
- Cross-threading the plug — always start it by hand, several full turns, before using a wrench.
- Over-torquing — cracked insulators from too much force are a classic DIY error.
- Forgetting to check the gap — even pre-gapped plugs can shift in shipping.
FAQ
How often should I replace the spark plug on the Harley-Davidson Fat Bob 114?
Follow the factory interval in your owner’s manual, and inspect it at every service. Replace sooner if starting gets hard or the engine runs rough.
What are the signs of a worn spark plug?
Hard starting, rough idle, misfires under load, and worse fuel economy. A quick look at the plug usually tells the story.
Can I use iridium plugs?
Only if your manual lists one as an approved equivalent. Stick with the 6R12 unless Harley-Davidson says otherwise.
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Dan Heiman
Writer at Helmet Today, covering helmet reviews, fit and riding safety.
