Why two different types of sparkplugs?

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Mar 30, 2010 | 06:23 PM

blueduramax

Thread Starter

Cruiser

Joined: Feb 2010

Posts: 119

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From: Oregon

Why two different types of sparkplugs?

While getting to know my 2004 Road King Classic I discovered that the front cylinder has a 10R12A plug and the back cylinder has a 6R12 plug. Is there a logical reason for this and should I change them to 6R12's? The front one seemed a little wet and they both looked like they were pretty old.

Mar 30, 2010 | 06:28 PM

ashmarc

Road Master

Joined: Dec 2009

Posts: 1,223

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From: Las Vegas, NV

Stock plugs for my '06 Ultra are the 6R12's........not sure why you would have different ones......

Mar 30, 2010 | 06:43 PM

BamaVan

Advanced

Joined: Dec 2009

Posts: 71

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From: Alabama

Somebody prob just replaced it with wat was available at the time

Mar 30, 2010 | 06:48 PM

jimsreddyna

Road Captain

Joined: Dec 2006

Posts: 516

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From: Fenton, MO

I don't know if the 10r is a colder or hotter plug than the 6r? Maybe the front cylinder was running a little hot or it was fouling out so they tried to compensate with a different heat range plug?

Mar 30, 2010 | 06:54 PM

dawg

Seasoned HDF Member

Joined: Jun 2006

Posts: 28,562

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From: Vermont

V-rods and Buells run the 10R12A's as stock. TC's run the 6R12's

Mar 31, 2010 | 03:56 AM

emwolb

Ultimate HDF Member

Joined: Aug 2008

Posts: 5,413

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From: eastern oklahoma

used to be an old trick on the nucks, pans, etc, to use a different heat range on the rear cylinder because it didn't get as much cooling air. in todays world though, 6R12's work just fine. and don't buy into any of that crazy gimmick three electrodes, splitfire or any of that crap. 6r12's work.