Spark Plugs
Seasoned HDF Member
Joined: Aug 2009
Posts: 9,228
Likes: 3,239
From: west Michigan
Standard Harley Davidson spark plugs gapped at .040 every ten or twenty thousand miles. I got over 30K out of my original plugs and replaced them just because. In the old days, engines with breaker point ignitions required fresh spark plugs to run right. Modern electronic ignition systems are easy on spark plugs so they're not much of an issue in stock engines.
Seasoned HDF Member
Joined: Sep 2007
Posts: 9,206
Likes: 372
From: Central Illinois, Between I-80 and I-74
Check and gap them with every service. You will know when to change them, and probably do it long before you have to.
The stock Harley plugs are fine, not a whole lot of money to save going aftermarket.
The stock Harley plugs are fine, not a whole lot of money to save going aftermarket.
The stock 6r12 supposedly has ion knock sensing capability that aftermarket and gold/platinum H-D plugs don't. If the dealership is convenient I'd stay with stock plugs for the few $ cost diff. Like others said, stock plugs last a long time but I did change mine at 6k miles when I went to 103" /cam/head/tune..
Seasoned HDF Member
Joined: Mar 2010
Posts: 38,300
Likes: 6,325
From: Honah Lee
Should run fine with stock number plug. In fact if you look in manual Harley probably says no substitutions. They do not mean just a Harley plug , what they mean is correct number. If you go to another manufacturer always make sure it's plated.
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Outstanding HDF Member
Joined: Jan 2010
Posts: 2,916
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From: earth
The plugs themselves don't have ion sensing capability, but the electrical characteristics of the OEM plug are factored into the function of the "ping" sensing circuit, so changing plugs could have an effect on the system. I've used HD 6R12's, 6R10's and the Autolites, I have noticed no obvious diference in performance among the three. I can't validate my claim with dyno sheets for comparison, so take it for what it's worth. Only thing I can tell you for sure is the OEM plug works fine, and the ignition sytem is one of the things the Factory got right with the late model EFI Twin Cams; the ignition system is very robust and not much room for improvement. As far as life expectancy or interval for replacement, what ever you decide, you will probably change them long before the end of their useful life.
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Road Master
Joined: Oct 2009
Posts: 862
Likes: 3
From: East Tennessee
Plugs
The plugs themselves don't have ion sensing capability, but the electrical characteristics of the OEM plug are factored into the function of the "ping" sensing circuit, so changing plugs could have an effect on the system. I've used HD 6R12's, 6R10's and the Autolites, I have noticed no obvious diference in performance among the three. I can't validate my claim with dyno sheets for comparison, so take it for what it's worth. Only thing I can tell you for sure is the OEM plug works fine, and the ignition sytem is one of the things the Factory got right with the late model EFI Twin Cams; the ignition system is very robust and not much room for improvement. As far as life expectancy or interval for replacement, what ever you decide, you will probably change them long before the end of their useful life.