Spark plug gap - Harley Davidson Forums

Touring Models Road King, Road King Custom, Road King Classic, Road Glide, Street Glide, Electra Glide, Electra Glide Classic, and Electra Glide Ultra Classic bikes.
Sponsored by:
Sponsored by:

Spark plug gap

Thread Tools Search this Thread
Old 04-12-2010, 08:40 PM
Electra1 Electra1 is offline
One Of The Regulars
Thread Starter
Join Date: Jun 2008
Location: South Dakota
Posts: 129
Thanks: 0
Thanked 0 Times in 0 Posts
Default Spark plug gap

Hello out there. Replacing my plugs in my 2005 FLHTCUI tonight. I have always gapped them at .040 in, but tonight I couldn't remember so I looked it up and found that I can gap anywhere from .038 to .043 inches. Now a questioned came to my mind and it might be a stupid question, but here it goes. What gap in the range given would give me the best bang for the buck? I mean should I be setting the gap at .038 or to the other side at .043? Or is right in the middle the best? My plugs look good at .040 in, but my mileage is on low end at 36mpg. I have stage 2 big bore with SE203 cams. Using the standard HD6R12 plugs. Thanks for the time in advance.
Electra 1
Reply With Quote
Old 04-12-2010, 09:20 PM
dawg's Avatar
dawg dawg is offline
Club Member
Join Date: Jun 2006
Location: Vermont
Posts: 21,520
Thanks: 0
Thanked 88 Times in 80 Posts

From nightrider site:

Performance and Technical information on setting spark plug gaps.
Are plugs pre-gapped from the factory?

Yes and no. A spark plug part number might fit hundreds of engines, and although the factory will typically set gap to a pre-selected setting, this gap may not be the right one for your particular engine, and may not take into account modifications that you may have performed to the engine.

Insufficient spark plug gap can cause pre-ignition, detonation, even engine damage. Too much gap can result in a higher rate of misfires, noticeable loss of power, plug fouling, and poor economy.

When you raise compression or add forced induction (a turbo system, nitrous, or supercharger kit), you must lower the gap (reduce gap about .004" for every 50 hp you add on an V-8 engine, for a Harley-Davidson V-2 engine every 12.5 hp you add). However, when you add a high power ignition system (such as those offered by MSD, Crane, Nology,) you can open the gap back up about .002"-.005".

As an example, let's use a hypothetical `96 350 Chevrolet LT1 engine build-up. The standard gap is .050" for an un-modified LT1. We'll add 150hp Nitrous, so we must lower the gap about .012" to .038". We then decide to add that killer MSD 6A/Crane Hi-6 box and, using our guidelines as outlined above, we can now open the gap up .002"-.005" to about .040"- .043". By following this basic guideline should get you very close.

Further experimentation may be necessary, but by always starting with a larger gap than it thought necessary to reduce the risk of detonation, you should be safe.
Reply With Quote
Old 04-12-2010, 09:22 PM
Mick's Avatar
Mick Mick is offline
Road Warrior
Join Date: Jul 2006
Location: Rochester N.Y.
Posts: 1,532
Thanks: 0
Thanked 2 Times in 1 Post

If you look at the gap while it's at .040" is the negative (side) electrode at a perfect 90 degree angle to the center electrode?
You would get the best , highest quality , longest lasting life from the plugs when the electrodes are a perfect 90 degrees.
The result is that the spark will travel around the center electrode and wear it evenly all the way around.
If your old plugs are worn perfectly around the center electrode , that would show that they were set that way
to begin with.
That being said , if the gap was better wider , low RPM spark quality would be better than high RPM.
If the gap is better smaller , the opposite would be true.
Generally a larger gap would produce better fuel mileage efficiency than a smaller gap.
This is how cars have been leaning towards for years.
A larger spark is friendlier to a lean fuel mixture most of the time.
If you want to try it both ways , just "carefully" bend the side electrode so the gap is a perfect 90 degrees for each gap that you try.
I have done this at my shop for years with great success.
Reply With Quote
Old 04-12-2010, 09:59 PM
Quadancer's Avatar
Quadancer Quadancer is offline
Elite HDF Member
Join Date: Nov 2009
Location: Acworth Georgia
Posts: 4,053
Thanks: 0
Thanked 4 Times in 4 Posts

You ever try drilling a hole in the electrode? I've been wondering if that works. i would expect that it does nothing, since electricity follows the closest (easiest) path.
One thing I want to do is index the plugs, but keep forgetting to; I also suffer from less FE than I want to have. By indexing, I mean marking the plug to show the direction of the electrode, and adding washers to make it point towards the center of the cylinder after tightening it. Or towards the intake valve.
Reply With Quote
Old 04-12-2010, 10:32 PM
Neckball Neckball is offline
Outstanding HDF Member
Join Date: Apr 2005
Location: Charlotte, NC
Posts: 3,095
Thanked 2 Times in 2 Posts

With what most of us have in our tool box, you can't get an accurate enough measurement to know if you're really at .038" or .040". Get it close to the middle of the range, you'll never know the difference between the low side and the high side if you tried it.
Reply With Quote
Old 04-12-2010, 11:09 PM
Electra1 Electra1 is offline
One Of The Regulars
Thread Starter
Join Date: Jun 2008
Location: South Dakota
Posts: 129
Thanks: 0
Thanked 0 Times in 0 Posts

Thanks all for the responses. My feeler guage gets me close to .040, but like was said how do I know I might be off plus or minus a few inches. Looks like staying where I have them set will be fine. Thanks again.
Electra 1
Reply With Quote

Related Topics
Thread Thread Starter Forum Replies Last Post
spark plug gap cajun1957a Touring Models 5 01-30-2017 07:52 PM
Plug gap for Twin Cam 88.....? SurfGuitar141 Softail Models 2 04-28-2011 06:21 PM
plug gap confusion emtjayb Sportster Models 3 12-27-2008 11:10 AM
Are HD 6r12 pre gaped? Demonsmasher Sportster Models 6 06-04-2008 12:28 AM
Spark plug gaps Z6969 Dyna Glide Models 4 10-19-2006 09:37 PM

2000, 2005, 2008, big, bore, davidson, forum, gap, harley, hd, king, plug, road, spark, sparkplug

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Forum Jump

Featured Sponsors
Vendor Directory

All times are GMT -5. The time now is 02:02 AM.

  • Ask a Question
    Get answers from community experts
Question Title:
Your question will be posted in: