Wednesday, August 27, 2008

Spark Plugs 101

Here's one that is frankly on the verge of being a little embarrassing. When I bought my first Knucklehead nearly 30 years ago it had a set of Champion D14 plugs in it. For about the next 20+ years, whenever I replaced plug on any of my Knuckles I would simply go to my local Napa and buy another set of D14s.

Then somewhere along the line, I became aware that the Champion D16 was the correct cross reference for the Knuckle. So, after all those years I was faced with no longer being able to remember which number was right and which was the one I had used for so many years. Have I mentioned lately that its no picnic getting old?

So for the last couple of years, every time I have been asked for a spark plug for a Knuckle, I have not been able to remember which was correct; D14 or D16. That all changed the other day when I was researching something else. I happened to run across this in Palmer's "How to Restore Your Harley Davidson":


  • "Of the original Harley-Davidson 18mm spark plugs, the No. 3 is most often used. For a motor used mostly on the highways at fast speeds the No. 4 plug is more desirable."


And a few lines later:

  • "A cheap alternative to the No. 3 and No. 4 18mm plugs are Champion D-14 and D-16 spark plugs, respectively."

Kind of reminds me of the old line "I used to think had made a mistake once, but I found out I was wrong." So, as it turns out either the D14 or D16 is an acceptable plug in Knuck, depending on the use.

I do think I have uncovered a small mistake in the Palmer book though. All the catalogs I have show the D16 as the replacement for the No. 3 plug. This is backed up by a Champion spark plug catalog that I have. The D14 is colder than the D16. The confusion no doubt stems from the fact that in Champion's heat range numbering the lower the number the colder the plug, and Harley did it the opposite way. That's a very minor discrepancy given the huge scope of his book. I have trouble not transposing stuff in just one short blog entry! The important thing is that there are interchanges for both the No. 3 and the No. 4 Harley plug.

With that in mind, here is a page from the Knucklehead service manual.


This drawing illustrates spark plug heat ranges pretty well. A spark plug's heat range refers to how well the plug dissipates heat. A colder plug dissipates the heat quicker, and so runs cooler. Therefore, a hotter plug will help keep deposits from fouling a plug if used in an over rich or oil consuming engine. On the other side of the coin, a colder plug will be less prone to overheating and causing pre-ignition in a high compression engine or one that is run hard (Both high compression ratios and prolonged high speed will generate more heat).

Interestingly, the factory manual states that:

  • " In some cases best results may be found using a colder plug in one cylinder than the other. In this case it is usually the front cylinder that takes the colder plug as this cylinder is not as likely to foul a plug at low speed."

I've never tried it, but it seems logical enough. So next time you inspect your plugs and find they are not both the came color, it may be worth your while to try a different heat range! Now you know.

5 comments:

Jack Hester said...

Good information, Lee. Once again reminds me that we need to crack these old manuals more often than we do. Information within is still valid for today. I was just admiring the color of the inside of my pipes, on the '59 FLH, yesterday while I waited on the tow vehicle to come and pick me up. 90 miles from home, and the front exhaust valve stuck, and is bent I believe. Anyway, it'll be back on the road by next weekend. My point though, I noticed the front pipe to be grayish brown, while the rear pipe was tan brown. This old bike is tired and worn, but this it the first time it's left me on the side of the road, since acquiring it in '74. 90 miles and AAA would not pay. $300 out of pocket, but it's in the shop.

When I do fire it the next time, it will have a colder plug in the front. Will comment on the color, after the Charity ride, 20 Sept. No major rides before then. Just local. Thanks for the post.

T J said...

I have owned a 1947 EL since 1997, been through many changes. I would like info on mild performance work that I can do to my KNUCKLE myself. e-mail is boileranimalfromhell@yahoo.com. thanks t j starling

Wally Waters said...

Have you had experience with using 18mm to 14mm spark plug adapters on Knuckleheads

St. Lee said...

I have always done so for my drag bikes where it is especially important to have a wide selection of heat ranges available. The very first one had adapters that "Gumps" Riley made from some 18mm spark plug casings(only certain brands had enough "meat" in them to allow this). For most street applications I have always been able to get along with the old 18mm plugs. One thing to consider is that it has been claimed that you should use one heat range colder than normal when using an adapter due to the interruption in the heat dissipation path caused by the adapter. I have no way of knowing exactly how valid this is, but I run a pretty cold plug in the drag bikes (both then and now) anyway. Possibly a more important note is that if you chose to use the adapters from V-Twin, be aware that the threaded portion is too long and needs to be shortened so it doesn't protrude into the chamber. Just screw the new 3/4 reach plug into the adapter and machine off the portion of the adapter that protrudes past the threads of the plug. This will put both the adapter and plug to the same depth as the old 18mm plug was. If doing the modification for street use I think I would try to pick a plug with a heat range close to that of an Iron Head Sportster (you can't use a Sportster plug because it is too short on the reach).

Wally Waters said...

Thank You for the quick response and Kind Regards, Wally Waters