Spark Plug Gap for 81 400L??

[Guest]

Getting parts in order to do the spring tune up of my 81 400L, just wondering what the correct spark plug gap is. Anyone have this info?

[JEEPRUSTY]

Assuming the engine is the same

http://www.mtsac.edu/~cliff/storage/gs/GS400_Complete_Manual.PDF

should provide information.
I do not know the answer as I always leave the gaps as they are from the factory.

Welcome to the site.

[koolaid_kid]

If you are taking fresh plugs out of the package, they are already correct.

[GSX1000E]

.024"-.028" (.06-.07mm) they are almost never the right gap out of the box, check them. (.030-.033")

[JJ]

I was taught to never assume that any spark plug is gapped correctly out of the box. In fact, it has been my experience that they seldom are. A single plug usually covers several different applications, and they don't all call for the same gap. I always check the gap and adjust as necessary when installing new plugs.

[Nessism]

If you are taking fresh plugs out of the package, they are already correct.

Agree. After checking the (new) plugs on my last 5 GS's and never finding one out of spec, I don't bother checking anymore. The gap is not that important anyway.

[fyarl666]

Thanks for all the info. Went and picked up new plugs today, one was gapped properly, the other was not. Quick fix to gap properly, but lesson learned, never assume plugs come factory gapped properly.

[JJ]

Agree. After checking the (new) plugs on my last 5 GS's and never finding one out of spec, I don't bother checking anymore. The gap is not that important anyway.

You have been lucky. How far does the gap have to be out of spec before it becomes important? What would a .045" gap do? How about a .015" gap?

[tkent02]

Since I started buying NGKs about 40 years ago, they are mostly gapped right if the little cardboard tube is still good. Other brands, not so much.

Even so, I always check.

[GSX1000E]

You have been lucky. How far does the gap have to be out of spec before it becomes important? What would a .045" gap do? How about a .015" gap?

"To measure is to know." (Now where have I heard that around here? Perhaps it is just a selective quote/suggestion?)
I always measure them myself. If you want perfection, YOU MUST MEASURE THEM.

[Nessism]

"To measure is to know." (Now where have I heard that around here? Perhaps it is just a selective quote/suggestion?)
I always measure them myself. If you want perfection, YOU MUST MEASURE THEM.

Does making snide comments make you feel superior?

"Perfection" is meaningless when it comes to spark plug gaps.

[tkent02]

You have been lucky. How far does the gap have to be out of spec before it becomes important? What would a .045" gap do? How about a .015" gap?

Would run. The .045 maybe miss a little at high RPM, maybe not. The .015, maybe lose a little fuel efficiency. No big deal, not usually all that critical. Sometimes it is. Some ignitions are different than others. A weak spark may have trouble with the .045. The .015 may foul easier in an old engine.
Played with this a long long time ago, seemed like an efficiency thing more than something that had to be perfect to run at all. I think a wider gap makes the engine more efficient until it gets so wide starts missfiring. There is probably more to it that that, or new cars with their super powerful ignitions would have 1/2 inch gaps.

[JJ]

I remember that until the seventies and emission laws, spark plug gaps for cars was almost universally .032".

When manufacturers started leaning out engines and lowering compression ratios, spark plug gaps opened up to between .040" and .050". The theory being that the larger gap, combined with electronic ignitions that delivered more voltage, would expose more of the air-fuel mixture to the spark, increasing the chances of ignition.

I don't know why anyone would intentionally run a spark plug gapped less than the spec calls for.

[koolaid_kid]

The gap needs to be narrow enough for the spark to make the leap across the gap, and wide enough to cause ignition of the fuel/air mixture.
Period.
Suggesting that the factory gap might be as narrow as 0.015" or as wide as 0.045" reveals the author's lack of experience with real world parts.

[JJ]

The gap needs to be narrow enough for the spark to make the leap across the gap, and wide enough to cause ignition of the fuel/air mixture.
Period.

Correct. While both ends of the range will work, there is a smaller range within it at which ignition is most effective.

Suggesting that the factory gap might be as narrow as 0.015" or as wide as 0.045" reveals the author's lack of experience with real world parts.

Wrong.

I didn't say that I have found factory gaps as small as .015" or as wide as .045". Those were two extremes that I asked Ed about when asking what effect they would have on performance. I screw up enough without you putting words in my mouth, thank you.

What it reveals, if anything, is a few decades of experience dealing with spark plugs as they are bought in the real world. I have changed hundreds of plugs in my lifetime, and have found at least one or two out of a set of eight will be out of spec right from the box. Not far, grant you, but out of spec. As stated before, one spark plug may, and usually does, have several different applications. Not all of them will call for the same gap.

While some manufacturers take care and put a plastic or cardboard sleeve around the electrode end of the plug to protect it, some do not take this precaution. During packaging, shipping, and in-store-handling the unprotected electrode end of the spark plug can be banged around and close the gap.