i have an '04 corvette and the owners manual says the spark plug gap should be 0.40, and most forums say 0.60. what's the reason for the difference?

 

In 2008 GM released TSB 03-06-04-060B, Information on New Spark Plugs and Gapping

So if you use iridium plugs, gap to .040. If platinum, gap to .060.

 

I bought ac delco iridiums- 41-993 which are gapped at 0.60. maybe i should go with the 41-985 gapped at 0.40 instead?

 

Up to you.

As you've found through your own research, a lot of people run all manner of different combinations of electrode material and gap in an LS1, and, all things being equal, it's fine. You're not gonna hurt anything.
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Given a reasonably healthy, naturally aspirated, engine, going to a tighter gap with iridium (IOW going with the recommendations in the TSB) might possibly help you squeeze out a little better fuel mileage.

The corollary to all that is that it becomes a matter of great import with boost and such.

 

Thanks for your impute, i just took out one of the plugs, and they are what i believe to be the originals. They are 41-485 iridium gapped at 0.40. I'll stick with them.

 

I changed the plugs and wires on my newly purchased used car, just to know for certain when they were changed out. I also went with the AC Delco iridiums, gapped at .40 replacing some good-looking, probably original plugs gapped at .60. I was able to hear a very slight improvement in the evenness of the idle, but with no observed change in the instrument readings. change was very slight, I have musically trained hearing, but I don't think that mattered much in this case, the new plug gap just evened the sound out a bit.

The plugs came out of the box pre-gapped at .40, and six out of eight were correct. They might have been dropped in shipping, mimicking the old school way of changing the gap, or missed by quality control, who knows. The critical issue was not to mar the interior electrode surface when adjusting or checking the gap, Problems with the electron path and a less than smooth electrode surface are why plugs now come pre- gapped. You might want to double check the electrode distance on your pre- gapped plugs, preferably without touching anything in such a way as to leave a scratch. Any peak or a point will attract an electron more readily than a flat surface, and you only want the spark at the gap..

 

You need another zero in there, my friend. Not being a smart ***, but I hate when I see wrong decimals!! It should be .040", and .060". Your decimals put the gap at over 3/8" and one at nearly 5/8". Personally, I'd go with .040", as I think .060" is a bit much. .040" is what I run. Just saying, and not trying to be an ***, although the wife says sometimes I don't have to try!!!!!

 

Your wife must be related to mine

 

Could be a "wife" thing, generally speaking! To bad they don't come with a scan tool, as many would show fault codes, and we could simply clear them! Suddenly, I'm hearing Louis Armstrong singing 'Wonderful World!!!!

 

The larger the gap the more tendency for a misfire---

 

 

That's what she said!!

 

Had a bit of of mismatch myself when replacing plugs recently. Manual for 98 says 1.524mm or 0.060in plug gap while the iridium plugs I was putting on had a .043in gap. Make sense now that everyone pointed out the difference iridium and platinum gaps.

Agree with everyone else saying replace the plug wires. Mine completely came apart replacing.

Also, on plug #7, do get a 1/4in swivel joint and socket with long extension. Most 5/8in spark plug sockets will be made for 3/8in drive.

 

The original platinum tips were only gapped to .060 for as long as the platinum pads stayed bonded to the electrodes. Six out eight on my car were missing when I pulled them, with gaps of .080 or larger.
Still ran well, but at idle sounded like a pretty healthy cam. Kind of regret changing them now.

 

Yeah noticed the platinum tipped ones were at .078 when I pulled them out too (only used them for 24 years!🤣, they were from factory). I should have replaced them sooner.

 

Put in some double iridium plugs. You'll get at least 40 years out of those!!!!!