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LS1 spark plugs B4C 41-985 vs replacement 41-101 & GAP

This is a discussion on LS1 spark plugs B4C 41-985 vs replacement 41-101 & GAP within the General Help forums, part of the LSx Technical Help Section category; Hello all. New to LS1s and this forum. (Not to Chevys, 63 YO and had many). Need help on this. ...

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    Question LS1 spark plugs B4C 41-985 vs replacement 41-101 & GAP

    Hello all. New to LS1s and this forum. (Not to Chevys, 63 YO and had many). Need help on this. I just got a 1999 B4C optioned Camaro with 127K. One police dept owned and not maintained very well, but driven by what seems to me as one person. Changed rear end oil and additive, changed trans oil and filter, changed engine oil - synthetic - and filter, and then got to the spark plugs. Took the ORIGINAL 41-985 ACs out that had 127k on them, was surprised how good they looked, and tried to get replacements. AC put out 41-101 years ago and scrapped the 41-985s. Bummer. By the way, the ORIGINAL plugs in this car were set st .060 from GM , NOT .040!!! No electrode wear to be seen with the naked eye and a good light. So I know they were .060 to begin with. After hours of looking online for 41-985s and hearing every comment imaginable about the different plugs on market, I went to Advance to see what plugs they had and what they looked like. The counter guy took a real interest in this and brought out every NGK, and AC 41-101 he had. I will not use Autolite, Densco, etc. But have been considering Accel - pricey, and would like to know more about E3 plugs, never heard of them. Anyway, I decided on a set of Platinum NGKs (Iridium next like original maybe?) because the thread length was about the same - I would NEVER use some of the NGKs he brought out that were for this car because the threads were way longer on some and I'm not risking hitting pistons and blowing up a fine aluminum motor - , the electrode insulation (hot vs cold plug) was about the same, and the GAP was EXACTLY the same (.060) as the 41-985 I brought with me. The electrode length of the AC 41-101s were about 1/4 of an inch shorter (hotter plug) than the original 41-985s - I did not like that! Put the NGKs in and the car ran worse. Had misses - surprised a service engine light did not come on - and was very hard to start after I shut it off. Yes, I disconnected the battery before starting with the NGKs. Today I put the old 41-985s back in and the car runs exactly like it did before I changed them and still has a somewhat rough idle, but does not miss when accelerating and starts just fine. I would get the 41-101s and probably keep the gap at .040 as they come, but one person on this forum said he got 4 to 5 miles less a gallon and had backfires upon deceleration with them. Don't want that. Any opinions and thoughts welcome. One bottom line I want to throw in is: GM did a great injustice to these 1999 LS1s by not continuing to supply the exact plugs gapped exactly as they came from GM.

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    They simply changed the P/N on the plugs. I had the same issue when I replaced the plugs in our '07 Suburban last year and went looking for 41-985's. No biggie, just bought the 41-101's and all is well.

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    NGK TR55 plugs should work. Gap to .59 - .60


    Also have run the TR5IX as well no issues with with fitment.


    Have you cleaned the MAF? A lot of misfire and idle issues can be attributed to a dirty MAF. Especially if a K&N type filter is used.

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    No, haven't cleaned MAF. Good idea. But I mainly want to change 15 year old plugs with 127k on them and don't want to spend lots of bucks experimenting - already shot $35 on the NGK platinums that did me no good- and they are a PITA also to replace. I'm assuming the TR5IX are Iridiums and what do they come gapped at? Because I've heard don't regap iridiums, but I don't see why not if you are careful and don't damage the electrodes. The TR55 plugs are regular copper coated and regular tipped? What do they come with as far as the gap? Somebody before me recently replaced the stock air filter and this car has no mods. Thanks. Bob.

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    Spaz is My Mentor SMWS6TA's Avatar
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    The gaping for Iridium's is different from platinum or copper core plugs. The electrode doesn't move like the others. It's fixed to the correct height from the factory for the best spark, plus it's a harder metal than copper. Most ppl will crack the porcelain, that's why there's no gaping.

    TR55's I've just bought were about $2.50 a plug. All but 3 were already gaped to .60, the others had a .54 gap. So those will be gaped to .60.


    Still have stock exhaust? There are some tricks that can help you. #8 cyl is by far the worst one to do. Let us know if you are on stock exhaust so I can post up some methods to help.


    Scratch that I just reread you have no mods...so.



    Loosen/remove the coil on #7 & 8. This should help get to the plugs to remove. Install 7 from the top and 8, sorry but with stock exhaust jack it up and go from the bottom. You may need to remove the starter for more ease of room.

    If there is any justification for LT Headers it's because it makes it 1000x easier to do plug changes.
    Last edited by SMWS6TA; 08-19-2014 at 03:06 PM.

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    Ok, sounds like tr55 will work. I will also check out the tr5ix and see what they come gapped at since it sounds like you really should not gap these iridiums. Still a little confused on why you can't regap them because if you use the old style gapper like I've got, you are only bending the arm of the electrode, you're not touching the porcelain and electrode centers if you're careful, so why not? Is the entire electrode "arm" you would normally bend to adjust made of iridium too? I thought just the small contact in the center of the "arm" at it's end- round, slightly raised protrusion pointing downwards and toward the other electrode - was iridium. I know the needle like contact in the center of the insulator is iridium. Thanks for comment and help.

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    Spaz is My Mentor SMWS6TA's Avatar
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    put my plugs in last night and discovered the easiest way to install plugs. No headers, coil packs in the way. So easy



    I'm reinstalling my heads after having some work down. But it was fairly easy with no headers in the way..

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    Yeah, it's certainly not a fun thingto do with stock exhaust and all the rest in the way. PITA

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    Quote Originally Posted by wildrq View Post
    Yeah, it's certainly not a fun thingto do with stock exhaust and all the rest in the way. PITA
    I run 41-101's at 0.040" and it runs great.

    Get #8 from the bottom, the rest go pretty quick.

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    Quote Originally Posted by wildrq View Post
    By the way, the ORIGINAL plugs in this car were set st .060 from GM , NOT .040!!! No electrode wear to be seen with the naked eye and a good light. So I know they were .060 to begin with.
    If you did a search back about 12-13 years ago you would see that GM did a revision on the spark gap of LS1 motors. This was about the time NGK came out with the Tr-55 iridium. The recommended gap was reduced to .040 from .060. I believe there were ignition issues that prompted it. Also the fact that the OEM AC plugs were prone to losing the platinum button welded onto the ground tab. The rest is history.
    Last edited by SSnow; 09-01-2014 at 05:03 AM.

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