Xdavid1999

So here is one of my many problems. I bought a classic car(1962 Impala SS) it was sold to me as having a LS1 4.8 that was built up. A few months later I was having some issues with it running after it was turned off. Long story short I pulled the casting number off the block and it comes back as a2005 Corvette LS2 6.0… Great whatever at this point… nothing I can do about that now. My problem is I pulled the old spark plugs out and there shorter than the new ones I just bought. About 3/16-1/4” shorter. The plugs that came of the car area NGK-BR7EF. I researched this plug and its not supposed to be in this car. The only info I can find on the plug is through a racing site that says to use that plug on high horse power, supercharged, turbo or a nitrous motor. The plugs I bought are for a 2005 corvette ls26.0L. So now my concern is what the hell do I need to run in this car? Can I put the longer plug that it says I need in it? The casting number on the back ofthe drivers side block is 12568952 and the stamped number on the front of the passenger side head is 29L215610. I pulled the valve cover off and everything looks like a stock GM head. Any advice would be appreciated.

79LS1FOX

iTrader: (11)

I would go with AC 41-110's keep in mind your heads are not cathedral port, which is what was used on the 2008 Corvette, they're LS3 style heads with larger chambers so you may have to 1 or 2 ranges hotter, but I would start with the AC 41-110s.

Xdavid1999

I was contacted by someone asking if they were the stock heads... The head number is 0821, people are stating that's an LS3 head? Anyone have any input on that?

1 FMF

iTrader: (1)

sparkplugs.com is a decent website and will give you technical specs on spark plugs.

Off top of my head the NGK TR55 fits the LS1, it has
thread size = 14mm
thread pitch = 1.25mm
seat type = tapered

these 3 things are needed for it to actually fit into the head.
from there the next biggest thing is the reach. If the reach is too long and it contacts the piston that's bad. If reach too short then the plug won't perform its best, but the engine will run.

The reach of 18mm or 0.709" is very common, I have these size plugs in my LS1, my 3.0L boat engine, a 351w boat engine, and I have TR55-IX in the 8.1L in my truck.

After reach the next big thing is heat range, which is always subjective but if you stay within the middle and use the most common you're almost always fine.

The NGK-BR7EF or the BR## plugs I have in my outboard engines, don't remember off top of my head their specs but pretty sure you don't want to use them in automotive cylinder heads. So cross reference the above specs based on NGK-TR55 to find spark plugs that are most likely to be correct for the application, and once you have it then you'd be getting the same plug but only in a different heat range either 1 or 2 steps hotter or colder.

Xdavid1999

So even if its an LS2 block with LS3 heads that people are trying to tell me now I could basically use the same plug? NGK vs AC I don't have a preference, but like you said I'm scarred if I use the wrong plug that it will be crushed by piston. So in reality I gotta figure out what heads these are before I screw everything up!

1 FMF

iTrader: (1)

2006 corvette will have the 6.0L LS2 motor.
2011 corvette will have the 6.2L LS3 motor.
you'll see they both call for NGK TR5 plugs, which is the same as TR55 just a half heat range colder.

and you could take a straw or other piece of plastic, put into spark plug hole and hand rotate that cylinder to top dead center. measure the depth and compare to spark plug to verify.

look up piston deck clearance for ls2 and ls3. both motors have the same piston stroke of 3.622" or 92mm.

wssix99

iTrader: (5)

I understand that this designation has to do with the block used and not the other internals. I'd check the block casting and see what it is. If it's a LS1 - then you haven't been short-changed.

Do you know who built the engine? They may be the only ones who have a definitive answer for you.


There are multiple head options for the LS1 block. As well, just because you have a certain casting - that doesn't tell you if any modifications have been made to it or the internals below it.


I would say the safe bet is to replace the plug with what was there, using the same gap.

I would say the ideal situation would be to find the builder and understand what their specs/recommendations are for the part.

1 FMF

iTrader: (1)

the reach of NGK-BR7EF is also 0.709"
the difference is out of the box the BR7 comes with a gap of 0.028" - like i said outboard motors I buy them at West Marine.
Not sure i'd want to go getting advice/recommendation from whoever chose those plugs...

Xdavid1999

I called the NGK tech line and she said stick with the same plug or drop down to the ngktr6 but don't go any lower unless I know exactly what's in the motor. The car runs fine on the plugs that are in it. I'll try the 6"s and see how it reacts. Does anyone know how to identify LS heads or have a chart? The head has a 0821 casted into it which some are telling me is an LS3 head? I know I've got an LS2 block from the casting on the block which is 12568952. We scoped the sparkplug holes and looked down in the cylinders and the pistons look flat with luckily no marks and the walls are spotless. From what I see it looks like a stock block. The only other thing I cant tell is if the cam is stock or changed... Going to check the compression next.