NGK Spark Plugs

[Unreal]

I know this has been discussed before, but I did a search and could not come up with a definitive answer.

What NGK plug should I use in my L72. 840 heads, small dome pistons 9.75:1 static compression. 4346 Carb. Runs on premium pump gas.
Car will be street driven and probably not raced. With my A/C box in the way, it's a PITA to change #8, so I really want something that will last!!

Thanks,

[Chevy454]

R5671-7 (parts guy will probably show 4091)

BTW: click here

[Unreal]

Thanks Rob. I knew I had seen it somewhere here, and I searched under your screen name. Somehow I missed it.

[Chevy454]

No problem...just remember, the "-7" is the heat range, and NGK's run backwards of normal plugs...

[Unreal]

Rob,
I went to install the R5671-7 plugs today, and they are the 5/8 hex version, not the 13/16 hex. I guess I shoulda checked them out when I bought them!!

Anyway, if I read your other post correctly, I should either use GR5 (stock 2851) or GR4 (stock 2635)

Did I read correctly? Would you recommend one over the other for my application?

Thanks,

[Unreal]

Just called Advanced, and they listed the GR5 for an L72, and have them in stock. Since I dropped my compression to 9.75 to run on pump gas, do I need to go to one range hotter? Would that be GR4?

[Chevy454]

The 5/8" hex won't matter...in fact, it makes them easier to get a socket on...seems I had 'em listed as 13/16" hex, instead of the correct 5/8", but I fixed it: click

The hotter plug may indeed be better...

[Unreal]

Well, the thread diameter was smaller, too. I figured the 5/8 must be for 14mm and the 13/16 for 18mm....not sure about that, but I know I would have needed a LOT of pipe dope to get them to seal!!

Just called Advanced, and they stock the GR4's. Thanks for your help.

[Unreal]

Just out of curiosity, did the 291 head, which replaced the 840, use the smaller thread diameter spark plugs?

[JRSully]

the 291 used the 840 plug for the early part of the model year, then switched over to the peanut plug