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 (http://www.yenko.net/ubbthreads/showflat.php?Cat=0&Board=UBB10&Number=322242&Searc hpage=1&Main=321345&Words=5671+Chevy454&topic=&Sea rch=true#Post322242)

[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 (http://www.yenko.net/ubbthreads/showflat.php?Cat=0&Number=322242&page=0&vc=1#Post3 22242)

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!! http://www.yenko.net/ubbthreads/images/graemlins/smile.gif

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

[Chevy454]

They both used 14mm threads...in fact, if it's an original carb'd sbc/bbc then it's gonna be 14mm threads...the seat design, thread reach, hex size, etc, may be different, but the threads are gonna be the same. If the 5671s don't fit, I'm betting the GR5s won't either...

As for the switchover from the gasket seat to tapered seat design, iirc it was in May...I'll have to check my notes to see exactly which week, but we have a set of 840s that share the 1.88 exhaust valve casting size but have the gasketed plug seat...and I've heard of 291s that are just the opposite.