What are best spark plugs for a 1951 331 c.i.

[hearn]

What brand and number have folks found to work best in the 331 V-8?  Thanks for your suggestions.  Jim

[Rockfish39]

ACDELCO R45S Professional; Conventional
Gap .035

assuming that the ignition system hasn't been converted to HEI, magneto, or subjected to other high performance mods...

[Jay Friedman]

I agree with Rockfish39.  In my '49 with the same motor I have also found that AC R45S plugs work best.  Autolite 86 plugs also work well in my car.  You can buy these in any auto parts store.

AC 48 and 46-5 plugs were used on 1951 331 motors (and on my 49) when they were new, but they are no longer manufactured.  NOS AC 48s and rarely 46-5s can be found on ebay and elsewhere, but unless yours is a show car I would not use them.  John Washburn, who writes the Washburn's Wisdom column in the Self Starter wrote on article on NOS spark plugs a year or so ago.  He tested various brands of NOS plugs on a machine and found that even though never used a certain percentage failed the test.  It seems that plugs can wear out merely by lying around on a shelf for many years. 

[Caddy Wizard]

Autolite 86P, IMO...

[walt chomosh #23510]

Art,
  Would your Autolite selection of plugs include the 1955 model?....as always, your opinion is valued and respected....walt...tulsa,ok

[Caddy Wizard]

Art,
  Would your Autolite selection of plugs include the 1955 model?....as always, your opinion is valued and respected....walt...tulsa,ok

Yes, I run the Autolite 86P in a 55.  In fact, I run that plug from 49 to 55.  Looks good.  Performs very well.  The platinum tip just makes it last a little longer (no power advantage).  Since I am usually working with a hotter spark with a Pertronix system, I usually open them up a tad to about .040 or so.

[Lexi]

Art I installed a set of NOS AC 44s gapped to .035 in my rebuilt 1956 365, (now with Pertronix). I don't seem to have any issues. Would you still open them to .040 or just leave alone?

As for the 331 V8 plug discussion, I have seen various NOS sets on Ebay. You may have to look for a while but they should turn up. Yes, John's article is still a factor now when I buy old plugs. Clay/Lexi

[Caddy Wizard]

Art I installed a set of NOS AC 44s gapped to .035 in my rebuilt 1956 365, (now with Pertronix). I don't seem to have any issues. Would you still open them to .040 or just leave alone?

As for the 331 V8 plug discussion, I have seen various NOS sets on Ebay. You may have to look for a while but they should turn up. Yes, John's article is still a factor now when I buy old plugs. Clay/Lexi

I would open them up to .040, especially if you are running a hotter coil.  One of the advantages of the Pertronix is that you get a hotter spark.  That higher voltage can jump the larger gap easily and give you a hot ignition flame.  You should find that the idle is better with the larger gap and it should have a tad more power. 

[Lexi]

Art, as always much thanks. My NOS plugs have compression gaskets on them, for an effective seal. I seem to recall back in the day taking plugs out, cleaning and re-installing, (without replacing the gasket). Is there a re-use period, like good to go for 2 or 3 removals or should new gaskets always be installed, once plugs have been removed for service, prior to re-installing them? Also, do you know if the 331 engine plugs came with those compression gaskets up to the '55 year as well? Clay/Lexi

[Caddy Wizard]

Art, as always much thanks. My NOS plugs have compression gaskets on them, for an effective seal. I seem to recall back in the day taking plugs out, cleaning and re-installing, (without replacing the gasket). Is there a re-use period, like good to go for 2 or 3 removals or should new gaskets always be installed, once plugs have been removed for service, prior to re-installing them? Also, do you know if the 331 engine plugs came with those compression gaskets up to the '55 year as well? Clay/Lexi

I have never bothered to replace the gaskets/sealing rings on the plugs when removing and cleaning them.  Typically, the gaskets are aluminum and can be reused with good effect.

[Lexi]

I kinda thought that. Good to have your perspective on this. Thanks again Art, your insight and assistance is greatly appreciated. Clay/Lexi