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Which spark plug for 1956 322

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I thought i would replace the plugs which i did. I couldnt find replacements easily at the time so i looked at the cross reference chart and purchased some NGK plugs that supposedly were the equivalent.


The AC Delcos that were in the car were toast but i noticed they had about 1/4" longer reach than the NGKs, meaning the Delcos fit further into the combustion chamber.  Not sure what effect that has on the running of the engine.


In any event, can anyone tell me which AC Delco plug should be used on the Century?  The shop manual says to use a R44 but search on the internet shows all sorts of variations of this - such as R44T, R44S and R44LT and the list goes on...

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So i found an AC Delco prefix reference chart. A prefix letter may or may not be used. R for example means its a resistor type. The first number denotes the thread size so 4 = 14mm. The second number denotes heat range (0-1-2-3-4-5-6-7-8-9-10) so 5 is mid range. Then there are various suffix letters which can refer to all sorts of things. For example suffix S means the plug has an extended tip. And L means long reach and XL means extra long reach. Unfortunately this does not help me to know what reach should be used.

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Most of my old AC catalogs show 44 or R44 but one does show " or R44S extended tip" . You didn't say what number was in it but going up one heat range to the currently available R45 might be worth a try. You should also be able to find R44S on ebay if you watch for them. Years ago the extended tip was considered by many has half a heat range hotter.



Edited by Dave39MD (see edit history)
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My 55 did not like AC R43 plugs...fouled after 20 miles.

I have been using Autolite 85 for the last 60.000 miles with excellent performance, changing every 30,000 miles.

Never use platinum or other exotic plugs in a car that does not have fuel injection/computer controlled; the first time you flood the engine the plugs are useless.

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The "reach" of the plug is the thread length.  That needs to be the same.  One thing I didn't like about some of the AC plugs is that they tended to have a "boss" that extended past the threads, into the combustion chamber.  Champions and others didn't do that, for example.  In my Camaro and our '69 Chevy pickup, I had much better durability from the old Ford Motorcraft plugs of the correct heat range.  I usually liked to go one notch colder so that I could advance the timing a few degrees more, which would raise the plug's actual temperature to what it would have been with the normal plug and normal timing settings.


The extended gap plugs have a wider heat range then those that don't have that feature.  Run a bit hotter at lower rpms to better resist fouling, but then the increased mixture flow at higher rpms will cool them off.  Some claimed that using extended gap plugs, in the place of normal gap plugs, was like effectively adding a few degrees to the base timing, as the plug was nearer the center of the combustion chamber and would light the mixture sooner, as a result.


I'd rate the NGK V-Power plug as "slightly exotic", but not in the same ballpark as their Iridiums or similar.


I found a spark plug designation decode chart on NGKs many years ago, online.  EVERY aspect of the plug has a designation for it.



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