The Stovebolt Site Forums The Shop Area Engine Shop AC spark plug number please?

I just got my rebuild completed and am buying my first spark plugs for the 30 over rebuild. NAPA gave me R43 AC spark plugs. The service manual says another number. What is the proper AC number for a 235 that is 30 over and was the original engine to my 1955 1st series 1/2 ton 3100? I know the R means resistor type, but what is the rest....44 or 45 or 46 and what does that mean? Thanks - Scott

scdascda,

The 43 is the heat range of the plug. My '62 327 uses AC R44's. I can't remember right off which way the numbers go for 'colder' or 'hotter' plugs but the term hotter or colder refers to how quickly heat can transfer from the tip of the plug to the body. Hotter plugs are used when idling a lot (i.e. delivery, taxi, or police) and colder plugs are used when the engine is worked hard (i.e. highway trucks or racing.) The back of the AC sparkplug catalog is a gold mine of information on the meaning of all the digits in the plug number.

I run an HEI in a '69 350 and run AC R44SX plugs. Resistor, 44 heat range SX = Extended tip and wider gap (.060) for the HEI.

AC R45S or Autolite 86 gapped at .045" for electronic ign. AC R45 gapped at .035" for points. The AC R44 is hard to come by, you'd have to cross to a Champion part number. The R43 is the coldest, R44 is warmer, R45 is warmer still. Usually the second number is the heat range. Autolite 85s are cooler than 86s.... Each plug mfg has its own code for heat range, plug thread size, tip length, resistor, stock gap, etc.

I knew I had some plugs in my truck cabinet marked "plugs for truck". Went out to see what number they were and it says autolite 65. Maybe these are not what I will want to install someday in my '62 235 engine?

chiphead:

I didn't know that a different gap is used for electronic ign. I'm running a pertronic unit, should I be setting my gap at 0.45 instead of .035? If so why is this done? (just curious) Does it make a difference in the way the engine runs?

Thanks
George

Thanks for the info! I intend to use the truck for around town plus going 45 miles to the junk yard at 50mph. I guess the 43 would work fine, don't you? Thanks - Scott

I USED TO RUN AC 43'S (NO RESISTERS).

SINCE INSTALLING MY HEI UNIT I USE R43'S GAPPED TO .060

*organ music*

Today's second reading is from the book of Fire, Chapter 1, verses .035"-.060".

And lo,

The HEI or electronic ignitions can carry more coil primary current and switch it faster than the breaker points. The electronic ign coils are wound to make use of the extra current, and thus generate a higher voltage at the secondary (plug) winding. Thus you can open the gap wider and the higher voltage will jump the gap and produce a fatter spark.

Gap depends on plug design, voltage available, fuel mixture and compression ratio. The higher compression tends to "blow out" the electrical arc we call a spark. Its either close up the gap so the lower voltages can throw across the plug gap, or add more voltage. So thats why high-compression motors run better with electronic ignition. The cylinder pressure can't blow the arc out as easily. Rule of thumb I've adhered to is .035" for points, .045-.050" for HEI.

From what I understand, the .060" gap was specified in the late '70s to light the lean fuel mixtures in low CR engines of the time. Lean fuel mixtures and bicycle-tire cylinder pressures are why the '78 corvette had a whopping 180 HP. EGR my donkey.....

The .060" gap has been seen to cause problems above 10:1 CR. The larger gap coupled with normal wear of ign system (cap, rotor, wires, plugs) and high CR can cause misfire. The voltage at the plug wire has to peak higher to jump a .060 gap than a .050" gap. Thus arc leakage from wires and carbon tracking can shunt the juice to the block before it jumps the plug gap. Ever wonder why the race cars use 8 or 10 mm plug wires? Insulation.

can I get an amen....

cm,chip,stu,olblu,46,and all,

Thanks for comments and info. Autozone confirmed R43 that napa gave me. Checker said. ...It is getting so only gm dealers can get AC plugs. Thanks again - Scott

I bought spark plugs for my 1952 216 engine from NAPA yesterday. When I opened the box, three of the plugs were R43, and three of them were R43S. I learned that the S means "extended tip", and those plugs do have a tip that sticks out a little bit more than the R43.
Does anyone one know if it's OK to put the R43S plugs in my engine? If so, does it matter that I've got a 50/50 mix?

The extended tip plugs were brought in after these trucks were bilt. They are for o-head valve engines only. Practically all car co's went to them because it provides a better heat range. The flow of gases cools the tip. It is recomended they be retrofited to any o-head valve engine. Billy, I would take the R43 back & get R43s.

AMEN!

Thanks for the explanation.

Ok, I'll do that.
Thanks for the info.