Different generations of AC R43T spark plugs?

[Mark E.]

I ordered eight AC R43T spark plugs from RockAuto.com. All eight received are R43T, but came in three different types of boxes.
One is physically different with a longer insulator and its box shows significant shelf wear. Note the different AC logo as well (no "Delco").

Any guesses about the vintage?

[Jeffrey S.]

Re: Different generations of AC R43T spark plugs?

Mark,
Check under the flaps of the boxes and you might find a date "clock". The number in the center will be the year and the first number around the circle will be the month. This actually is the date that the box was printed but it will get you the general area of date. My guess is that the top box is from the 1990's and the others are more current. Probably made in Mexico.
Jeff

[Mark E.]

Re: Different generations of AC R43T spark plugs?

No date clock found. But one flap on the oldest looking box has a red "95".

Evidence that I may be first R43T customer RockAuto has had in years. Did they even exist then? Or maybe they buy out old stock from various sources?

[Duke W.]

Re: Different generations of AC R43T spark plugs?

Those plugs have a very slow sales rate because AC heat range "3" is only suitable for continuous high output operation with taper seat plug heads, such as race track hot lapping or heavy hauling. It looks like one box has been on the shelf for decades, but all are functionally equivalent despite the difference in boxes and artwork on the plug.

AC heat range "5" is best for all Corvette engines that only see normal road and highway use with only brief periods of maximum output demand.

Duke

[Mark E.]

Re: Different generations of AC R43T spark plugs?

Those plugs have a very slow sales rate because AC heat range "3" is only suitable for continuous high output operation with taper seat plug heads, such as race track hot lapping or heavy hauling. It looks like one box has been on the shelf for decades, but all are functionally equivalent despite the difference in boxes and artwork on the plug.

AC heat range "5" is best for all Corvette engines that only see normal road and highway use with only brief periods of maximum output demand.

Duke

Given your practical experience, I'm sure you're right.

Why does the CSM specify R43T? Bad spec? Maybe in the real world we discovered a hotter plug works better?

What did the factory install in a 1970 LS-5?

[Duke W.]

Re: Different generations of AC R43T spark plugs?

In general GM installed colder than necessary plugs back in the day - typically 4 in small blocks and 3 in big blocks. The 44s continuously fouled in my 340 HP SWC in normal around town driving. About 1965 after I learned about spark plug heat ranges, I installed 45s and plug life was vastly extended. Once or twice each summer when I drove out to Kent to spend the day hot lapping the track, I installed 43s.

If anything we probably drive these cars more modestly, today, so you don't need a "cold" plug because they rarely, if ever, see extended high load operation. That's why I recommend heat range 5 for all vintage Corvette engines that only see normal road and highway driving, including extended 80 MPH freeway cruising where the engine is still basically loafing at no more than about 30 percent of maximum output.

In the AC nomenclature system the first digit, 4, means a "conventional" 14mm thread with 3/8" reach and a gasket seat. The second digit is the heat range, which typically runs from 1 to 6, and the higher the number the hotter the plug runs, but some manufactures use the opposite convention - the lower the number the hotter the plug. Various suffixes indicate other geometric features like S for extended tip and T for taper seat. All plug brands have Web sites that explain the nomenclature and have cross reference charts to other brands.

The insulator needs to get up to about 700F to burn off deposits, but over 1300F can cause preignition that can lead to detonation. The lower the average load, the hotter the plug needs to be to maintain the minimum tip temperature, and AC heat range 5 or equivalent in another brand is the Goldy- Locks plug - not too hot, not too cold... just right.

Duke

[Mark E.]

Re: Different generations of AC R43T spark plugs?

Thanks Duke.

[Jeffrey S.]

Re: Different generations of AC R43T spark plugs?

Hi Duke,
Just to clarify: the second number relates only to the heat range. Any reach or thread configuration (full or partial threads) or tip extension or any other characteristic can have the same heat range. Any plug with no suffix has a 3/8" reach. Any other reach will be identified by a letter suffix.
Jeff

[Joe L.]

Re: Different generations of AC R43T spark plugs?

Given your practical experience, I'm sure you're right.

Why does the CSM specify R43T? Bad spec? Maybe in the real world we discovered a hotter plug works better?

What did the factory install in a 1970 LS-5?

Mark------

The original spark plug installed for 1970 LS-5 was the R43T.

As for the plugs you pictured, the box on the top of the photo is the oldest. It's probably from the late 80's to mid-90's. This plug was manufactured by the AC Spark Plug Division of GM, although not necessarily in the USA. The other boxes are much more recent. By this time, the AC Spark Plug Division of GM as a manufacturing entity was "a thing of the past". So, the plugs in the newer boxes were not manufactured by AC or, even, any GM manufacturing operation. They were out-sourced to other spark plug manufacturers who applied the "ACDelco" logo on them. Due to different manufacturers (AC versus outsourced), the tooling used to manufacture the plugs was different. Thus, the difference in configuration.

[Mark E.]

Re: Different generations of AC R43T spark plugs?

Thank you Joe.

[Mark E.]

Re: Different generations of AC R43T spark plugs?

Three days ago I requested an exchange for a matched set of 8 plugs from RockAuto. I already received a replacement box of 8, and they don't want me to return the original 8.

So now I have 16 R43T plugs, one of which is physically different but functionally the same. And all of which, according to Duke, are too cold.