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post #1 of 4 (permalink) Old 06-21-2018, 05:36 AM Thread Starter
Join Date: Jan 2018
Location: Auckland New Zealand
Posts: 41
Spark plug recommendation

Could someone please recommend a suitable spark plug for a stock small block 350?
It currently has what looks like some old Autolite 26

Last edited by NZPhil; 06-21-2018 at 05:59 AM.
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post #2 of 4 (permalink) Old 06-21-2018, 10:12 AM
Join Date: Jan 2006
Posts: 12,381
I would use a new Autolite 26. You know that works.

AC-Delco spark plugs went out of business in 1962. GM buys the plugs that are labled AC from the cheapest vendor when they let contracts. Autolite, Champion (both owned by Ford) as well as NGK, Denso, or Bosch are what you will find inside an AC-Delco box today.

I just love reading about how a champion plug is trash compared to an AC plug, or an AC is much better than a NGK plug. A plug is a plug, despite what E3 says. They all work the same way; with the only difference being the heat range and the choice of metal used as the cathode in the plug (I prefer a platinum or iridium metal as they last a lot longer than copper, even though they cost more initially). All plugs made today are resistor plugs due to computerized ignitions that do not like static any more than your radio does.

Big Dave
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post #3 of 4 (permalink) Old 06-21-2018, 02:48 PM Thread Starter
Join Date: Jan 2018
Location: Auckland New Zealand
Posts: 41
Thanks Dave, unfortunately when I got the car it was running but not road worthy so how well it was running is a little unknown. I’m ok with using the same plugs but would probably prefer to upgrade if you can offer a recommendation. Thanks
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post #4 of 4 (permalink) Old 06-21-2018, 03:36 PM
Join Date: Jan 2006
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Like I said Chevy listed an application for your year as being an AC-Delco R45TS plug. This is a 5/8th inch hex tapered seat plug.

R tells you that this is a resistor plug. A resistor plug has a tiny air gap inside the plug to prevent voltage from bleeding off early as the plug ionizes the air in the combustion chamber prior to the full coil discharge without the air gap.

The 4 stands for a 14 mm thread on the spark plug body, where it threads into the head.

5 is the heat range: though a 3 to a 7 will "run" in the engine but I do not know what your engine needs as to it's heat range (higher number used in a clunker that burns oil to prevent fouing, a lower number used in a high performance engine that is being fired by a high performance ignition system. If you pick the wrong heat range you could find yourself in detonation as the spark plug becomes a glow plug.

T stands for a tapered seat. Older plugs had an aluminum washer that was crushed by the flat end of the plug sealing onto a flat machined head. This required a bottoming tap with a flat faced cutter attached which is an expensive tool. So to save money GM went with a 45 degree cutter on their taping tool that is less likely to break and made the machining process faster (and cheaper even though it didn't seal as well).

S stands for an extended tip which could be problematic if you had a domed piston (but you don't). To run an extended tip and a domed piston you have to first index your plugs so the dome can not hit the tip.

Here is the complete nomenclature list for AC-Delco plugs:

The AC Delco spark plug numbering system

Big Dave
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