I purchased a huge box of NOS parts for just a few dollars recently that came with many sets of plugs which were a bonus. I had thought the R45S and T were the same but was surprised to see how much fatter the S plug is (see pic). From reading the archives if "4" designates thread size and "5" heat range would these plugs be otherwise interchangeable? I use R44T now and am going to use the R45T's as a step hotter plug. Does the R45S plug have any Corvette applications? Also The 45S plugs each came with the Zinc washers in the boxes--If I can use them do I need the washers? The box had NOS 308R caps in it so I can't lose even if I can't use the S's.
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Re: AC Spark Plug R45T Vs. R45S 73 L-48
"T" stands for taper seat plug which the later chevy heads used. i believe that started in 1970. the"S" stands for extended tip and the gasket is needed to be used if the heads do not use the tapered seat plugs. you can not use the gasketed plugs if your heads take the tapered seat ones,they are not interchangeable -
Re: AC Spark Plug R45T Vs. R45S 73 L-48
The R45S is an acceptable replacement plug for most pre-'71 SBs with non-taper seat spark plug heads. The OE AC plug was 44 until the switch to resistor plugs for additional EMI control circa 1969 when it became R44. All these non-taper seat type plugs should be installed with gaskets and heat range "5" is best for normal road driving regardless of engine power rating.
The "S" suffix means extended tip. They were not OE on most vintage small blocks, but some prefer them, and they may have more resistance to fouling. There have been occasional reports that extended tip plugs interfere with SHP domed pistons, but this is usually only an issue if the block has been decked, heads milled, and a thin gasket is used so quench clearance is at or below Chevrolet's recommended .035".
The R45S is still readily available and inexpensive.
The "T" suffix indicates a taper seat plug, which was used on SB heads beginning in MY 1971. These are sometimes referred to as "peanut plugs", and they are NOT interchangeable with the earlier type gasket seat plugs. They do not require gaskets, and I believe they are also currently available. If your '73 still has the OE heads, the R45T is a good plug to use.
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