AC spark plug suffixes decode

[Charles P.]

Last week there was a good thread about spark plug heat ranges and number decoding. (4 is thread size, second number heat range) My question is what do the the letters after the numbers mean. I see R45T, R45TS, R45SX, R45NX etc. If an R45 ACNITER is recommended on a base engine 72 or 73 what would the correct suffix be?

[Joe L.]

Re: AC spark plug suffixes decode

Charles-----

The suffix refers to the seat style and tip style. Earlier plug models (like the R-45) did not use a suffix code since all were gasketed type. A "T" suffix refers to a tapered seat (i.e. "peanut" plug). The "S" refers to extended tip.

[Charles P.]

Re: AC spark plug suffixes decode

If R45 is the plug best suited would T or tapered be best or correct?

[John H.]

Re: AC spark plug suffixes decode

Charles -

If your '72-'73 has the original cylinder heads, they're machined for the tapered "peanut" plugs ("T" suffix).