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The suffix "M" stands for special design electro. That is the difference... If the letter "M" was used as a prefix it would indicate for Marine use such as a boat engine....
You should use the R45TS spark plug on a 1992 RS Camaro.
Spark Plug (ACDelco R45TS) [Gap @ .035” for points distributor, .045” for HEI distributor]
0.035 inches for CR43TS or R43TS AC Delco spark plugs
check with your parts house to get the gap for your particular application.
Spark plug gap for a 1977 Corvette using ACDelco R45TS is .035
R45TS
Factory plug TYPE is R45Ts plug. Gap is 0.045
AC CR43TS
72-75 AC R44T 76-80 AC R45TS
A/C plug # R45TS and it gaps at 0.045 That is the factory plug.
.045 " is the correct gap.