Re: Correct spark plug for 1999 Glastron 5.7L Mercruiser
I worked for GM for 28 years. Let me explain the AC part number that you mentioned. First of all, I own a service manual for the v-8 mercruisers which recommends the MR43T as the stock plug. You had a R43TS in the boat when you acquired it. Here is the explanation of the AC part numbers
The R means it is a resistor plug which reduces radio noises.
The 43 is the HEAT RANGE. The next hotter plugs would be R44TS, R45TS etc.
An M in the prefix designates a MARINE application.
The L in the suffix means LONG REACH
The T in the suffix means TAPERED SEAT
and the S in the suffix means Extended Center Electrode
So, the previos owner substituted an automotive application with an extended center electrode for the original marine plug. The heat range was correct. I wouldn't use the long reach if I were you. Anyway, I am replacing mine with the MR43T which is the recommended plug. Hope this helps.
