The chart that JENZ38 put up is great. The operative question on AC 46 spark plugs or AC R46 plugs is the R46 plugs has a resistor material inside of the spark plug insulator that was used to help reduce ignition noise in radios, etc. Both plugs in my opinion can be used interchangeably. They are both the same size and heat range. For plugs with an S after the number, the chart says extended tip. This is easy to see because those of us that have been around spark plug manufacturing call it a skirted plug. If you look at the area below the threaded portion, there is a smooth land that extends from the end of the threaded area to the end of the spark plug shell. If the number of the spark plugs are the same, the heat range is the same. Just a little more reach. On the comment about having AC-46 plugs in their car and they got some AC-48 & AC-49C plugs, these are plugs that run hotter. If you compare the tips by looking from the bottom electrode area, the higher the number the skinner the insulator around the electrode, therefore the hotter plug. The C after the number indicates that it was a "commercial" sparkplug. In the older days if you had a car/truck that was burning a fair amount of oil and fouling a sparkplug, you could put a hotter plug in that cylinder or maybe the whole engine to nurse it along until it was either overhauled or disposed of. So if the car had a regular AC-45 plug and over time it started to foul out the plugs, you could put in an AC-46, or AC-47 plug in depending on how much the plugs were oil fouling out.