D15 II Spark Plugs?

OK, as pertains to my original question, here is what I think I learned.....

As horsepower goes up, a slightly cooler plug is used......so that may explain why the cooler plugs were spec'd for the larger 160 engine over the 143.

Heat range of plugs determined by changing tip insulation and distance heat must travel to disperse heat to the block.  The extent to which you can crank up the temp inside the cylinder just by making subtle changes to the plug tip seems bizarre to me. Especially considering how many times a plug fires per second.

As to the D15 II and 160 engine......new replacement plugs from modern lineup would be Autolite 303 or 295. Cooler plug is the 303, a resistor plug, and would be used as the OEM replacement for a tight engine, or recent rebuild. If that fouls......the slightly hotter non resistor Autolite 295 can be used to help burn off the oil to resist fouling.

Start with the cooler plug......no long term harm to engine from fouling. But a too hot plug will do damage to pistons.

Given the state of this engine, 5,200 hours on original engine and burning a bit of oil with plugs fouling quickly, ought to be using the 295's.

Conclusions from an old dog still learning new tricks.

Hubert (Ga)engine7 wrote: I put a Pertronix ignition system in it but it crapped out within a few months while I was in the middle of bush hogging.

You probably should have put that coil in at the same time the Petronix was put in.  If the install was only a few months old, it would have been replaced under their warranty.  I've got three older veh's with Petronix, and never have had an issue on any of them...