This summer, I had intermittent pinging issues with my propane slant six, but when cooler weather arrived, I sort of put on the back burner. Then, I switched to a 205 thermostat. The pinging returned with a vengeance. In a move of semi desperation, I changed the plugs out to a very cold racing plug (NGK), and the pings went away, even when the motor's very hot and working very hard.

The problem I have now, is that the racing plugs are not resistor plugs and the electrical noise is, well, it causes issues with my crank trigger igntion.

I was running NGK UR6's, which were far, far too hot and pinged badly, not quite as bad as UR4's, though.

I'm using the R5673 series, but I'm unable to find a very cold resistor equivalent. I'd appreciate any input. Thanks.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

I'm not entirely sure that this crosses correctly. Take a look at AC plugs. R42TS should be as cold as it gets.
Fortunately, they're inexpensive.

Getting a plug colder than an NGK 6 range for applications like this isn't very good. There was a reference to a couple of colder plugs, but they don't turn up as the correct reach.
NGK BPR7FS might work, but you'd have to check it out to see if there would be problems with it.
Others in range would be Champion RV7YC, T22PR ND, or Bosch HR6BC. Maybe. I don't know.
Heat ranges are not on a standard scale, and the numbers don't necessarily line up with each other in cross referrence charts.
Now that would be a great place for the OEM's to demand standardization. Huh?

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Part of Tyrde-Browne Racing

I run AC Delco 45's in my 10:1 mill the engine likes something between UR4 and UR5...for some reason the R45TS has good uniform coloring across all 6. I've tryed 44's in a lower comp mill and worked good...
I'd be curious to see how the 42's work for you.

-D.Idiot

I'm going to drive a couple thousand miles on these and see what happens. I've managed to get the interference mostly ignored, so it runs fine (just can't use the CB), so I'll see what happens to the plugs themselves.

It runs fine, and I don't see how the plugs could foul on propane, so...???

I'll go back to the parts house and use their spark plug cross reference and see what it says about the AC's. I've considered trying Halo plugs, to see if they'll behave in this situation.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

update. I drove to 350 feet above sea level today and it started pinging again, while hauling down the interstate at 75.... I cranked the power block down (leaner) and pings went away. Didn't seem to hurt the power any.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.