Re: save a thread on cyl heads

[r-c]

For your application you might want to consider the J4C plug. In general any of the old engines that used a J6J now J6C can use a J4C, simply because we have better oil. If you idle a lot the J6C is better.
With inserts, interestedly, OMC had a heil coil part number that was brass with chrome plating as they thought it transfer heat fairly equal to the stock thread. While a stock heil coil is stainless steel. I never saw much difference.
The bigger steel inserts do hold heat in and change the plug heat range in some applications.
Run the engine and read the plugs, go from there.