Hi Ross,
Since I just installed some 386s, & obviously don't want to hurt my engine, can you expand on that a bit? According to Champion's info on their aviation plugs, they have a heat range from 25 (cold) to 50 (hot). The recommended plugs for my IO-360-B1B (parallel valve, 8.5-1 compression) being 37, 38, and 40, or just on the hot side of midrange. The Autolite chart I was able to find shows the 386 as being 1 'step' above the center of a 6 step heat range for their plugs.
Our dilemma is, how to relate the two ranges. I can tell you that an REM37 aviation plug has a *very* long insulator, and a very extended tip, compared to an aviation REM38. I have *not* examined how deep into the barrel each insulator goes; I just read your post. The REM 38 and the Autolite 386 both have nearly flush tips. As soon as possible, I'll check insulator depth into the barrel on both plugs.
Charlie