I tried an Autolite AR3910X (magic spark plugs; Wallace) about a month ago and noticed that it’s not easy to adjust the gap. I found that my engine, with a stock plug, starts better with a wider gap.
The idea for this plug came from cutting standered plugs in order to raise the compression a bit. Some of the shoulder material is removed in order to place the plug further into the combustion chamber with the home made cut plugs. Also people remove the washer. Just don’t run the plug into the piston. The 3910 design has more material that takes up more room. Also the 3910 claims that indexing is not needed.
So did I see any improvements? The first test didn’t show any improvements if anything it may have run a bit off.
This weekend I opened the gap up a bit with a cut off wheel and Dremel. Still needed a harder pull to start the engine. I pulled the brush hog around for over an hour and thought it lacked a bit in power. So I shut down and reinstalled the stock Chinese plug. It started better and the air/fuel ratio was much wider. Power seemed to be restored.
Maybe later I’ll open up the gap more but right now I would say stick with a quality standard plug that is gaped and indexed for your engine.
The other issue with this plug is the hex size is 5/8" and I cannot use the plug wrench that comes with the engine…