As mentioned the 186 heads are gasket 13/16 plugs, not the taper 5/8 plug (R45TS)
I prefer the extended tip plugs that have the "S" on the end of the part number. Example R45S All this does is put the electrode slightly deeper into the combustion chamber for a more complete burn and better combustion process. If you run an R45 for example, same plug, but you'll clearly see the ground strap and electrode are somewhat hidden up inside the plug. We used to call these plugs oil burnin plugs because back in the day we used them on engines that were burning oil, less likely to foul the plugs since the electrode was hidden up inside a bit LOL.
These days I use Autolite. For the 186 heads you'll want Autolite 85's or 86's. The interesting thing about these is that they are already extended tip plugs and as far as I know that's the only way they come from Autolite, they don't make the non-extended tip (oil plugs
)
The only difference between this and their racing plug is a shorter ground strap that exposes a bit of the electrode. I cut the ground strap back a pinch on the standard Autolite plugs for the same affect.
