Here is thre different gapless plugs. Left to right
NGK BUHW (used)- NGK BUHX (new)- Champion L20V (new)
All have 1/2" long threads.
The BUHW has a wider gap and no booster gap
The BUHX has a smaller gap with booster gap
The L20V has a bevel edge at the gap with no booster gap
I just installed the Champions. I wonder if it will do a better job of not fouling because of the bevel edge. Some swear by Champions. I'll post the results.