Who's running NGK ZFR5N in there Slant?

GFlo said
I also have the HEI conversion, so I gapped them a bit higher in the 0.036-38" range (and you can certainly go higher).

I am not running HEI conversion (yet.) Mine are gapped approximately between 0.030-0.035" and looking at the spark plugs I appear to have been running a bit rich even with a NOS carb mostly leaned out with limiter caps. I just discovered my voltage regulator appears to be bad and disconnecting it appears to drop my idle speed a bit (which is fine because with the apparently bad VR my idle speed seems to be a little high sporadically 800-950..then 900 through 1150 at steady warm idle. I will tinker with idle after VR replacement then see if maybe I need to increase spark plug gap?

GFlo said
The plug tubes act as the spacer between the plug and head.

Anyone please make sure you clean your tubes as well.

GFlo said
FYI - these plugs are the latest Dan Stern recommendation (he's tried a lot of them).

No specific tuning specifications. If I recall these spark plugs are for medium temperature range medium performance range.