Thanks for the replies. I tried researching by plug construction and couldn't find one with a 5/8" hex and a crush ring or gasket seat; only in a taper seat. At least in the short reach as used in our engines. www.sparkplugs.com is a pretty good site for researching spark plugs. Oh well, I was hoping someone knew of one. Looks like I will have to machine the heads for taper seat, or buy other plug caps for the RCEXL ignition I have that is for 5/8" hex plugs. (I know taper seat plugs will "sort of" seal in a gasket seat head, but it is a bit questionable. Better to machine a small taper.)

AV8TOR