On big blocks running 14mm, 3/4" reach, 5/8 hex plugs with Hooker 2-1/8" diameter pipes clearance issues exist between plugs, plug boots and wires and the header pipes, as seen in other threads. There was a Tech tip in Jegs about using Champion RC12YC plugs for this application because they are about 1/4" shorter than other plugs. This turned out to be true and made life easier. There was another brand plug named as well...NGK BKR5E.
In trying to solve a misfire problem at 6000rpm, I was looking for a colder heat range plug...the next colder one in the same Champion line (RC10YC). Couldn't find it easily, so I went with the NGK plugs, but also in one range colder (BKR6E). While these plugs didn't solve the mis-fire problem, to my surprise, they are shorter than the Champions by another 1/8"!
Yippee!
In trying to solve a misfire problem at 6000rpm, I was looking for a colder heat range plug...the next colder one in the same Champion line (RC10YC). Couldn't find it easily, so I went with the NGK plugs, but also in one range colder (BKR6E). While these plugs didn't solve the mis-fire problem, to my surprise, they are shorter than the Champions by another 1/8"!
Yippee!