What say we back up here, and define a few things, and examine just what all was done to theoretically correct this?
Sounds like you did not check it they way I described above.
1. A tiny crack in the porcelain filled with moisture
2. The permanent arcing has created a so-called creepage path on
Ayuh,.... It's gonna be covered right,..??
Just use ACX, or BCX plywood,... The glue in both is the same as yer Marine graded plywood,...
The 3-cylinder 25D calls for the B7HS-10 plug. So does the 25C3 2-cylinder.