Intermittent ign. prob. FOUND--porcelain cracking
The porcelain was cracked on my 3,4,5 plugs! The cracks ran the long way in the insulator between the threaded steel part and the wire end. The insulator seems fine up around the anode. The plugs did Not have screw on lugs.
The plugs were Autolite 3924's. The engine is a 258 w/ 4.0 head. The 3924's are one heat range colder than spec. I just installed a set of 3923's which are two steps colder.
What caused this cracking? I am thinking that the plugs were too hot, and couldn't expand because of the fixed (non-screw on) lugs? The 3923's have the screw on lugs, we'll see how they do.