I have a 4.2 with the GM HEI dist. Getting ready to change the plugs. Per Performance dist they recommend .50 to .55, but I've seen some messages lately that suggest others may go up to .60. Even noted a improvement from .55 to .57.
You should use the stock plugs originally designed for the head. If you widen a plug that was designed for .035 (stock) to .060, the ground electrode will be deformed - the spark won't jump correctly. It wants to jump from a sharp edge to a sharp edge. That far open it'll jump to the middle of the ground electrode, not the tip's edge.
The plug needs to match the head, not the source of the spark. Don't use plugs designed for the HEI in another engine.
Don't use aftermarket or "gimmick" trash, no plats, splits, multi's, colorful or other junk. They are designed to sell, not to use.
Use the stock original plug - then you'll have the right heat characteristics, indexing, and position.
Sooo - best way - open them up to .045 - replace when they get to .050.
Going from .035 to .045 makes a nice difference in idle, but still lets it rev higher than you want to go.
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