When you can see noticeable wear on the ground strap and the center electrode as well as damage to the center porcelain insulator
Or when the motor starts missing due to bad plugs or wires
That seems reasonable. Hard to visualize wear on the center electrode, because they are so small/odd shape to begin with and don't exactly recall, and the "little" erosion I do see, seems like a regap would suffice?
The one's I'm looking at are NGK, truck runs fine, coil on plug 90K on Plugs, 500K on truck. I was going Autolite Plat, because they were so cheap?
