My best friends brother has a 97 f150.about every 3 months he has to change the #3 plug.The engine developes a miss.When he removes the
plug the porcelin has a crack the entire length of it.We know it is not
from installation because the the crack is not around the plug.
It does not matter who puts the plug in it cracks the same.Three different
have installed the new plugs.As stated above the crack is the entire length
of the porcelin not around it.Does anyone have solution?

 

I installed Denso Iridiums in my 97 when i had 1 crack like you are saying.
They are in the truck now 2yrs with 28,000mi and still going strong.A little expensive but i think worth the money because i dont enjoy changing these plugs.

 

My 97 F-150 4.6L blew the #7 a lot just as you described. I quit using the Motorcraft plugs and started using the Bosch Platinums and the problem disappeared.

The Denso Iridiums are tough plugs and would be a great choice.

 

Has ur friend changed the wires and coil packs. Might have a problem before the plug itself. I had to replace a coil pack and wires right before I traded my 97 in.