Recently purchased a '93 supercab 4x4 F-150 and bought some parts to do some neglected maintenance for this weekend.
Bought the recommended Motorcraft ASF42C plugs and just couldn't figure out why they didn't want to thread into the head...until I compared them to the old plugs which was a AWS32C. After a confusing hour on the internet it seems that I have GT-40 heads on my truck...I think.
Do the GT40 heads have casting numbers somewhere?
Thanks,
Russ
(feeling lucky in San Antonio)

 

Look at the end of the head. There is a raised pad. If it has 3-lines on it it is a GT-40 head, if it has 4 lines it is a GT-40P.

 

Thank you Strange.