Spark Plug Guru's

I meant to pose this question yesterday, but i had to get to a party...

yesterday i was changing my plugs and i went to autozone (because i know they carry autolites), and i got the plugs they said go with my year mustang... now, as you all know, my stang is carburetted and i don't know the exact year of the block. So, i get home with these plugs designated to fit the 1989 ford 5.0 (autolite 25's), but when i take out the first plug, i notice that the thread pattern is different. i look at the plugs and it turns out that the ones in the block are autolite 104's, not 25's.... i said "hmmmm..." and still tried to put the 25's in - they don't fit..


so, any of you guys know what kind of blocks the Autolite 104's fit in, or better yet, what kind of blocks the 25's DON'T fit in?


i'm real curious.

well, they're gt40's, but i figured it's all the same? so is that the reason?

...i think that might have bee a seriously "duh" moment. hah, whoops :x

no no no, i already have the autolite 104's.. i went out and bought them yesterday.. gapped them to .054 and i love them. they're great. but i was just curious why the 25's don't fit but the 104's do? what motors/heads to people run with 104's?