Spark Plug Guru's


New Member
Jan 15, 2008
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.
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AUTOLITE 764 is what I am reading on teh other stang sites for gt40 heads.

If they are gt40 ask for plugs for a 93 cobra.

If GT40P ask the parts guy for plugs for a 98 explorer with a 5.0, and use those
Don't get autolites, get the OEM motorcraft plugs from Autozone.

The correct copper Motorcraft plug for the GT-40 head is the SP-447 plug.

The reason i now say pick motorcrafts over Autolite is that the specs on the motorcrafts are tighter than Autolites. Autolite's resistance values can vary while Ford want a specific tighter range. As a result, the motorcrafts tend to be more consistent from plug to plug.
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?