Okay, the guy I sold my '73 302 LDO to was trying to change the plugs, and two different parts stores gave him different size plugs than what's in there. I'm not just talking about the socket size, I mean the thread pitch as well. Anybody got an idea here? Does this puppy maybe have aftermarket heads? Which had the 5/8 plug and which was 13/16, and which is correct?
The new heads, GT40P, GT40, etc have a deeper and smaller plug. Not short and shallow, like the stock ones. Different size socket also. Not sure if they confused it with those...
So, either the car has late model heads and the parts store is giving us the right plug (less likely,) or the car has original heads and the parts store is giving us the wrong plug (more likely.) Interesting, but not surprising. Gotta love SoCal. It took two days and trips to six parts stores to find the correct front brake cylinders, and finally found that CarQuest had the right ones, but they were catalogued as something else.
Stock plug should be the big plug, 13/16" Hex with a 18mm thread What part number plug was in it? Why doesn't he just purchase the same part number?
parts guys nowadays probably have someone else to do thier tuneups. they try to cross reference plugs and usualey give you the wrong number. oldtimers usually have the number in thier head.
I know some 302's have the standard size spark plugs that everyone is use to and there are some like the 70 model DOOE 302 heads and the DOOE 351W heads which have the BIG style spark plugs just like the inline 6's. Dont know if this is whats going on but it may be something to consider.
Is the engine the original? If it is, it will most likely have the later heads, which use the smaller plugs. The "small" plugs are 14mm thread and 5/8" hex. "early" plugs were 18mm thread and 13/16" hex. I think the switch from "early" to "late" was around 1972 for the 302's, IIRC. I have some '72 heads both ways so that makes me think that '72 was the transition year.
