I have set of rebuilt heads that I have questions about. The casting number on them is 4006452. My research says that these are 400/440 heads used from 1976-1978. The problem is they have the smaller spark plug hole. I thought that these would use a plug like a RJ12YC. Instead, a plug like a RC12 will fit from my 93 dakota 318. The parts stores I visit cannot find a 440/400 equipped vehicle that required the smaller plugs. They say all 400/440's in that time period call for RJ12 or RJ14. Has anyone run across this before? What are the correct plugs for these heads? Any help will be appreciated.

 

the autolite part# is ap85..85 with a .35 gap.. that refrences with your rj12yc.. rc12 crosses with nothing that i can find... sounds like a champion plug..im sorry

 

Today I tried the AP85 plugs. They will not thread in. I have confirmed that it is a 14mm thread. The problem is the length of thread. Even when I try to screw in my compression tester, I have to use the adapter with the longer thread. Were there any 400/440 engines built in '76-'78 that used the longer threaded plug? If so, what plug number.

 

not that im finding i went thru the autolite data base. and they are all the same plug.

 

i had the same problem, i have the same casting number on my heads. mine came out of a 78 motorhome. They are 81.5cc heads and they use a plug with the 5/8" socket. I finally just went to parts store and asked for a plug for a 78 corvette. These are the right fit but i was not sure of the heat range. I found a guy at my Advance auto parts store that used to have several mopars and he sold me the plugs that he said were the right ones. I am at work right now but I will look when I get home to give yout he number. They work just fine. Also, the corvette plugs worked fine too.

 

Hey thanks Ram440. When I saw motorhome , I did more digging into the books. The correct plug is a Champion RV9YC. Its a taper seat plug and fits the 440-3 engine. I wounder why Chrysler put those in motor home engines? By the way, I kind of miss Florida, I was stationed at MacDill for 6 years.

 

that number looks familiar, it may be what i have in mine now. I will confimr back.