Topic: need spark plug help
in Forum: C3 Engines
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Frederick, MD - USA
Joined: 9/8/2003
Posts: 3398
Vette(s): 1969 convertible L71 427/435 4-speed black interior
Whoever installed those heads on the car simply may not have known better. They could have bought them at a swap meet and didn't know the difference. Either way, find out what they're original application is and go from there.
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Duncanville, TX - USA
Joined: 11/8/2003
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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
The original heads for this year should have the tapered seat plugs. Possibly the heads have been swapped for earlier ones that do use the gasketed plugs. The casting # will help, as well as the dates. Check HERE for locations of the casting and Date codes.
Joel Adams
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thanks for all the help. i will pop a valve cover off this weekend and post the casting number.
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Willow Grove, PA - USA
Joined: 10/27/2006
Posts: 232
Vette(s): 1974 Stingray Coupe LS4 454 Dark Brown Metallic with Deluxe Saddle interior
My 74 454 takes the r44t's just fine.. do you mean they dont even thred into the head threads at all?
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
What he has is the older style, 13/16" hex plugs, with flat seats, and a metal gasket. Most(all?) of our cars should have the shorter, 5/8" hex, tapered seat plugs. The older plugs have longer threads than the tapered seat plugs do, and that is what he is saying when he says they won't thread into the head, The threads just won't reach down in there far enough.
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Former Member
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Willow Grove, PA - USA
Joined: 10/27/2006
Posts: 232
Vette(s): 1974 Stingray Coupe LS4 454 Dark Brown Metallic with Deluxe Saddle interior
oh good grief!
Gotta wonder what heads they are for then!
any chance at one time the heads were cross threaded bad enough that someone tapped ALL the threads and spot faced the seats so all the plugs would be the same...
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Noper...read the first post closely...Someone has change the heads on his car to early heads. Those take the larger plugs. 
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
the head casting numbers are 3917215 gm5 c 11 8 pass and 3917215 gm6 c 18 8 pass. i think they are 67-69 396-427 closed chamber. i got ac r43xls plugs and they seem fine but didn't start it yet i am in the middle of replacing the exhaust.
thanks for all the info guys.
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
#3917215 heads were used on:
'68 427 Corvette & Passenger cars, 390/400HP, oval ports
'68 396 Camaro, Nova, Chevelle, Pass.
These are the early heads, with the big plugs.(The tapered seat plugs were first used in 1970.)
They should also be closed chambered, but I'm not 100% sure on that.
'68 427 Corvette & Passenger cars, 390/400HP, oval ports
'68 396 Camaro, Nova, Chevelle, Pass.
These are the early heads, with the big plugs.(The tapered seat plugs were first used in 1970.)
They should also be closed chambered, but I'm not 100% sure on that.
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
in Forum: C3 Engines
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