Hey guys my cousin has an 84 gmc that he drives daily but needs some attention its a 305 with more then likely stock heads that needs love I have two sets of center bolt heads here that he was gonna use to send out to machine shop one set is a 350 other is a 305 both center bolt valve cover style . Difference if he was to go with the 350 heads and these 305 also WILL they work which ones a better option to get redone also. gm14102183 and 14101081pictures below
1984 gmc 305
1 - 11 of 11 Posts
Joined
·
15,298 Posts
The problem with center bolt cover heads is first I’m going to assume these are Swirl Port heads from 87 through 95. They use a slightly modified standard intake bolt pattern with the bolts along the plenum. Pre-1987 all the intake bolts are 90 degrees with the head’s machined intake surface. Starting in 87 going through 95 the bolt pair adjacent to the intake’s plenum were changed to an angle of incidence to the head’s machined intake surface to 72 degrees. The problem this will cause is the 1984 intake will not bolt up to 87-95 heads even though looking at them it’s the same bolt count in the same bolt to bolt locations. Given this is where the exhaust heat crossover is this is a highly stressed area that needs to be securely fastened.
This can be worked by clever people by egging the center holes of the intake manifold and milling/gringing a new flat surface for the bolt head to register against. There used to be an angle washer kit for this, but it’s been a long time since I hunted for any. Or plugging the screw holes in the head with a bolt shank that is held in with superglue, Lock-Tite, or epoxy then drilling and tapping new holes at the needed angle into the head. Of buying a new 4bbl intake from ProComp or Professional Products that have an adapter that will fit the 55-86 or when flipped over the 87-95 bolt angle.
The other problem is the 87-95 heads have no pushrod guidance, these heads use self guiding rockers, which is a simple, for once, substitution with a set of 87-2002 Gen I or Gen II SBC rockers.
The exhaust side is typical 7 bolt Gen I/II SBC as are the front accessory mounts.
You can see if this was going to be a money saving head swap it ain’t.
Just a note, the L30 and L31 (305/350) Vortec of 1996-2002 production use a way different Fordish SBF pattern of four bolts per side straight down.
Bogie
This can be worked by clever people by egging the center holes of the intake manifold and milling/gringing a new flat surface for the bolt head to register against. There used to be an angle washer kit for this, but it’s been a long time since I hunted for any. Or plugging the screw holes in the head with a bolt shank that is held in with superglue, Lock-Tite, or epoxy then drilling and tapping new holes at the needed angle into the head. Of buying a new 4bbl intake from ProComp or Professional Products that have an adapter that will fit the 55-86 or when flipped over the 87-95 bolt angle.
The other problem is the 87-95 heads have no pushrod guidance, these heads use self guiding rockers, which is a simple, for once, substitution with a set of 87-2002 Gen I or Gen II SBC rockers.
The exhaust side is typical 7 bolt Gen I/II SBC as are the front accessory mounts.
You can see if this was going to be a money saving head swap it ain’t.
Just a note, the L30 and L31 (305/350) Vortec of 1996-2002 production use a way different Fordish SBF pattern of four bolts per side straight down.
Bogie
Thanks bogie have not done any diagnostics just blue smoke on startup and consuming oil and could use a carb rebuild . Issue is this is his only vehicle so has to be done in a timely manner so that's when he seen my heads and figured send them to the shop and then we can head swap them and upgrade intake
Joined
·
15,298 Posts
If it’s oil smoke from bad guides a head swap will work but if the smoke is from bad rings it will get a lot worse once compression is restored.
Other than that your heads with a proper intake manifold and a set self guiding rockers will work.
Bogie
Other than that your heads with a proper intake manifold and a set self guiding rockers will work.
Bogie
just throws the smoke on startup . So go with the 305 in photo for the 305 or the 35o heads in photo also? If I go with the 350 heads how will it be different from the 305 heads also? Do you recommend a manifold that would work with these heads?
Joined
·
2,801 Posts
One simple test that would help give you an indication of ring condition.
If you get a significant increase in compression after squirting oil in the cylinder, the oil is helping to seal up what could be bad rings in those cylinder(s). Replacing heads won’t fix the bad rings.
I know you want to reuse the heads you have, but unless they are already in good condition and ready to install it might be a poor use of your money. Fixing up old smog-era heads just results in old smog-era heads that are in better condition. It doesn’t do much for overall performance, especially if your longer term plan is to use a new cam and other upgrades. More aggressive cams and modern intakes need good flowing heads and higher compression, and most older heads just don’t provide it.
Bruce
- do a standard compression test on all cylinders and record the values
- repeat the test, but squirt some oil in each cylinder before testing it
If you get a significant increase in compression after squirting oil in the cylinder, the oil is helping to seal up what could be bad rings in those cylinder(s). Replacing heads won’t fix the bad rings.
I know you want to reuse the heads you have, but unless they are already in good condition and ready to install it might be a poor use of your money. Fixing up old smog-era heads just results in old smog-era heads that are in better condition. It doesn’t do much for overall performance, especially if your longer term plan is to use a new cam and other upgrades. More aggressive cams and modern intakes need good flowing heads and higher compression, and most older heads just don’t provide it.
Bruce
This is meant in jest, but spending money on a 305 is like re-using a kleenex. Just throw it away and get another one.
The minute you hit $400 in repairs, it's cheaper to get a good running engine (even a 350) from the junkyard and drop it in.
Using 350 heads on a 305 will take the low compression and make it worse. Stick with 305 heads for a low-buck 305 repair. I also agree on complete diagnostics, at the very least a compression test.
The minute you hit $400 in repairs, it's cheaper to get a good running engine (even a 350) from the junkyard and drop it in.
Using 350 heads on a 305 will take the low compression and make it worse. Stick with 305 heads for a low-buck 305 repair. I also agree on complete diagnostics, at the very least a compression test.
Joined
·
15,298 Posts
EGR intake or not? I forgot that most aftermarket intakes may or not have this feature and you guys may need it.
Bogie
Bogie
1 - 11 of 11 Posts
Join the discussion
