Olds 307 heads on Olds 350

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ChevyHeaven

Not-quite-so-new-guy
Apr 23, 2013
31
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So right out the gate I have a blown head gasket on my 72 olds 350. I kinda jumped the gun one day when I took it for a quick emergency drive with the PCV valve unplugged and the breather closed off(please don't make me explain why) I'm assuming it just built up enough pressure and blew the gasket. Didn't really get hot so I think I should be ok to just replace the gasket and move on.
I do however have a perfectly nice reconditioned set of olds 307 heads, on the off chance that the heads on my 350 took a some damage, what would the harm be in putting on my 307 heads?
I've researched it a little bit and have heard things like it wont add much performance, the intake valves are smaller, there swirl port heads etc. Considering my goal right now is more reliability than performance I should be safe right? My only fears are I'll either just put my 350 heads back on and have problem's, or I'll use my 307 heads and have problem's.
Suggestions?
 

King_V

Master Mechanic
Jul 17, 2013
307
5
18
Sicklerville, NJ
Agreed - I don't know what the valve sizes are on the 5A heads, or if there are port size differences, but the combustion chamber is about the same size as your original 1972 7A heads.

Dang . . the font-size-change thing isn't working!!

The 1972 350 had 7A heads, but while it was a capital A next to the 7, it's smaller than the 7 and written almost like a subscript. The A in 5A and 7A for the 307 heads is the same size as the number it's next to.
 

ric85442

Not-quite-so-new-guy
Nov 19, 2013
25
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1
King_V said:
Agreed - I don't know what the valve sizes are on the 5A heads, or if there are port size differences, but the combustion chamber is about the same size as your original 1972 7A heads.

Dang . . the font-size-change thing isn't working!!

The 1972 350 had 7A heads, but while it was a capital A next to the 7, it's smaller than the 7 and written almost like a subscript. The A in 5A and 7A for the 307 heads is the same size as the number it's next to.

5A valves r intake 1.875 exhaust 1.507 64cc I believe the 7A valves r the same but has a bigger 67cc witch lowers compression ratio and the ports r way smaller
 
Oct 14, 2008
8,806
7,746
113
Melville,Saskatchewan
I have a set of 5A heads on my 94 4x4 with TBI, 76 Olds 350 short block, 204/214 cam, Sanderson headers and junk dual exhaust. The 4L60E and 3.73 also help launch. I removed the air tubes from the head. It far out performs the original swirl port sbc 350 and equals or betters 4000 rpm and down the LS 5.3 from 2003-04 I have drove a lot. The LS vehicles had a slight gearing advantage due to shorter tires. In a car that sees any revs they need a lot of work.
 

ChevyHeaven

Not-quite-so-new-guy
Apr 23, 2013
31
0
0
Well I'm lucky enough to have the 7A heads.
I'm pullin my 350 out this summer for a bit so I just need it to make it through the winter safely.
Whats the worst that happen to a 350 with 7A swirl port heads?
 

King_V

Master Mechanic
Jul 17, 2013
307
5
18
Sicklerville, NJ
Slight loss of compression, major loss of any ability to rev since the 7A swirl port heads have ports that have been described as coffee-straw sized....
 

ChevyHeaven

Not-quite-so-new-guy
Apr 23, 2013
31
0
0
Would it be anymore restricted than a 307 normally is? And would it do any damage to my 307 heads?

I'm thinking I may just have to take my chances and replace the head gasket and hope for the best, just want to cover all my basis. I'm clinging to the hope the 350 heads didn't take much damage and a head gasket will be the end of it.
 

King_V

Master Mechanic
Jul 17, 2013
307
5
18
Sicklerville, NJ
Remember, that the swirl-port roller 307 peaked horsepower at 3200 RPM. Now you're asking a motor with 43 more cubes to breathe through those same heads.

Here's a blurb from http://www.442.com/oldsfaq/ofhed.htm#Heads 1964 - 1990:
The 5A heads have the 'std.' older larger regular SB size ports, not the miniscule nostrils that the 2A, 10, 6A, 7A heads have. Port size mismatch occurs with these heads and most factory and all aftermarket intake manifolds. Earlier SB's used rectangular ports, about 1.25 x 2". The 260 and later 307's, starting with the #6A heads, have teeny tiny ports, same width, but height cut down to about equal to width. So, imagine the A/F coming down the intake, all happy in its 2" tall port, when wham 1/3 of it has to STOP or cram into the rest of the space left by the port there in the head.

Starting in mid 1985, Olds switched to a different head (coded 6A and 7A). They, for some unknown reason, shrunk the size of the intake ports. The result was a reduction in horsepower by about 10 on the "Y" motor. The other consequence is the ports don't match the larger size on the Performer and other aftermarket manifolds, causing "reversion". In other words, the air/fuel flow bounces back into the manifold, creating backpressure and (probably) worse performance than the stock intake. I'm guessing, but Edelbrock and other performance parts makers most likely don't find it cost effective to produce a manifold for an engine/head combo produced for only three years (86-88).
 

1SlowBu

Greasemonkey
Jun 17, 2011
125
0
0
Why?
 
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