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I am hoping someone may be able to help me think through an upcoming project.
I have a 1986 Cutlass with a 307 LV2 that has the notorious 7A swirl port smog heads. I happen to have a pair of 7 heads (with a subscript 'A') from a 71-72 350 that I am considering putting on the 307. Before I spend the money to have the heads crack-checked and rebuilt, I am wondering if they will even work well on the 307.
I See that the bore of the 307 is 3.80", while the bore of a 350 is 4.06", so do do I need to consider the valve-to-cylinder wall clearance?
Both engines have the same stroke, so the valves should not contact the pistons.
Is there anything I am not thinking of? I do have a couple of performance intakes and some higher-flow exhaust manifolds to help the 350 heads breathe easier, I just need to make sure they will work with the stock 307 block.
Thank you in advance to anyone who can lend any advice!!
307s use 1/2" head bolts, so you'll be drilling out holes to make them fit.
mid 60-something CC chambers for 307, and I think 7a heads were from a 72?? I think, so chamber size should be about the same, mid 60s.
If you can find some 5A heads from an 85 and prior 307, you'll be a whole lot better off, but yeah, I think you could make them work if you don't mind hogging out the holes. Just know they'll effectively be 1977-up 350 heads with 1/2" bolts from there on out.
I don't mix and match, so I don't know for sure, but with a little hogging on the bolt holes you may be able to make 'em work. Also, if using the original valve covers, you can opt to drill holes for the extra bolts on the 350 heads or just fill in the gaps with RTV. I prefer the 10 bolt covers myself.
The early heads used 7/16" bolts, the later motors used 1/2" head bolts. You'll need to drill out the head bolt holes on the early heads. You'll also need to change the cam to use the flow rate of these heads. You'll need different intake and exhaust manifolds since the 7A heads have tiny ports and manifolds to match. The 7A heads have larger ports for both intake and exhaust.
This pic below shows the older 307 and 350 "normal" sized intake ports on the gasket and it has 85Y and 86-up 307 7A heads on the engine. You can see how much of a mismatch using the peanut port parts off an 85Y or any 86-90 307 would be. (1985 VIN 9 only had flat tappet cam and the bigger ported 5A heads). The aluminum intake for the 86-90 307 was marked with an A5. You want an A4 or earlier 307 intake or any 350/403 intake that has the bigger ports for your earlier 7a heads. Early 307s also came with cast iron intakes but had the larger ports. All VIN 9s came with cast iron exhaust manifolds that will fit nicely on the 350 heads. Those stainless steel tubular pieces of crap exh manifolds should be avoided at all costs.
If I decide to go this route, I would have the machinist bore out the head bolt holes when they overhaul the heads, so thank you for the reminder. I do have a set of 10 bolt hole valve cover I would use, since those seem to do a much better job at not leaking oil.
Also Joe, would changing the cam be necessary? I understand what you mean since keeping the stock cam is not going to take full advantage of the higher flow heads, but since the valves are larger, wouldn't that get me some improvement over the stock heads?
If I decide to go this route, I would have the machinist bore out the head bolt holes when they overhaul the heads, so thank you for the reminder. I do have a set of 10 bolt hole valve cover I would use, since those seem to do a much better job at not leaking oil.
Also Joe, would changing the cam be necessary? I understand what you mean since keeping the stock cam is not going to take full advantage of the higher flow heads, but since the valves are larger, wouldn't that get me some improvement over the stock heads?
Not necessarily. The roller cam 307s are optimized for low end torque and gas mileage, period. That means everything from the cam profile to the valve sizes to the CR to the carb to the ignition timing to the gear ratios. Changing just the heads without changing everything else is likely to produce no benefit for a lot of work.
I did not take that into consideration, so I will have to dig around and do some more research on cams and pricing in that case. I am simply looking to liven up the engine a little, but not dump alot of money into it to make a ton of power. If that were the case, I would swap in a 350 or 403...
I do have two edelbrock performer intakes (one has a higher carb mount than the other, I think it is a 7111 if memory serves me) as well as some 350 exhaust manifolds, so I am not concerned with allowing the 350 heads to breathe correctly.
I would be interested in the 7A 86 heads if you choose to remove them. I have been screwed once on here for someone who had pics of heads and never sent them. But I see you have been on here along time, so let me know. Jeff
Jeff - If I decide to go this route, I will let you know if I decide to sell the 7A heads and send you a message. Fair warning, it will be a few months at least before I really dig into this project.
Also, am not sure where you are located at, but I have found 7A heads online before, as well as in some local junkyards.
In factory form the ‘72 heads have around 68-70 cc chambers. I have no idea what size the 307 heads are. So you might lose compression using the 350 heads.
Also, are the alignment dowels different diameters?
In factory form the ‘72 heads have around 68-70 cc chambers. I have no idea what size the 307 heads are. So you might lose compression using the 350 heads.
Also, are the alignment dowels different diameters?
The 7a heads on the 72 are 64 CC. They're around 67 cc on the 307 5A heads, and 64 cc on the 7A heads, so I'm thinking that in itself will be somewhat of a wash as your compression won't be affected much, if at all. 1.75 intakes and 1.5 exhaust valves for either 307 head aren't great, though.
Not sure about the dowels as again, I haven't done any mix and match with different era heads.
Another thing to do is put a 307 head gasket on the 7a head to see how much will seal. If you can find a Vin 9 307 or a very mild aftermarket roller cam, that would work best. Good luck.
Thanks everyone for the advice, I really appreciate the help! I will see if my machinist can check the cc size of the 350 heads just to be sure. I know that is something I could do myself, but I am sure he already has the tools and experience to do so.
If I can remember when I start on this project, I will post updates and pictures (also if I have any more questions haha).
The 7a heads on the 72 are 64 CC. They're around 67 cc on the 307 5A heads, and 64 cc on the 7A heads, so I'm thinking that in itself will be somewhat of a wash as your compression won't be affected much, if at all. 1.75 intakes and 1.5 exhaust valves for either 307 head aren't great, though.
Be aware that these are the blueprint minimums for NHRA stock class, not the as-cast volumes. Untouched Olds heads typically run 2-4 cc larger than the published numbers. And of course all bets are off if the head has ever had a valve job or been milled.
Be aware that these are the blueprint minimums for NHRA stock class, not the as-cast volumes. Untouched Olds heads typically run 2-4 cc larger than the published numbers. And of course all bets are off if the head has ever had a valve job or been milled.
the heads I started with were 66 cc,s. Memory serves me correct I believe nhra minimum was 59. I cut mine down to 60 and a .034 head gasket. Math wise it was 10.25 compression.
Thanks everyone for the advice, I really appreciate the help! I will see if my machinist can check the cc size of the 350 heads just to be sure. I know that is something I could do myself, but I am sure he already has the tools and experience to do so.
If I can remember when I start on this project, I will post updates and pictures (also if I have any more questions haha).
Merry Christmas everyone!
No problem if it's a few months. These are for a new build that will take a bit to get going.
The only head gasket that will work on a 307 block is the 307 head gasket. The sealing bead around the cylinder bore is to small to seal the larger chamber 350 heads.
the 350 head gasket won’t work on a 307 block because it’s larger diameter cylinder sealing ring runs across the 307 blocks water passages
CANADIANOLDS brings up a good point, but I am sure I have heard of others putting 350 heads on a 307. Are there custom head gaskets out there for this? Or do others just modify the 350 head gaskets for the 307 block?
December 20th, 2022, 10:57 AM
