1967 L71 3904391 Cylinder Head Differences

[Richard M.]

All of my research has been fruitless......

On these two distinct 391 square port heads on separate listings on ebay right now, I see the area surrounding the exhaust crossover area is different between the two. The crossover is needed for the divorced choke operation correct? One has no crossover at all and no surrounding openings, the other has the crossover port, with open areas above and below it. These openings go nowhere, but surround the port(for cooling?). Why are these differences seen on different heads with the same casting number?

So.....on the one that has the openings, what is supposed to cover the upper opening on a 3x2 435hp? The aluminum manifold does not cover the upper openings on either side. Is it the intake gasket? The JG states the gasket should fit the profile of the intake, not the head. But if the gasket is cut to fit the intake, the upper opening is exposed.

The 1st pictured head(no x-over, no openings) is J 10 6, with no single hump. Full ebay link here.

The 2nd pictured head(with x-over & openings) is A 21 7 with single hump Full ebay link here.

Rich

[Joe L.]

Re: 1967 L71 3904391 Cylinder Head Differences

All of my research has been fruitless......

On these two distinct 391 square port heads on separate listings on ebay right now, I see the area surrounding the exhaust crossover area is different between the two. The crossover is needed for the divorced choke operation correct? One has no crossover at all and no surrounding openings, the other has the crossover port, with open areas above and below it. Thes openings go no where, but surround the port(for cooling?). Why are these differences seen on different heads with the same casting number?

So.....on the one that has the openings, what is supposed to cover the upper opening on a 3x2 435hp? The aluminum manifold does not cover the upper openings on either side. Is it the intake gasket? The JG states the gasket should fit the profile of the intake, not the head. But if the gasket is cut to fit the intake, the upper opening is exposed.

The 1st pictured head(no x-over, no openings) is J 10 6, with no single hump. Full ebay link here.

The 2nd pictured head(with x-over & openings) is A 21 7 with single hump
Full ebay link here.

Rich

Rich------


I have no idea what the cylinder head without the cross-over passage is. I can tell you this, though: NO PRODUCTION cylinder head used on any Corvette EVER used a non-cross-over passage cylinder head. Absolutely NONE.

The cross-over passage is required for divorced choke operation. However, even 1965 L-78 which did not use a divorced style choke had the cross-over.

The intake manifold gasket covers the "exposed" portion of the cross-over.

[Richard M.]

Re: 1967 L71 3904391 Cylinder Head Differences

Joe.... I was just about to go to bed(midnight here), and just saw your reply. I've spent the last several hours trying to figure this out and gave in and asked here after much archive searching and book reading with zero results. You hit it on the head.... as usual.

So the gasket covers it up. Good. I guess the JG is right too? So I'll paint the gaskets and head surfaces on this engine silver as it states.

Many thanks. I can sleep now.

Rich
p.s. I asked the seller with the "no crossover" head about it. Hmmm, now I'm wondering could it be a special prototype or ?

[Richard M.]

Re: 1967 L71 3904391 Cylinder Head Differences

I just noticed looking very closely that the intake gasket is still installed and covering the port. Goes to show I must be really tired.

Ok, but the answer is that the gasket covers the opening above it on mine so I'm good now.

[Clem Z.]

Re: 1967 L71 3904391 Cylinder Head Differences

All of my research has been fruitless......

On these two distinct 391 square port heads on separate listings on ebay right now, I see the area surrounding the exhaust crossover area is different between the two. The crossover is needed for the divorced choke operation correct? One has no crossover at all and no surrounding openings, the other has the crossover port, with open areas above and below it. These openings go nowhere, but surround the port(for cooling?). Why are these differences seen on different heads with the same casting number?

So.....on the one that has the openings, what is supposed to cover the upper opening on a 3x2 435hp? The aluminum manifold does not cover the upper openings on either side. Is it the intake gasket? The JG states the gasket should fit the profile of the intake, not the head. But if the gasket is cut to fit the intake, the upper opening is exposed.

The 1st pictured head(no x-over, no openings) is J 10 6, with no single hump. Full ebay link here.

The 2nd pictured head(with x-over & openings) is A 21 7 with single hump Full ebay link here.

Rich

any intake gasket that is used to block the heat riser crossover that does not have a stainless steel insert will burn thru in time. any metal besides stainless steel will burn thru between the exhaust heat and exhaust pressure

[Dick W.]

Re: 1967 L71 3904391 Cylinder Head Differences

Rich, after enlarging the first picture, I believe the crossover has been welded up. Look closely at what appears to be grinding marks.

[Clem Z.]

Re: 1967 L71 3904391 Cylinder Head Differences

Rich, after enlarging the first picture, I believe the crossover has been welded up. Look closely at what appears to be grinding marks.

dick the first picture still has the intake gaskets on the head. those blue lines are felpro "print o seal" that are used on their gaskets. another trick is to fill the crossover with high temp furnace cement to help smooth out the exhaust port in the head by the exhaust valve

[Dick W.]

Re: 1967 L71 3904391 Cylinder Head Differences

dick the first picture still has the intake gaskets on the head. those blue lines are felpro "print o seal" that are used on their gaskets. another trick is to fill the crossover with high temp furnace cement to help smooth out the exhaust port in the head by the exhaust valve

That is evidence that I am blinder than a bat.

[Richard M.]

Re: 1967 L71 3904391 Cylinder Head Differences

That is evidence that I am blinder than a bat.

Me too Dick......After Joe and I talked about it I went back just before calling it a night and noticed it's the gasket still on it. I was tired AND blind. I Posted again but you must've missed that one.

I asked the seller but then noticed it they sent another note apologizing for my brain-fart.

So I guess I'm all set. I found another thread deep in the anals of the archives this morning that also cleared it up. Gasket covers the opening around the port, not the port itself like Joe replied.

https://www.forums.ncrs.org/showthre...asket&uid=9491

So good ole' me as usual has to understand why they have the openings. I say it's to keep the head area cooler around the port. I betcha!

Rich

[Joe L.]

Re: 1967 L71 3904391 Cylinder Head Differences

Me to Dick......After Joe and I talked about it I went back just before calling it a night and noticed it's the gasket still on it. I was tired AND blind. I Posted again but you must've missed that one.

I asked the seller but then noticed it they sent another note apologizing for my brain-fart.

So I guess I'm all set. I found another thread deep in the anals of the archives this morning that also cleared it up. Gasket covers the opening around the port, not the port itself like Joe replied.

https://www.forums.ncrs.org/showthre...asket&uid=9491

So good ole' me as usual has to understand why they have the openings. I say it's to keep the head area cooler around the port. I betcha!

Rich

Rich------


I did not mean to say that the gasket covered the entire exhaust cross-over passage. The gasket only covers the "peripheral opening" which surrounds the cross-over passage.

The only PRODUCTION Corvette big block that ever used a blocked cross-over passage in the intake manifold gasket was L-88/ZL-1. The gasket was the same as used for other rectangular port big blocks except that thin stainless steel plates, spot-welded together, blocked the cross-over passage.

Some aftermarket gaskets with a blocked heat cross-over passage use only the gasket material, un-punched for the cross-over, to block the passage. As clem describes, these are very short-lived.

[Richard M.]

Re: 1967 L71 3904391 Cylinder Head Differences

Rich------


I did not mean to say that the gasket covered the entire exhaust cross-over passage. The gasket only covers the "peripheral opening" which surrounds the cross-over passage.

The only PRODUCTION Corvette big block that ever used a blocked cross-over passage in the intake manifold gasket was L-88/ZL-1. The gasket was the same as used for other rectangular port big blocks except that thin stainless steel plates, spot-welded together, blocked the cross-over passage.

Some aftermarket gaskets with a blocked heat cross-over passage use only the gasket material, un-punched for the cross-over, to block the passage. As clem describes, these are very short-lived.

Joe, yes I understood what you meant. I feel a bit foolish I didn't notice that the gasket was still on the head. My eyes were focused on the center port and nothing else.

I have another gasket set here. It the VC 15168 with the correct seals with round tabs and "GM" on the tab. I may change over and use those. they look better and will do a better job of covering the chamber above the port.

Rich