Magnum Heads On 340

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bobscuda67

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I rebuilt my 68 340 block in the early 2000s and used magnum heads on it. After having one set with cracks I just bought new bare castings.
Back then all the Mopar magazines said that the 340 hipo exhaust manifolds bolt right on and they were right except you eight huge exhaust leaks. I made some plates that mostly sealed up the leaks but here I am 20 years later still fighting leaks on this thing.
Has anyone used the 340 manifolds on magnum heads and got them to seal? How did you do it?
 
Just food for thought, but make DARN sure that there's no interference between the downleg of the manifold (esp. driver's side) and the head. I have a Magnum slated for install in a '68 Dart, and when I mounted up the early 340 manifolds, the driver's manifold outlet was hitting the bottom part of the head. It wasn't a huge amount, but enough to probably prevent the manifold from sealing no matter what gasket was used; and I'm not a fan of doubling up on gaskets. I ended up sandwiching a header flange between the manifold and the head, which took away the interference. I didn't see any interference on the passenger side.
No pics of HiPo manifolds on a Magnum right now, I'll see if I can get some next time I'm at the shop.
What I DO have is a pic of an early A manifold (I know it's not the same, but it illustrates the point of interference) on a Magnum head.
1715643187578.png

Ignore the chunk out of the manifold. That's the result of someone tightening down the manifold on a set of later LA heads which also interfere. Ka-tink.
Something I've pondered doing, but haven't actually tried yet; is to try to find some 318 port-sized header flanges, which more closely resemble the Magnum port sizes. A little quality time with a die grinder should match the flange up to the port of the head, and the extra material of the flange around the ports should allow the 340 manifolds to seal up just fine...
 
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Thanks I did catch the interference between the head and manifold and ground down the head.
Using a flange was something I thought about too.
A magnum flange with extra meat around it to catch the LA manifold but I wouldn't want it to be very thick. Maybe 3/16" would be about right.
If someone had a CNC they could cut some that would bolt on and fix this problem.
 
You also have to grind on Eddie 318-340-360 RPM aluminium heads and manifolds.
I did a little on each, so as not to have to grind to much in heads or manifolds.
It is due to the fact that they used the same casting as late heads where there is a threaded port below the manifold for smog cheap.
Note, grind to much off and ya screwed.
The small and large port Mopar Comando Heads were also cast from 360 Magnum castings but machined for LA
Same deal.

18527486_22573996.jpg


eddie2.JPG.jpg
 
I thought about having them brazed but didn't
want to molest my 340 manifolds.
I can't believe I'm the only person that has run into this.
 
Have you established specifically on which ports and where the leaks are? There are several possible reasons for leaks and you can't fix anything until the cause is identified. Need to start with straight, flat surfaces on both parts at a minimum. I can't remember the brand, but there are thick, layered metal gaskets that conform to irregularities and should band-aid any misalignment. Personally, I would start by correcting any irregular surfaces. If two metal parts are flat enough and have enough clamping force, you can get a gas-tight seal that will withstand hundreds of PSI, without using a gasket. Can't achieve that with the bolt pattern on an exhaust manifold but it will be a lot easier to seal if the faces are fully in contact.
 
Thanks I did catch the interference between the head and manifold and ground down the head.
Using a flange was something I thought about too.
A magnum flange with extra meat around it to catch the LA manifold but I wouldn't want it to be very thick. Maybe 3/16" would be about right.
If someone had a CNC they could cut some that would bolt on and fix this problem.

SendCutSend can cut stuff for you. I have a pattern I gave someone here recently for this exact reason.

Can't promise it is a perfect fix, I have the same problem (Magnum R/T heads with 340 manifolds) and my adapter plates seem to be ok. But I have leaks I haven't identified so it could be them.

I think I used 14 or 16ga plate for mine, I didn't want to push the manifolds out too far.

capture-png.png
 
Have you established specifically on which ports and where the leaks are? There are several possible reasons for leaks and you can't fix anything until the cause is identified. Need to start with straight, flat surfaces on both parts at a minimum. I can't remember the brand, but there are thick, layered metal gaskets that conform to irregularities and should band-aid any misalignment. Personally, I would start by correcting any irregular surfaces. If two metal parts are flat enough and have enough clamping force, you can get a gas-tight seal that will withstand hundreds of PSI, without using a gasket. Can't achieve that with the bolt pattern on an exhaust manifold but it will be a lot easier to seal if the faces are fully in contact.

The problem is the bottom of the port.

parts-018-jpg.jpg


That line at the top of the gasket port is the bottom of the cylinder head. There isn't any material there to squish the gasket and create a seal. Some Magnum heads drop down further, mine and the OP's did not.
 
I have seen threads on this forum with this discussed before and I believe the EQ heads have extra casting material around the ports to allow 340 manifolds to seal. The magnum heads from Hughs may also have the extra material.
 
Yes, that's the problem at the bottom of the port with stock replacement magnum heads.
The 340 manifolds are a large LA port and the magnums are a small port head.
I made some 16 gauge block off plates like Dion R made and they leak slightly around both sides of the plate. it's not much around each port but you multiply it times 8, it's annoying.
I talked to a friend of mine tonight and he has a machined straight edge and I'll have him
check my manifold for flatness tomorrow.
The exhaust port on the heads from Hughes are like mine, not enough pad under the port to seal the 340 manifolds. I think the 273/318 manifolds would fit fine, but who wants those on your engine.
 
SendCutSend can cut stuff for you. I have a pattern I gave someone here recently for this exact reason.

Can't promise it is a perfect fix, I have the same problem (Magnum R/T heads with 340 manifolds) and my adapter plates seem to be ok. But I have leaks I haven't identified so it could be them.

I think I used 14 or 16ga plate for mine, I didn't want to push the manifolds out too far.

View attachment 1716249845
What material are you using? Soft copper?
Or a composite like Comedic uses on their exhaust gaskets for headers?
Something else?
 
What material are you using? Soft copper?
Or a composite like Comedic uses on their exhaust gaskets for headers?
Something else?

Steel with a Magnum gasket on one side and an LA gasket on the other. They aren’t gaskets themselves.
 
The manifold is warped .008 on the center two ports.
I'm gonna try the Remflex gaskets and not have the manifold cut.
 
I have remflex on the homemade headers in my bb wagon. They are the best I have ever used. I will use them again when I get the 360 magnum in my dart. Worth the cost.
 
I used both. Plates against the head and the gasket between the manifold and the plates.
Haven't driven it yet but plan to today, when the fog clears, to the gas station.
 
Thanks I did catch the interference between the head and manifold and ground down the head.
Using a flange was something I thought about too.
A magnum flange with extra meat around it to catch the LA manifold but I wouldn't want it to be very thick. Maybe 3/16" would be about right.
If someone had a CNC they could cut some that would bolt on and fix this problem.
Have some cut with a water jet at a good fab shop. I had some small spacers fabed from a gasket for break backing plates.
 
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