MD7A Exhaust Manifold Mystery Hole

[cas206]

Here is a picture of the MD7A exhaust manifold that I'm cleaning up. It is a raw water system that mixes with exhaust. Notice the plugged up hole in the center.

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Here is a close up:
[IMG][/IMG]

For most of the plugged up openings, it was obvious that they were stuffed with crud that needed digging out. However, this center hole has me puzzled. The gasket has an opening in it around this hole and the engine block has an opening that matches the exhaust manifold hole, seen here. Seems obvious that it needs dug out right? However, the "stuff" in there is a lot harder than the build up in the rest of the manifold. I can see what looks like flakes of metal in the junk I've managed to dig out of it . The edges of the opening is rather crudely cut compared to the other openings. I'm afraid to dig any further into it without knowing why it seems different than the other crud I dug out.

Any ideas?

[jeffjd]

What did you end up doing. I just pulled the head and had the same question. Here are some pics

The First pic is the manifold then the head. This is really weird because the exhaust manifold hole seems to be solid metal. Three holes on the manifold and four on the head? The only thing I can think of is the head was used on different engine models.


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[cas206]

I never got a definite answer. What I concluded was that it must be a built in anode of zinc or some other material. After cleaning out the exhaust and water channels in the manifold, I couldn't find where that plugged up port would connect to any channel inside the manifold. There was definitely a softer metal material beneath the layer of crud inside that port. I left it as is and put it back in operation. Engine is running fine.

[jeffjd]

Thanks for the reply - I will do the same and leave it as is.


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[gjordan]

I have an MD6B head, and an MD7A head in my shop. I will try to take a look tomorrow to see if something makes sense. ______Grant.

[jeffjd]

Gjordan

That would be great. My head is at the machine shop now and it would great to have this answer prior to assembly. Thanks


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[hpeer]

Fascinating esoterica.

I have a spare MD7A in the basement with the head pulled off. I'm on vacation for a couple of weeks so it's not available to look at. Can't wait to look and see what I find in mine.

[jeffjd]

My understanding is that the MD7a does not have a sacrificial zinc? Has anyone added a sacrificial zinc by drilling and tapping a hole in the exhaust manifold? Is this a horrible idea?


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[gjordan]

I went and looked at the MD6 head and manifold and the MD7. The 7 manifold has a very clean,hard metal plug in that spot. Almost looks like S.S. The 7 was fresh water cooled, so all of the head and manifold passages are pretty clean. The 6 was raw water cooled, and the opening on the head was almost plugged with rust, and the plug in the manifold was barely recognizable. Why there is a passage to nowhere, I dont have a clue. My guess is that possibly these engines were used in some other configuration that required a different cooling system, but that is just a wild guess. The plug is not like what we often call a freeze plug. It is not rounded or domed. It is flat and flush with the surface of the manifold. Unless the plug looks to be corroded enough to start leaking, I would clean it, and paint it with some rust preventative primer, and not worry about it. Just another mystery of the GREEN DEMONS. _____Grant.

[jeffjd]

Grant thanks! That is my plan.


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[gjordan]

Jeff, just out of curiosity, when you get your head back from the machine shop, could you let us know what was done, and how much it cost. That is after you recover from the sticker shock. Good Luck. _____Grant.

[jeffjd]

The quote is about $200 to weld the groove the salt water eat away at the point the head gasket failed, valve job, Hydro test and surface it. This include no parts and I estimate the parts will be $450. I will post a summary with actual costs and lessons learned in a new thread next week when the project is finished.... Or I order a beta.

Anyone know how to post a link to a different thread.




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[jeffjd]

Head work costs $750- $800

The machine shop was $250. They welded the Head where the water created a channel, cleaned everything up. Deck the head to make sure it was flat. Ground the values and reset them, and replaced the injectors sleeves.

They also put Heli coils in the exhaust manifold where the thermostat housing bolds it. This was worth the $50, because about 1/3 of the threads were left.

Parts cost about $500 and included.
Cam seal $45 was leaking
Head gasket $120
Injector sleeves $100
Exhaust gasket $45
Other gaskets and seals $35
Other parts, lube, rags, washers, bolts, jb weld for the channel on the block $60
Shipping $60 (three shipments)

Engine starts with lots of smoke, but I am dealing with water leaks between the water pump and thermostat housing. So it has only run for a couple minutes. I had to call it a day and will fix the water leak in a few days.

I don't know if there is a trick, but lining up the pipe from the raw water pump is difficult without creating leaks... I did it before when I rebuilt the water pump.


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[JD-MDR]

Did you guys ever figure this out. Please respond as I just bore and chipped that hole with a masonry bit and chisel. I sure felt and looked like carbon. I hope I didnt ruin my manifold

[cas206]

When you finished cutting it out, where did it connect? In my totally uninformed opinion, if you broke though into the exhaust chamber, then it's probably toast. If I recall correctly, the matching hole in the engine block was part of the cooling water channel.