1974 OMC exhaust manifold

[74 Fantasy]

I really could use some expert advice and words of wisdom if anyone can help. We have a 1974 Fantasy tri-hull with a straight chevy 6, 165 OMC which we have owned for 38 years and are taking a long shot at replacing the exhaust manifold. The last time we used it we notice water in the oil and we drained it out and put clean oil in did not use it anymore. It has been suggested that before undertaking the head maybe it is the manifold since it has never been changed. The boat is only used in fresh water and we have always tried to take care of it. We really can?t get another boat, so we just keep working with the one we have. I know it sounds ridiculous hanging on to it, but another summer?s use would be great and we are kind of attached to it.

I would like to know the best way to go about removing the 4 ?V screws? and nuts on the 2 studs undone and reducing the chances of breaking them. Thus far I have my doubts if it can be done after 40 years and the boat is a very long way from any kind of parts store or boat shop. If there are some specific tools that might help if and when disaster happens while trying to remove them we would like to pack them along. We want to avoid welding broken off bolts to get them out, since that isn?t possible, otherwise it?s a couple hundred miles on the road. From what I have come across easy outs sound pretty risky also, especially if you break them off.

Also I would like to know about sealant on the gaskets, yes or no and the best thing to use. Manifold to head, both end caps and manifold to elbow since we will be replacing these parts.

I added a couple of photos. We have not started taking it apart yet but know it is going to look pretty bad inside after 40 years.

Any words of experience would be very much appreciated.

Thanks,
Robin

 

[southkogs]

Welcome aboard Robin:

Have y'all checked compression on the engine? I have the same engine as you do, and to be honest ... the head was pretty easy to do. I'm a mediocre mechanic on a good day and the whole project took me about 5 hours (total not including having the head machined and cleaned up).

I would personally rather replace the right parts than guess at replacing the wrong ones.

The manifold on mine came off very easily without any special tools. I hit the bolts with some penetrating a couple of hours before I started turning, and then just applied pressure consistently. Some people (as I understand) don't use sealant on the manifold gaskets at all, but I used the RTV that the auto-parts guy suggested. I never removed the upper elbow, just unhooked the clamp.

 

[74 Fantasy]

Thank you for the insight. We figure at this point if we replace the manifold and it turns out that it?s the head by the time we get done fixing everything maybe we can get a few more years out of the boat. We are hoping that we can change out the manifold in a day and taking the head off and having the gone through would take some time at the right shop and that will take some research first, since we do not live in the state where we keep the boat. Maybe plans will change after we start tearing it apart, but we can always hope for the best. Just some foresight and being prepared helps sometimes.

I was not able to add the photos since I haven?t posted enough times and my file was to large, maybe in the future I can add some.

Thank you again so much for the helpful advice,

Robin

 

[southkogs]

You should be able to post photos now ...

Well cared for that inline six has a lot of life left in it.

 

[74 Fantasy]

No Title

Hello, I attached a couple of photos and need some insight on the heat exchange tube where it goes through the manifold. I was hoping that maybe you might remember about where the tub goes into and through the manifold, the holes drilled in the manifold are larger than the replacement tube we have and think there must be additional parts, (spacers, grommets?) On the parts pages it is listed as an assembly. We aren?t sure if we can remove the old one and use it or what we need where the tube goes in and out the manifold. Since we won?t know until we drive the 300 miles to our boat and I have not been able to find any information or pictures online I was hoping you might remember something that would help.


Thank you again,
Robin

 

[southkogs]

Wow ... boat looks beautiful, and your engine is cleaner lookin' than mine is

I didn't disassemble that part, because I didn't need to. I even left the carb mounted when I removed everything. Everything that I can find calls it an assembly, but I think that's only because of the connectors on the ends. Nothing particular about it in my manual, so I'm thinking there's no extra parts.

 

[74 Fantasy]

We have packed our tools and boat parts and will be undertaking the work on our 74 Fantasy soon. Just hope it goes alright and I can let you know the outcome in mid August when we return.

Thanks for the support and advice.

Robin

 

[74 Fantasy]

No Title

Repair update.....I wanted to let you know the results of our repairs on the 1974 OMC 165 manifold. As you can see from the photos 41 years of use and rusting away from the inside was the problem and that would be an understatement. We were astounded by what we found after taking it apart, but not surprised after researching and learning from knowledgeable people with past experience and the helpful advice we received through the forum.

One of the best tips I found was to take 4 bolts about 2 inches longer then the ones that go through the manifold into the block/head and cut the ends off. As you remove an existing bolt replace with a cut off longer bolt, do all 4 this way to support the weight of the manifold so you can just slide it out on them. Then you can guide the new gasket and manifold into place on them, then remove them one at a time and replace with the bolts that hold the manifold in place. This helped tremendously since the manifold is heavy and then we did not have to worry about trying to start or strip any bolts. The nuts on the 2 studs weren?t any problem either.

Taking it apart went well, no problems and we did not break any bolts. They came apart fine after 41 years, just sprayed with penetrating liquid wrench the day before we started. All the parts went back together easily, drained and flushed with new oil about 4 times prior to using the boat. The boat ran great and no water in the oil. We caught some fish and enjoyed the remainder of our vacation.

Thank you again for the resources and support the forum provides.

Robin
74 Fantasy

 

[southkogs]

Congrats! Great to hear a good end to the story.

 

[wire2]

Sounds good, but the story may not be at an "end". You mentioned having water in the oil. There are no oil passages in an intake/exhaust manifold. It may be leaking in the head. (or, hopefully not, a crack in the block). The same water that cools the engine flows through the exhaust manifold passages surrounding the raw exhaust ports, so you'll find similar heavy rust in the head and block. But if you're just squeezing out the remainder of this season with it, keep your fingers crossed and enjoy!