How to plumb 1-piece OMC exhaust manifolds on a Mercruiser 4.3L?

[Caddy_Kid]

deleted by author

 

[Bondo]

How to plumb 1-piece OMC exhaust manifolds on a Mercruiser 4.3L?

Ayuh,.... Ya Don't,.....

Those 1 piece manifolds are a tinkin' time bomb, waitin' to ruin yer motor,.....

 

[kmarine]

I recommend spending a little money on the correct aftermarket manifolds now to save you lots of money in the future. Those manifolds were not designed for that manifold thermostat housing. The amount of water flow from the thermostat housing regulates the manifold flow. You currently are gambling with your engine.

 

[Caddy_Kid]

deleted by author

 

[Scott Danforth]

Admittedly I don't know much about these manifolds, but I'm assuming that OMC was able to somehow make them work with their version of the 4.3L engine. I would be curious to know what they did with them plumbing wise in order to make them function?

Currently the boat needs a valve job, some rot repair, upholstery, stearing shaft replacement, trim tab solenoid, wax, convert trailer to bunks, and now I'm looking at what, around $600 plus shipping for complete exhaust manifolds and risers? I would really prefer to make what I've got work (OMC did somehow, could I replicate that?), or it really seems like I should just unload the whole thing on someone else and be done with it.

No OMC never got them to work right. Replace your manifold, these were junk from day one and its a miracle they lasted this long.

 

[Caddy_Kid]

I'll keep an eye out this winter for a used set then I guess. New looks like they around $600 or so for aftermarket (manifolds, risers, gaskets, etc), is that about right?

I'm also not sure what is meant by "dry joint exhaust system" and I do see that on occasion there are 1-piece Mercury manifold sets for sale that are take offs and appear to be of a post 2000's vintage, but it sounds like those have similar issues? Or would they even work with my setup? I found GLM makes a replacement aftermarket set up in cast iron and aluminum, but have read mixed reviews on the install process and quality control on those. Any pointers on where I should be looking for the best value? Thanks.

 

[kmarine]

you could look for a used omc thermostat housing.

 

[Caddy_Kid]

I can certainly keep an eye out for that as well. I will have to do some research to find out what their part# is and/or a picture. I'm guessing it has two in and two out ports, rather than two in and four out, but that's just a guess.

 

[bruceb58]

Buying used manifolds is like buying used underwear.

 

[Caddy_Kid]

I just sort of figure unless I buy a new boat, everything on it is used anyway. Of course I really don't seem to find much in the way of used manifolds, so perhaps that option is a moot point reguardless

 

[bruceb58]

The problem is you have no history with a used manifold. You have no idea if they went through a freeze or how old they really are. Not worth risking a blown engine on old manifolds

 

[Scott Danforth]

buying old manifolds is like buying old brake shoes for your car. or using old oil you found in a ditch to put into your motor

Manifolds are a consumable item. they wear out due to corrosion over time. about 5 years in salt water, and about 20 years in fresh water.

 

[Caddy_Kid]

I figure I will have the heads off this winter, so that's as good a time as any to check out the manifolds. They certainly are expensive to replace and heavy to ship. But it is true that I have no idea of the provenance of the manifolds that are currently on there as far as year, condition, where they were used (salt or fresh), that sort of thing.

In other news, after looking at OMC thermostat housings and plumbing diagrams for the one piece heads, it looked like I should be able to simply move the thermostat from inside the housing to the top of intake manifold (same location as a vehicle) since that is where OMC had it.

I gave that a shot and it seems to operate just as it should. I did move the temp sender from the thermostat housing to the front of the intake manifold (just below the thermostat) and verified that it is reading correctly on the gauge with a contact thermometer. Although not the best solution, that should at least get me on the water for a test run or two before winter hits. After I have had it to the lake and run it, I will be better able to make a decision as to which direction I will go with the entire project.