5 bolt vs. 6 bolt exhaust manifold on 3.0 L mercruiser

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I have purchased a new long block engine to replace the current motor SN # OM640085.The reason for the new motor is exhaust gasket failure and water entry into the block. The new motor has a 6 bolt exhaust manifold pattern same as the old motor. I removed the old exhaust manifold and it has a 5 bolt pattern. I bought a direct replacement of the old manifold containing a 5 bolt pattern. I have noticed on several web sites selling exhaust manifolds a discrepancy in the bolt patterns between the manifold and gasket. My question is this correct or was a mistake made at the factory when I bought the boat new back in 2003? Will it be acceptable to bolt the manifold to the new motor using the 5 bolt pattern?
 

Bt Doctur

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according to that serial number you have 2 bolts and 3 studs
 
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BT Doctor, you are correct concerning the studs and bolts. I have the concern about the number of mounting positions .
 

Bt Doctur

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If I`m not mistaken there are only 5 holes in the 4 cylinder exaust manifolds
 
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Bt Doctor, you are correct on the hole count on the old and new exhaust manifold , but the engine heads both old and new have 6 tapped holes.
 

Bt Doctur

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because the heads are car heads and car manifolds use 6 bolts
 

Scott Danforth

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its because the original 1963 2.5 liter motor came in cars as BtDoc indicated, and the 3.0 industrial motor also gets installed in fork trucks, trash pumps and generators with dry exhaust. the dry exhaust has 6 holes. the marine exhaust blocks one of the holes.
 

Bondo

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Will it be acceptable to bolt the manifold to the new motor using the 5 bolt pattern?

Ayuh,.... Welcome Aboard,.... Boat manifolds are so huge 'n massive, they don't need 6 bolts as they're so rigid, 4 or 5 will provide the equal clampin' forces,....
 

stang32

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Ayuh,.... Welcome Aboard,.... Boat manifolds are so huge 'n massive, they don't need 6 bolts as they're so rigid, 4 or 5 will provide the equal clampin' forces,....



well not for nothing but im working on a boat now that has 4 bolt holes & a leaking manifold gasket. i replaced the gasket & it immediately failed as well. the machine shop confirmed that the manifold is warped so the more bolts the better i feel. my question is , i see this [h=1]Barr Marine MC-1-96705 Manifold[/h]
which has 6 bolt holes, then i see [h=1]860235A03 - Exhaust Manifold[/h]
which has 5.

why and how is this ?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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well not for nothing but im working on a boat now that has 4 bolt holes & a leaking manifold gasket. i replaced the gasket & it immediately failed as well. the machine shop confirmed that the manifold is warped so the more bolts the better i feel. my question is , i see this Barr Marine MC-1-96705 Manifold
which has 6 bolt holes, then i see 860235A03 - Exhaust Manifold
which has 5.

why and how is this ?

to get a double-walled marine manifold to warp, there has to be a ton of heat put to it, as in a severe over-heat condition without water running in the manifold for a long period of time.

as for Barr manifold different then mercruiser manifold, does it really mater. and MC-1-96705 is a replacement for 96705A1 (1968-1982) no mention of reference to 860235A03 (1982-1995). so your comparing Brand A at vintage X to Brand B at Vintage Y in addition to hijacking a thread
 

Bt Doctur

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the exaust manifold is the same one used on the industrial engines and is not water cooled, hence the 6 bolts. The marine exaust manifold uses the same gasket but with 2 studs and 3 bolts.
 

Bondo

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the machine shop confirmed that the manifold is warped so the more bolts the better i feel.

Ayuh,... As Scotts says, a warped manifold has nothin' to do with how many bolts in it,... but Heat,.... Lotsa Heat,....
 
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