Riser Gasket

[GBGraham]

Hi, I have fitted a genuine Mercruiser manifold and a riser to my 3.0L and it came with a new gaskets. The manifold and riser has four water apertures in both but the gaskets that came with them individually only have 3. Is there a reason for this or is there gaskets out there with four slots cut out?

 

[Fill Ess]

Can you post pics? From the sounds of it you might have the restricted flow gaskets where some of the passages are blocked on purpose.

Did you keep the old one to see how it was positioned?

Something like this?

 

[GBGraham]

Can you post pics? From the sounds of it you might have the restricted flow gaskets where some of the passages are blocked on purpose.

Did you keep the old one to see how it was positioned?

Something like this?

View attachment 145778

 

[GBGraham]

This is mine, but 4 ports in the manifold and riser

 

[jmcdboater]

I’d cut the slot out and use the gasket since both manifold and riser have the fourth slot.

 

[GBGraham]

Thank you for the interest but out of curiosity I would like to know why there is so many difference's in gaskets, i.e. 2 ports with holes, 3 ports, 4 ports, especially when all the manifolds and risers have 4 ports?

 

[Fill Ess]

Thank you for the interest but out of curiosity I would like to know why there is so many difference's in gaskets, i.e. 2 ports with holes, 3 ports, 4 ports, especially when all the manifolds and risers have 4 ports?

It's to restrict flow by blocking off ports in some cases.

The newer style that I had (dry joint) have the two round water ports. And as you can see, they sell the gasket as open (L), restricted (M) and closed (R) depending on the setup:

 

[GBGraham]

It's to restrict flow by blocking off ports in some cases.

The newer style that I had (dry joint) have the two round water ports. And as you can see, they sell the gasket as open (L), restricted (M) and closed (R) depending on the setup:

View attachment 145850

Thanks for taking the time to reply and I can understand the theory of restricted flow but what alludes me is the reason why. What are the reason behind these varying gaskets to block ports in different degrees. Do you think it's a temperature control, if so what would be the criteria to have different amounts of ports open or shut off?

 

[Fill Ess]

That's one for the experts. I always looked at what it came with and replaced with the same. I had 5.0 MPI Mercs and they had the gaskets with 2 round holes where 1 is blocked off. Maybe w/o the restriction there is not enough back pressure? Like running with no thermostat?

 

[GBGraham]

That's one for the experts. I always looked at what it came with and replaced with the same. I had 5.0 MPI Mercs and they had the gaskets with 2 round holes where 1 is blocked off. Maybe w/o the restriction there is not enough back pressure? Like running with no thermostat?

Thanks, that would be my primary thought also except there 4 ports in both manifold and riser and my gasket has 3 aperture's, some 1, 2 or 4. But what denotes on which to fit?
Just curious

 

[Fill Ess]

Do you have the Merc serial number? If so start here. I think you have the 3.0? You can drill down to get the diagram for your setup and when you get to the exhaust diagram, it should show the correct gasket

https://www.boats.net/catalog/mercruiser/sterndrive/gas-engines/3-0l-181-cid-inline-4-gm

 

[GBGraham]

Thank for the site address, I'm sure it will get plenty use even though I'm Mediterranean based.
I have the correct one, 3 port gasket for a 4 port manifold and riser.