Mercruiser stainless risers

[...bill]

At the risk of creating yet another riser question, here goes.
Boat has twin 5.7L Mercruiser engines. 1996 vintage. Has the stainless risers. Freshwater (heat exchanger) cooled. Boat is in salt water 24/7.
Like others, the manifold/riser connection leaked. Year ago, replaced the gaskets with Mercruiser silver style gaskets and touched up the manifold mating surfaces. It got better, but still seeped slightly after the winter layup. Luckily, no engine damage from any of this.
After reading lots here, concluded it was time to yank the risers and manifolds. Manifold mating surface were marginal. Manifolds were rusted badly at the head those head/manifols gaskets were toasted (all 4), but luckily, no damage. No doubt, manifolds have expired.
Bought new Mercruiser manifolds and new cast iron risers. Manifolds are installed, and am about to install the new risers. Inspected the old stainless risers, and ahem, they look great. Like, fantastic. Had the old stainless riser mating surfaces trued thinking we should re-use these and return the new cast iron risers.
Then, read about how the stainless design is flawed as the bolts can't hold torque because they're short and may leak.

What to do? Re-install the old stainless risers, or use the new cast iron stuff? I have all of the parts to install the cast iron risers, but like the fact that stainless risers don't rot. Intend to use the Mercruiser silver blocking gaskets and new mounting bolts.
I know some folks would die for a set of stainless risers, but I don't want any leaks.

Wisdom appreciated.

Thanks,
...bill

 

[tpenfield]

I have the stainless steel (risers) elbows.

Are you running a FULL closed cooling system, or the HALF system. (i.e. coolant or sea water running through the manifolds.)?

The issue with the stainless steel is that, mated to the iron manifolds, the stainless steel wins the battle of galvanic corrosion and depletes metal from the iron manifolds. This is more of an issue with open cooling (raw water cooling) and HALF closed cooling systems, since the raw water runs through both parts of the exhaust system.

If you have a full closed cooling system, then I would re-use the stainless riser/elbows, because all the kids at school wish they had a set.

I just re-did the joints on my twin Mercruiser 7.4 MPI engines and went with both the blocking gasket and the block-off plate (see link below). Since the full closed cooling systems operate under about 14 psi pressure, I think there is a greater tendency for coolant/water to work its way through the single pliable gasket. I also used some gasket sealant (permatex) to hopefully further prevent water seepage. You can buy slightly longer bolts at the hardware store . . . I think I went up 1/4" in length.

http://forums.iboats.com/forum/engin...et-replacement

 

[...bill]

I have the full closed system(s). Indeed, after all these years, the old manifolds lost the galvanic battle.

From this time forward, I'll remove the risers as an annual inspection to check the mating surfaces.

Thanks for the inspiration to go with the stainless parts.

...bill

 

[Scott Danforth]

if you have a full system, make sure your bonded and use the stainless risers.

 

[...bill]

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[...bill]

tpenfield: I read your post about replacing your risers, thanks.
A few things I learned reinstalling these that might help others...
My stainless risers leaked at the manifold mating just as shown in tpenfield's pics. These had the original separator plate from Mercruiser; they leaked anyhow.
The root cause of the leaking is from at least 5 sources:
1. The exhaust manifold mating surfaces slowly eroded away and became too narrow to seal reliably.
2. The exhaust manifold mating surfaces are not flat.
3. The stainless riser mating surfaces were in pretty good shape, but did have some pitting in some areas of the mating surface
4. The stainless riser mating surfaces were not flat, especially around the critical center areas. They looked shiny and pristine, they were not flat.
5. Upon inspection, all of the mounting bolts showed metal loss at the location of the critical gasket mating area. Mounting tension varies during temperature cycling.

My fixes:
1. Gave up on the manifolds and got new. Yes, closed system, but the manifold mating surfaces deteriorate anyhow. Closed manifolds can last a long time, but not forever. Hopefully, should get 10 years from these.
2. Riser mating surfaces were machined. Removed almost 10 mils on 2 of them to remove pitting and to make them flat again (plenty of "meat" left to do this again and again). Can't stress enough how first impression of condition fooled me. Pretty/shiny does not equal flat. I know cast iron galvanically loses when in contact with stainless. In this case, the cast iron manifold clearly deteriorated, but the stainless surface also depleted. I'm guessing the stainless depletion is more likely from oxygen starvation type corrosion in this case. The stainless mating surfaces were compromised, but cleaned up nicely by machining.
3. I used a single Mercruiser blocking gasket. There?s no evidence I?ve found where these gaskets break through. Eliminating the blocking plate reduces mating surfaces from 4 to 2 which has to improve reliability of this connection. I see no harm in using the blocking plate if that?s your preference.
3. The mounting bolts. Mercruiser used "8 hard" bolts. I replaced all of the flange bolts and used the same. You?ll notice all the new cast iron risers have long (>5?-6?) mounting bolts. This is intentional as the long bolts act like torsion springs to maintain riser mating tension during temperature cycling.
The stainless riser mounting bolts are short and have minimal mechanical stretch compliance. Sure it?ll work, but long bolts are better.

Knowing all of this just adds to the maintenance list.

• Check the torque of the risers periodically.
• Every season (or maybe every other), remove the risers and check the riser/manifold mating surfaces for corrosion and flatness.

The old stainless risers are nice, but not zero maintenance.

...bill

 

[NHGuy]

You are taking a chance I wouldn't with skipping the blocking plate. I do understand the desire to reduce the number of joints. But you have removed the structure behind the gasket. Now you are trusting just a piece of thin metal mesh that's impregnated with sealer to hold back engine coolant at running temperature and pressure.

Also I see no benefit to longer bolts when they clamp a shorter distance. Unless there is a stack of washers or a spring bolt.

 

[tpenfield]

The single Mercruiser blocking gasket does have a metal foil that is sandwiched in between the gasket material. . . but I find that the pressure of a closed cooling system tends to deform the thin metal layer a bit. The bigger problem seems to be deterioration of the gasket material itself. Not sure if that is from the coolant or the sea water . . .

Here is a picture of one of the old single block-off gaskets from my recent work. the metal film later is intact, but the gasket material (gray sticky stuff) is/was deteriorated.

 

[...bill]

Longer bolts...
I was simply comparing the design of the cast iron risers and bolts to the stainless riser design.
The newer iron casting intentionally lengthened the bolts for springiness.
The stainless risers have short bolts, 1.5", and that?s just the way it is.

I'm "all ears" about the blocking plate. I haven't re-assembled yet, so open to the best practice.
I can use the blocking plates ( I have them), but would be inclined to use traditional gaskets with good sealer.
Local Mercruiser master mechanic swears by the improvement of the new gaskets and the elimination of the blocking plates. Just another opinion to influence me?
...Now I'm confused again...
In any event, I believe this riser connection has to be inspected (dismantled) at some regular interval to guarantee integrity. Even if the risers are in good shape.
...bill