Exhaust/risers and manifolds

jackson

jackson

Member
Nov 20, 2006
171
Narraganset Bay
Boat Info
2002 310DA
Engines
350 MAG Horizons/V-drives
Does anyone know when I should be worried about replacing exhaust manifolds and/or risers. The boat is a 2002 and I am in salt water. I have the 350 Mag Horizons which are fresh water cooled with 300 hrs and have the newer exhaust with barrel type mufflers. My mechanic checked them for me in August 2005 and said they look like new. I had him replace the riser gaskets while he had them apart. I have had no water ingestion issues. This is basically a peace of mind thing for me.....Thanks!
 
You have asked a good question............

It really does not matter whether the risers and manifolds are fresh water cooled or not. Most of the corrosion comes from inside the exhaust passage. This is because the passages are open to ambient air right off the sea water 100% of the time. Every time you use the boat, the cast iron riser and manifold heat up and cool down. That process of expansion and contraction dislodges the surface rust inside the exhaust passage only for it to rust all over again due to the salt air that migrates up in the system. So over time, the casting gets thin and sooner or later gets a hole in it and it begins to leak.

When a casting finally fails is during contraction as it cools. So one day your boat runs great, you put it in the slip or barn, then the next time you go to use it, the valves are stuck or a cylinder is full of water. In other words, you get no warning until you either need to repower of do a valve job. Checking the risers is a lot cheaper.

OEM Mercruiser cast parts are usually good for 3-5 years depending upon use, location, etc. So in the 3rd year of age, whether they are original or replacements, I'd spend the money to have the risers removed and checked. You can see in the manifolds by removing the risers so all you'll be out is an hour's labor and some gaskets.

After you check the risers the first time, I'd just do it every year.......but I'm conservative. There are people who wait and begin checking on the 5th year. If you do, then check them every year after that until they need replacement. If you do it in the 3rd year and they look new as your apparently do, you might get by waiting until the 5th year. But if you make it to year 5, I'd be sure to begin checking them every year.

Obviously, water ingestion is what this process is trying to avoid. To be thorough, it is also just as important to understand the riser gasket's role in this. The weak link in the Mercruiser exhaust system design is the riser to manifold gasket. They are thin and have some small segments on their sides, so they tend to dry and crack, and are prone to leaking. A fairly large percentage of the water ingestion issues seen on big block Mercs are a result of the riser gaskets failing. Checking the risers also means you replace the riser gaskets, which is a good thing. You should also make a visual check of the riser to manifold joint every time you check your engines. You are looking for drip or rust lines running down from the joint. That means you have a leak.....and they are probably also leaking inside as well.

It sounds like you are already on top of this, so figure your tolerance for risk, pick a schedule that makes sense in your circumstances and keep boating..........
 
My risers and manifolds were replaced 3 years ago just before I bought the boat. Osco's were installed, I'm pulling the risers for the first time in the next few weeks to inspect them. Spare Mercruiser gaskets were left on the boat, any reason why I can't use these when re installing the risers................if the risers are o.k.

John
 
The only reason not to use them is that they may have been superceeded by a different gasket or one made from a different material. Depending upon the engine you have, Mercruiser seemd to think this gasket was a work in progress for a while and every time you got a new one it was different. This has always been a trouble prone area, so for the cost of a trip to a Merc dealer, I'd at least check to see if there as a better gasket being supplied now. And, for the cost of a gasket, I'd use the latest one if there were a change.

You didn't ask this, but my previous answer concerning how often to check risers was for Mercruiser OEM castings only. Barr, Osco, Sierra, etc castings are all known to fail early in some applications. Merc OEM castings are more expensive but are superior to aftermarket ones. If you use aftermarket brands, you should be able to go 2 years before checking them, but the risers and manifolds should be checked in the second year and then every year thereafter until they need replacment.

You are wise to check risers on the right interval for the brand you have........sure beats a repower or valve job.
 
Thanks for the info Frank,

Osco has a 3 year warranty on risers and manifolds, so now is the time for me. I boat in the upper Chesapeake, mostly brackish water.

John
 
exhaust

Thanks for all the good advice. I skipped last season, so I will be sure to take care of it this season and every year after. Like I said, it is peace of mind for me so I would rather be safe than sorry.
 
I am planning on replacing our AJ's manifolds and risers this spring and appreciate all the info in the threads I have read. One question though: On reassembly is it better to install manifolds first, then the risers, or assemble the two together first and install in one piece? Or, does it not matter either way?

Also have priced both OEM and aftermarkets....was leaning toward aftermarkets but will go with OEM after hearing it from the "experts" (you get what you pay for!)

Thanks for all the help and advise,

Reel
 
I tend to think the weight of the risers and manifolds together would make it very impractical to put them on already assembled. Besides, the riser to manifold joint is a critical point. I've found it quite easy to put the riser on the manifold when it is already bolted in place right there in a perfect working position.
 
When putting on the gaskets between riser and manifold, do you need any kind of adhesive? Or do the gaskets go on dry? Thanks.

Doug
 
gaskets

Not sure if everyone does this, but the service manual states to apply a 1/8 in. bead of LOCTITE 510 Sealant around all holes on both sides of new gasket.
 
Was doing some parts research. Manifolds, risers, elbows. I understand the need to replace the manifolds and risers....do the elbows corrode at the same rate as the other 2? Do you, should you, always replace all 3, or do the elbows last longer?
 
elbows

Hopefully someone else has more of an idea, but I wouldn't think that the elbows would need to be replaced as soon as the others.
 
There was a good thread on this topic back in November with some good info and pictures from "Dave M." Still curious if the elbows deteriorate as fast as the other parts. I understand that in fresh water cooled boats only antifreeze circulates through the manifolds and risers and the elbows are where the sea water is introduced to leave the boat with the exhaust. My boat is raw water cooled so sea water runs through all the parts. Is an elbow failure as damaging as a manifold, gasket, or riser failure?

Thanks in advance!
 
I'll chime in with what I just found on mine.

My boat had a new set of rebuilds, a new hydro lift exhaust sytem, and a new set of manifolds, spacers, and risers installed around April of 2003. I've seen some terminology confusion in the past posts, so I'll add that risers and elbows are the same thing. What confuses people is thinking that spacers (height extenders) are risers. Technically, they're part of the riser, but the term riser is loosely about the exhaust water rising above the waterline of the boat, and then flowing over an elbow, and down to the exhaust muffler. whew!

All parts replaced were Mercury, done under the supervision of the Mercury rep, performed by Mercruiser master mechanics at Marine Max. Some of you will know why.

The boat sat idle until around November of 2003, when I purchased it. In fact it had 8 hours on the "new" engines. In November of 2006, I decided that it was time to inspect the manifolds and risers to determine their need for replacement, or at least more frequent inspection. These engines are run in Mid Florida Gulf temperatures, and salinity, all summer, most of the winter, and rarely at cruise speed for any extended period of time. The temperature of the engines does not fluctuate more than 5 degrees between idle, and running hard. They are both around 170 degrees.

I was not experiencing any leakage down the outsides of the riser to spacer joints, or the spacer to manifold joints. When I removed the risers, I found that the elbow portion was caked up with deposits that obstructed probably 50% of the exhaust opening. this was true for all 4 elbows. I'd like some idea of what that was, as I didn't see much in the way of water created rust in the exhaust passage of the manifolds, spacers or the risers. Could that be carbon? Or, was it 3 years of possible small salt water leakage that was pushed up to the top of the riser by the exhaust, but then got built up over time at the bend? It definitely wasn't rust, but it was hard as a rock and an ashy grey in color.

These manifolds spacers and risers are never fresh water flushed after use. The water passages in the manifolds, spacers and risers had lots of rust scale, and some debris, and had probably lost a fair amount of clearance for water passage. Still, the walls seem well intact, and thick enough to not scare me into reusing them, at least for a while.

I did find a small amount of rust in the bottom of a couple of the manifold port openings, where the exhaust first comes past the valve from the cylinder. The valves didn't appear to be rusted, or show signs of water getting into the block, but I think very small amounts may have pooled up in the manifold because of the way they are angled. Or, maybe just condensation of salt water coming back into the exhaust from the water lift muffler. At any rate, no real effect on the engine, or performance.

I thoroughly cleaned the manifolds and spacers, and resurfaced the mating points for the gaskets. I threw all 4 elbows away, and replaced them with new. I have only put a couple hours or so on the boat since doing that, but everything seems to be ok so far.

I'm guessing it's time to do the genset next. I'm sure that this one has never even been inspected, much less changed, especially with how hard it is to access. Looks like I'll be removing at least the exhaust portion of the starboard engine, and possibly the starboard engine. Anyone have experience with pulling the 7.0BCG from it's hiding place in the 370 with hydro lift exhausts?

It brings to mind my thought to cut an access in the starboard cockpit locker over the genset, to create a better access to the exhaust side, and top for changing spark plugs. THAT's a PITA!
 
370

Thanks for the insight. I'm hoping to open mine up this spring and find that they are still ok.
 
Interesting topic and replys.

Anyone know if there is any recourse utilizing the extended service agreement purchased thru Passport Services for a water issue that has been sucked into one engine cylinder and requires a new long block.

Anyone had experience with a claim to them.
 
Since my name was mentioned about a previous post with pictures, I will say that nearly all my information about this stuff has come from Frank. His explanation in this thread is very good, Thanks Frank! :thumbsup:

Also, for anyone inclined to do it yourself, I would encourage you to just do it. Once you do it, if your access is good, I think you will find it is an easy job, and gives you a lot of insight into what is going on in there. Checking once a year and putting in new gaskets is a good thing, and is not that difficult the second time.
 
Be sure if your doing it to get the stainless plate Merc is buying from Gil and using on all there exhaust as part of updates. My boat was fixed under warranty a few years back and I ripped it down this year just to be safe and found this plate. I went to buy gaskets and brought it with me and the guy was shocked I had them in my boat. Gil calls it a turbulator and owns the patten on it and Merc is now buying it from them for the valve overlap issues. Mine was done 3 years ago and everything still looks great so maybe this is the fix they needed. I cant believe mine was done three years ago and this has never come up anywhere. It is sold as a Merc part with a gil patten sticker on it.
 
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