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    Mercruiser 470 exhaust flapper-gctid371074

    Could someone knowledgeable in flappers (that just sounds weird) explain them to me.When my lower unit was rebuilt last year the wrench found what was left of my flapper laying in exhaust boot. So I bought a new one and based on general consensus it may not be "critical" but it should be replaced.As part of my fix-da-floor project, I have the piece of the bulkhead that hides access to the transom area off, and what a perfect time to replace the flapper as I have full access to the exhaust manifold.I have the type exhaust where there is a big square box at the back of the exhaust manifold and under the back of the box is a rubber boot that connects the manifold to the exhaust port (which looks like a long funnel down at this point) and in the top of that funnel is where the flapper goes.I took the box off (I have new gaskets and will have it cleaned up before I put it back on) and replaced the flapper....Back to the opening question... The flapper in its relaxed state seals the exhaust. This seems counter-productive... as water washing back up that funnel will just push the flapper out of the way. Pressure from the top, keeps it sealed.I have yet to find a drawing that shows the exact assembly I have, but the new parts matched the old parts so it seems right.Could someone just explain this to me. the a-retentive how-does-dis-work side of my brain is just not making sense of this.Top of Header

    [img]/media/kunena/attachments/vb/674478=26443-P1070353.jpg[/img]Rubber Boot

    [img]/media/kunena/attachments/vb/674478=26444-P1070354.jpg[/img]Disassemble

    [img]/media/kunena/attachments/vb/674478=26442-P1070357.jpg[/img]What was left of the old flapper

    [img]/media/kunena/attachments/vb/674478=26440-P1070348.jpg[/img]Nice shiny new flapper

    [img]/media/kunena/attachments/vb/674478=26441-P1070355.jpg[/img]Thanks in advance.Miles
    Aquatic Muse
    Mount Vernon, WA
    MMSI: 367498870
    '79 Bayliner Santiago w/ Mercruiser 470 power and drive

    #2
    Must be nice weather... was really hoping someone could double-check my work and if it's correct, explain how this works.

    Thanks in advance..
    Aquatic Muse
    Mount Vernon, WA
    MMSI: 367498870
    '79 Bayliner Santiago w/ Mercruiser 470 power and drive

    Comment


      #3
      Run a search on exhaust, youre bound to find a thread explaining just what youre asking! I just melted mine last weekend! Good Luck!

      Comment


        #4
        Thanks apexaro1,

        Been there done that...

        I actually have yet to find any specific diagram of the assembly. The principle has been explained numerous times in threads which I get.. "The reason for the exhaust flapper is to keep water from entering the motor thru the exhaust. When a wave comes from behind the boat, it pushes against the flapper and prevents water from getting in the motor" Makes sense, I'm just not convinced I have all the parts and that it's assembled as it's supposed to be.

        The parts I installed matched the parts that came out, but history has taught me not to rely or assume that the parts I took out were the original either. Add to that I can't see how exhaust gets out..

        Miles
        Aquatic Muse
        Mount Vernon, WA
        MMSI: 367498870
        '79 Bayliner Santiago w/ Mercruiser 470 power and drive

        Comment


          #5
          Ah- I think you are thinking upside down. The flapper points ears down and, yes flares out when the boat is not running. The exhaust pressure is high and pushes them together and viola, you are flappin. The engine you have, especially if you overheated it as I suspect, MUST have the flapper or you will seriously fry that last cyllinder. This will be the beginning of the end. It is not optional. Only get a mechanic who knows that engine. Flaky, but powerful and efficient. I had to work like Mister Scotty on the Enterprise to keep mine running, but loved it.

          Comment


            #6
            I ran a 470 for 25 years (1 rebuild) and only lost the flapper once. It's main purpose is to stop backwash from coming back into the exhaust manifold. It's not necessarily overheating that causes it to go away, just running the engine for a very short period of time with NO water flow. A tremendous amount of heat is generated with the motor running, and if there's no coolant flowing across the flapper, it will go away long before the motor overheats (which also won't take very long). The diagram that you show confuses me a little as mine seemed to rest on two "ears" which held it in place, and allowed it to open and close as desired. Memory serves me that open was the default position, with backflow closing it. I tossed my manuals for the 470 a couple of years back, but if you can find one on-line, they are a tremendous source of information. Some folks seemed to strongly dislike the 470, but I sure enjoyed mine. It seemed almost bulletproof!
            Bob Hawes.
            Kelowna, B.C.
            1998 Trophy 2052 WA
            4.3 Vortec, A1 G2

            Comment


              #7
              20miler wrote:
              Ah- I think you are thinking upside down. The flapper points ears down and, yes flares out when the boat is not running. The exhaust pressure is high and pushes them together and viola, you are flappin. The engine you have, especially if you overheated it as I suspect, MUST have the flapper or you will seriously fry that last cyllinder. This will be the beginning of the end. It is not optional. Only get a mechanic who knows that engine. Flaky, but powerful and efficient. I had to work like Mister Scotty on the Enterprise to keep mine running, but loved it.
              bhawes wrote:
              I ran a 470 for 25 years (1 rebuild) and only lost the flapper once. It's main purpose is to stop backwash from coming back into the exhaust manifold. It's not necessarily overheating that causes it to go away, just running the engine for a very short period of time with NO water flow. A tremendous amount of heat is generated with the motor running, and if there's no coolant flowing across the flapper, it will go away long before the motor overheats (which also won't take very long). The diagram that you show confuses me a little as mine seemed to rest on two "ears" which held it in place, and allowed it to open and close as desired. Memory serves me that open was the default position, with backflow closing it. I tossed my manuals for the 470 a couple of years back, but if you can find one on-line, they are a tremendous source of information. Some folks seemed to strongly dislike the 470, but I sure enjoyed mine. It seemed almost bulletproof!
              I understand what both of you are saying, but if you look at the pictures above... clearly the flapper can not point ears down. The beveled edge seats perfectly in that exhaust port where it narrows. Just to be sure, as the elbow is out getting cleaned, I went out and tried to push the flaps down... they are flush the pipe. You can also see in the before photo that the shows only the old axle, that is sits just at the point where the pipe narrows.I am beginning to think the old parts are the wrong parts.. but at the same time... the old flapper was found in the bottom of the exhaust bellows so it must have worked to some extent....I have looked online for a 470 manual for 4 years. I have yet to find one. All I find is the SECAT (sp) manuals which do not discuss this part in any detail. The replace parts I purchased were purchased based on engine serial number, for "sterndrives" and I'm wondering if maybe the last time the flapper was replaced, it was replaced with the wrong one... but then again... how would the engine run.As far as over-heating... I've had this boat for three years, and the engine was checked out completely. If anything it runs cools and it runs strong. I'm a fan of Moto Guzzi bikes, so I'm used to the extra care and feeding.I guess what i need is verification that the brackets and flapper in the above photos are the right ones for this engine, regardless of whether or not they are technically the right part number, because on all accounts of how the flapper should work... this one cannot flap downward.I just scribbled a side-view of how the flapper sits and reposted what it looks like with the flapper in place and the before picture as well. The sketch is out of scale, but you should get the idea.If these are the wrong parts (which I think they are), can someone point me in the direction of the right parts as the engine serial numbers will put it back to these parts.

              http://baylinerownersclub.org/media/....jpg[/img]

              http://baylinerownersclub.org/media/....jpg[/img]

              http://baylinerownersclub.org/media/....jpg[/img]
              Aquatic Muse
              Mount Vernon, WA
              MMSI: 367498870
              '79 Bayliner Santiago w/ Mercruiser 470 power and drive

              Comment


                #8
                Mileskb wrote:


                1..... I have the type exhaust where there is a big square box at the back of the exhaust manifold and under the back of the box is a rubber boot that connects the manifold to the exhaust port (which looks like a long funnel down at this point) and in the top of that funnel is where the flapper goes.

                2...... Back to the opening question... The flapper in its relaxed state seals the exhaust. This seems counter-productive... as water washing back up that funnel will just push the flapper out of the way. Pressure from the top, keeps it sealed.

                I have yet to find a drawing that shows the exact assembly I have, but the new parts matched the old parts so it seems right.

                3.... Could someone just explain this to me. the a-retentive how-does-dis-work side of my brain is just not making sense of this.
                Miles,

                1..... you have an exhaust manifold, a rear elbow or riser, and an exhaust tube or "down tube".............. as these parts are called.

                The "back-flow" prevention flapper installs in the down tube.... and up high near the top of it as you show.

                2...... This flapper should/will allow exhaust gasses to flow out towards the out drive, and with as little restriction as possible.

                Any surge of back flowing water (such as from a wake at/during low RPM) is stopped by this flapper.

                The flapper can only be effective when installed correctly and when working correctly.

                3.... Your brain is working.... and I agree with you. What you have here does not look right to me.

                One side of this flapper should offer more area than the other..... like an old style furnace damper..... of which you'll notice is hinged off-set! This is so that the out-flowing exhaust gasses will force it open.

                Conversely, if a surge of In-Flowing water was to contact the flapper, it should close the port off momentarily.

                I may be wrong here.... but when I look at this flapper, and in particular where and how it is hinged, I don't see how the exhaust gasses can open it.

                Perhaps I'm not seeing the off-set.

                Could it be installed up-side-down???

                Is this an OEM flapper?????

                Bottom line..... exhaust gasses must open it..... back flow must close it.



                Here is an example of an exhaust flapper/shutter that is off-set.

                The larger surface area (off to one side) allows this style to open with exhaust gasses, yet it closes with back flow.



                Here is an example of an exhaust flapper/shutter that is center hinged. (I believe that it's being shown bottom side up)

                The surface area of each side allows the exhaust gasses to fold it open in one direction, yet it closes with back flow.



                Merc shows several styles for the 470 down tube and flapper.

                Here's one style, and it looks different from yours.



                Here's another view for the 470.



                .
                Rick E. Gresham, Oregon
                2850 Bounty Sedan Flybridge model
                Twin 280 HP 5.7's w/ Closed Cooling
                Volvo Penta DuoProp Drives
                Kohler 4 CZ Gen Set

                Comment


                  #9
                  THANK YOU 2850Bounty...The top diagram is closest to what I have, not exact, but close but it shows a detail that honestly I just figured out before I checked the messages... The top diagram shows the flapper going in place... flaps down...That's the key!!! I think/hope..I will remove mine and reinstall. I'm going to take a leap at this point that I have the right parts... Based on the original bracket and the brackets available and the mounting etc... it is the "center" axle type...Anyway, I think this sketch I thought of in the middle of the night based on a previous post that referenced the flaps needing to be down explains it best.

                  [img]/media/kunena/attachments/vb/675836=26612-sketchnew.jpg[/img]Note that water coming up would force the flaps out/open to seal the pipe. Exhaust would push the wings down/together opening the pipe.Thanks for all the help on this. The two diagrams I have run across, but again, neither reflects the setup I have that seems to be stock. I have seen other engines with the same arrangement I have both in person and photos from folks selling rebuilds... (as well as seeing the arrangements in the diagram) but I have yet to see a "diagram" of my arrangement. Oh well..Keep your fingers crossed that the flapper can go in like my sketch
                  Aquatic Muse
                  Mount Vernon, WA
                  MMSI: 367498870
                  '79 Bayliner Santiago w/ Mercruiser 470 power and drive

                  Comment


                    #10
                    Yup, Simpley in upside down. Default it closed, running it is open due to exaust gasses. Been there donw that.
                    Boatless at this time

                    A veteran is someone who, at one point in their life, wrote a blank check made payable to "The United States of America," for an amount of "up to and including their life."

                    Comment


                      #11
                      For those who are still reading... thanks...

                      ... with an easy couple questions...

                      Should I use and high temp rtv or any gasket stuff at all on the cardboard-metal-cardboard sandwich gasket between the header and the elbow?

                      Is there a torque rating for the 4 fasteners (2 bolts& 2 nuts) holding down the elbow?

                      Thanks again for the assist...
                      Aquatic Muse
                      Mount Vernon, WA
                      MMSI: 367498870
                      '79 Bayliner Santiago w/ Mercruiser 470 power and drive

                      Comment


                        #12
                        I believe that the gasket sealer is Locktite 510, or equivelent, with a 1/8" bead appplied to both sides of the gasket. I cannot locate the torque spec for the 2 bolts and nuts, but believe it to be 25 ft./lbs. Do NOT over- torque these bolts. You can quite easily blow apart the casting on the manifold.

                        As an aside, look up: [email protected]. This chap sells cd's for almost anything at a very reasonable cost. I'm sure that he'll have the service manual for the 470.
                        Bob Hawes.
                        Kelowna, B.C.
                        1998 Trophy 2052 WA
                        4.3 Vortec, A1 G2

                        Comment


                          #13
                          Thanks Bob... I will send a note off to the person you referenced ASAP. I have the Clymer Mercruiser Stern Drive shop manual which has pretty good info and procedure, but it's a hard read because it has ALL of the stern drives from the 1964-1985 period in it and you have to sift through stuff that has nothing to do with the engine at hand. Also while it focuses on details that are necessary for tune-ups and even replacement of parts, it glosses over certain take-for-granted items like... what type of sealer if any etc.. A dedicated 470 manual would be OUTSTANDING!!!!

                          Thanks.
                          Aquatic Muse
                          Mount Vernon, WA
                          MMSI: 367498870
                          '79 Bayliner Santiago w/ Mercruiser 470 power and drive

                          Comment


                            #14
                            Yep, as I mentioned earlier a manual would be great. Still haven't come across one that shows this section in detail so I ran across something I hadn't noticed before..

                            Where the Elbow connects to the Header...

                            There is a metal plate that covers the water ports...

                            There are TWO cardboard gaskets that came in the replacement kit

                            There is a rubber? gasket on the manifold that has a tab on the front.

                            I didn't notice the rubber (or whatever its made of) when I disassmbled. It appears in good shape and it appears I could remove it to re-goop it without harm..

                            So the question...

                            What is the order of the gaskets.. It was hard to tell what was in there as it came off because it was all melded together..

                            My guess is from top down... Cardboard-plate-cardboard-rubber but this does mean that there is cardboard where there is water.

                            Another option is cardboard-cardboard-plate-rubber or

                            Maybe I have an extra cardboard... and it's just cardboard-plate-rubber. ??

                            I did find this diagram http://www.marinepowerservice.com/CR...5471%5C/14.gif which seems pretty straight forward...

                            In the lookup, the #45's are the gaskets and #46 plate... simple...

                            However... the (click) full view doesn't not show any additional rubber or whatever gasket on the manifold..

                            And... for those that note details... the back of the elbow that goes down to the exhaust (with the flapper) doesn't remotely resemble what's in my boat.

                            But for now... what's up with the gaskets?

                            -mkb
                            Aquatic Muse
                            Mount Vernon, WA
                            MMSI: 367498870
                            '79 Bayliner Santiago w/ Mercruiser 470 power and drive

                            Comment

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