RDE-18 powerhead to exhaust plate/gaskets

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    wyo307

    Replies: 141
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    #52645

    Ok now I am starting to get confused. The baffle plate I was refering to was the one that goes on top of the exhaust housing and below the power head. It is sandwiched between two identical gaskets. My understanding was that is the one that was omitted and then the one with the two tubes was to be replaced by a solid plate. Am I understanding this correctly?

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    dan-in-tn
    US Member - 1 Year
    Replies: 993
    Topics: 104
    #52646

    Better late than never on a subject! Just bought a kit off Ebay that might shed a little light on this subject. I haven’t received it yet, but should be here today. Hope you guys can make something out of it. I have 4 of these engines, but don’t have one tore down at the moment. Read & enjoy.

    http://cgi.ebay.com/ws/eBayISAPI.dll?Vi … K:MEAFB:IT

    Dan in TN

    Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 4037
    Topics: 416
    #52649

    Thanks Dan. That explains a lot! ๐Ÿ˜€

    So it’s an upgrade to replace the loading tubes with a flat plate and drill new holes for the idle relief on the ’55 and ’56 models? That’s easy enough.


    Attachments:

    olcah
    olcah
    US Member - 1 Year
    Replies: 318
    Topics: 68
    #52650

    Could you please post step 2 and subsequent in those instructions when you receive them? Looks like the number of holes and location is different.

    olcah
    olcah
    US Member - 1 Year
    Replies: 318
    Topics: 68
    #52712

    Dan in TN provided better photos of the kit above (Kit 376786):




    Where I think we are with this:

    RD18: Use carbon seal on lower crankshaft bearing
    RD18: remove the large baffle – reason – it was not used on RD19, or maybe because OMC went back to carbon seal on lower crankshaft bearing. Disadvantage – the large baffle deflects hot exhaust away from the exhaust leg. May cook paint on the port upper side. I am not going to use the large baffle on mine.
    RD18: replace the small tube plate with a solid plate. Likely this was to prevent water from flowing up towards the powerhead when the pressure in the exhaust varied.
    RD18: Drill thru poop hole. Then drill four holes thru the next cavity boundary. This is not easy. I will probably drill undersize and file to make a single oblong opening.

    RD17: Use carbon seal on lower crankshaft bearing.
    RD17: Cut 2 inch opening in pan so carb gets more air.
    RD17: IF the water outlet is a 1/8 inch hole in the first cavity, do same as for RD18. If not I think this is sticky. The first kit in this thread plugs water outlet openings in other locations and then drills a 1/8 inch hole so the water outlet is in the same place as RD18. Discussed briefly with Dan in TN. Dan thinks that the outlet should be moved. Mumbles remembers a problem. If outlets are not moved but the solid plate was used, outlet water would splash directly against the solid plate and maybe up to the powerhead. Maybe it would be better not use the small solid plate or do the drilling, but put together without the large baffle plate and just put the small tube plate back in. It is an original 1955 setup. Billy J describes it above.

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    immulmen

    Replies: 4
    Topics: 0
    #52742

    I have a RD18 and a RD19. It looks like this mod has been done to the RD18. The RD18 has 2 holes in the poop hole and the RD19 has 3. The 18 has RTV sealant where the power head bolts to the leg. I will get some photos of my RD18 when I pull the power head. The model number is RDE-18C and the holes almost look factory. Is it possible it is a later production and this Mod does not apply to mine? The serial number is 1476427.

    olcah
    olcah
    US Member - 1 Year
    Replies: 318
    Topics: 68
    #52775

    I don’t know. If you are going to pull the powerhead, you can check for the mod then. I would not pull the power head just to check for this mod.

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    immulmen

    Replies: 4
    Topics: 0
    #52776
    quote OLCAH:

    I don’t know. If you are going to pull the powerhead, you can check for the mod then. I would not pull the power head just to check for this mod.

    My thoughts where to see what type of seal it has.

    olcah
    olcah
    US Member - 1 Year
    Replies: 318
    Topics: 68
    #52777

    Oh, I see. That’s a good idea.

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    billy-j

    Replies: 87
    Topics: 8
    #52787

    Re: RDE-18 powerhead to exhaust plate/gaskets

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    Postby t2stroke ยป Wed Feb 08, 2017 11:37 am
    Better late than never on a subject! Just bought a kit off Ebay that might shed a little light on this subject. I haven’t received it yet, but should be here today. Hope you guys can make something out of it. I have 4 of these engines, but don’t have one tore down at the moment. Read & enjoy.

    http://cgi.ebay.com/ws/eBayISAPI.dll?Vi … K:MEAFB:IT

    Dan in TN
    This is the kit I bought When I was going over my 1956 30 HP. Johnson. I did not feel comfortable making this modification. I am sure the Evinrudes of the same years had the same setup also. This turned into quite a lively thread but the big question is what these modifications were trying to improve? We do not know. We all know these engines ran with no noticeable issue either way original or modified. I think it’s nice to be aware and understand when we see this on engines what was done and understand it was a modification. but again the motor will run either way with no issues that we know of. I myself on my 1956 30HP. left out the big baffle plate and used 1 gasket between power head and lower leg and used the 2 tube little plate.I have used this engine over 7 years with no issue. Regarding the paint on the port side my engine was original paint and on the port side the red was gone and down to the primer but that was when the big baffle plate was installed from the factory I think there was just a thin coat of red on the leg from the factory and it wore off. I touched it up with red after I took out the baffle and the paint is still there. Before I went over the engine my father and I used it many years in Canada for fishing trips maybe 20 years not babying it at all and some of the stuff probably was abuse but it always ran. My 1955 Johnson 25 HP. I am going to drill out the hole in the lower pan to 2 inches so it can breath and plan on using the original 2 tube plate I am not going to worry about drilling holes in the bottom of my power head because I do not understand what it is going to correct.The 1955 and 1956 Big twin are my all time favorite engines but we must remember these were years where they transitioned from the old school to the modern engines. Maybe even these modifications we are talking about were not well thought out improvements as later years they learned enough to design engines without this issue. Bill,

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    chris-p
    Canada Member - 1 Year
    Replies: 2458
    Topics: 157
    #52793

    The modifications all have their purpose.

    If anyone was not sure, the reason for drilling through into the exhaust cavity is for better starts. It relieves some back pressure on the exhaust. WIll the motor still start without this modification? Well yeah, but OMC tried to better the experience of the boater (while limiting warranty work as well of course) by providing these Service Bulletins. In my humble opinion, I would follow each of them, when restoring a motor in question.

    In ’57 the RD went to 35hp, which incorporated these improvements.

    Then the 1958 RDS 20 is in my opinion the best of all worlds mechanically speaking, as far as a 35hp OMC is concerned.

    olcah
    olcah
    US Member - 1 Year
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    #52810

    Two excellent statements of two different but logical points of view! Thank you Billy and Chris.

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    garry-in-tampa
    Lifetime Member
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    #52812


    Somebody told me an Elephant was a Race Horse designed by a committee.
    . . ๐Ÿ˜†

    olcah
    olcah
    US Member - 1 Year
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    #52823

    More than one way to skin a cat. ๐Ÿ˜†

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    billy-j

    Replies: 87
    Topics: 8
    #52829

    Re: RDE-18 powerhead to exhaust plate/gaskets

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    Postby Chris_P ยป Fri Feb 10, 2017 7:02 pm
    The modifications all have their purpose.

    If anyone was not sure, the reason for drilling through into the exhaust cavity is for better starts. It relieves some back pressure on the exhaust. WIll the motor still start without this modification? Well yeah, but OMC tried to better the experience of the boater (while limiting warranty work as well of course) by providing these Service Bulletins. In my humble opinion, I would follow each of them, when restoring a motor in question.

    In ’57 the RD went to 35hp, which incorporated these improvements.

    Then the 1958 RDS 20 is in my opinion the best of all worlds mechanically speaking, as far as a 35hp OMC is concerned. I have thought a lot about the factory approved 1956 modification for replacing the 2 tube original part for the replacement solid plate and hole drilling changeover procedure. With the original 2 tube design the start up exhaust pressure was relieved thru the 2 tubes also I would think some engine cooling water would also go down the tubes with the exhaust gases. With the solid plate I would assume no water would come out the (Poop Hole) Just exhaust gases from the drilled holes at idle or start up.I would also speculate that when you ran the engine at a faster speed the increased exhaust pressure would just exit the under water exhaust with either set up. I would think at high speeds the original 2 tube design still had some water coming down the tubes with some exhaust. The changeover design would force all the engine cooling water out the under water exhaust. So the engineers reason might have been to make the engine quieter at speed or they did not want the water going down the 2 tubes or maybe the chance of water going up the tubes ? Unless one of the old time OMC engineers chimes in we may never know. But again we all know the engines run good either way modified or unmodified. I would think it is more important to be aware of this modification and identify it if putting an engine together from replacement parts so modified parts are not mixed with unmodified parts on the same engine. Regards Bill,

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