RDE-18 powerhead to exhaust plate/gaskets

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  • olcah
    US Member - 1 Year
    Replies: 248
    Topics: 58
    #6268

    Assembling a 1956 Johnson 30 HP powerhead to its exhaust leg. I got the motor in Idaho. The bottom of the Idaho powerhead looks like this:

    There is one gasket between the powerhead and the leg:

    There was a loose (moved out of place) blanking plate when I initially removed the powerhead. It fits in the compartment directly below the plate in this photo.

    I opened up a badly rusted 1956 RDE-18 parts motor. There was a large aluminum plate with a gasket on either side.

    Instead of the blanking plate there was a plate with two open tubes. That plate with the tubes was also loose and out of place.

    The configuration of the parts motor is the same as the 1956 Johnson parts list. The configuration of the Idaho motor looks identical to the 1957 (?) Johnson parts list. (only one gasket no large aluminum plate and it shows the blanking plate.)

    I looked over the large aluminum plate. It is against the powerhead but it does not restrict the exhaust from the same path that it would take without the plate. The blanking plate does restrict the exhaust from mixing with the water discharge flow.

    Interestingly the parts motor has a lip seal at the bottom crankshaft bearing and is S/N 1332458. The Idaho motor has a carbon seal on the bottom crankshaft bearing and is S/N 1317840.

    Does anyone know what the large aluminum plate is supposed to do? I can’t see anything. I can put the Idaho motor together either way but the way I found it is simpler.

    Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 3797
    Topics: 367
    #52390

    It’s best to put the baffle plate back in when you reassemble the motor. One thing it does is redirect the fire and flames coming out of the exhaust ports. Without it, the exhaust housing would probably get really hot on the one side and all the nice Holiday Bronze paint would burn off. The little plate with the two tubes should be screwed back in place to. I’m pretty sure it directs the exhaust fumes at idle speed.

    You’re lucky. Your ’56 has the carbon seal and I got stuck with the crappy one!


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    olcah
    US Member - 1 Year
    Replies: 248
    Topics: 58
    #52393

    Mumbles, I was not too lucky – I could see a problem by looking up the exhaust. When I opened it up the bottom carbon seal had broken apart. The washer above the seal was worn thru and had scraped the top of the bearing. To replace the bearing I opened the crankcase and then found the bottom bearing journal of the crankshaft was rust pitted and had to replace the crankshaft. ๐Ÿ˜€


    olcah
    US Member - 1 Year
    Replies: 248
    Topics: 58
    #52395

    Thank you Mumbles and by golly the paint on the top of the exhaust leg on the port side does look fried!

    Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 3797
    Topics: 367
    #52397

    Well, it’s a lot better to find the problem now while the motor is apart instead of on a nice summer day while at the lake!

    Funny you mention the broken lower carbon seal as I took apart a gifted ’58 Big Twin powerhead today and it had the same problem. The bearing is OK and the crank polished up nice and is usable so I guess I dodged a bullet and got lucky!


    billy-j

    Replies: 80
    Topics: 7
    #52403

    I have a 1956 Johnson 30 HP. It’s my favorite big twin. I went over it about seven years ago with the help and direction of a man who specialized in mid 1950’s OMC. He told me to remove and not use the baffle plate just install the power head back on the leg with one gasket. He said he had a OMC factory bulletin that said to remove the baffle plate. They did not use it on the 1955 big twin or the 1957 it was probably something they experimented with then decided to eliminate it. I did what he recommended and discarded the baffle plate. I have used the engine all those years and never had a problem with it. It never overheats anywhere on lower leg.There are video’s of my engine and boat on You tube if you want to look ( 1956 Lyman and 1956 Johnson ) Cleveland Ohio. Also he told me of another factory bulletin modification for the 1955 Big Twin 25 HP. it was in regard to the engine getting enough air to the carburetor if you look at your front bottom pan casting where your electric start wires come into the engine you will see a 2 inch hole right in the middle. The 1955 engine only had a 1/4 inch hole there with the new design cowl of the 1955 and the rubber gasket the engine could not get enough air. The bulletin said to drill out the 1/4 inch hole to 2 inch to let the engine breath. I also have this engine and it was a dog on top end compared to the 30HP.It had the small hole I never would have known why it was slower if he would not of told me about it. You live and learn. Regards Bill,


    adam1961
    Canada Member - 2 Years
    Replies: 334
    Topics: 72
    #52426

    It seems to me that the additional baffle was an attempt to keep water away from the lip seal. I believe that OMC found that it was better to abondon the baffle and switch back to the carbon seal. Somewhere I have read a series of bulletins to that effect.


    olcah
    US Member - 1 Year
    Replies: 248
    Topics: 58
    #52427

    Thank you Adam and Billy, so interesting to see why things changed. The large plate might deflect heat away from the (tight) driveshaft compartment. I wonder if the bulletins said anything about using that small plate, either the blank one or the one with the small tubes. The 1955 parts list uses the plate with the tubes. The 1957 parts list uses the blank plate? Would that prevent water from coming out the exhaust tube vent? Maybe Frank Robb or Gary have more info.

    RICHARD A. WHITE
    RICHARD A. WHITE
    US Member - 1 Year
    Replies: 1464
    Topics: 113
    #52429

    You mean open this hole up to 2 inches???
    Anyone got a copy of that service bulletin?

    http://www.richardsoutboardtools.com

    Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 3797
    Topics: 367
    #52436

    This makes sense about the baffle plate. After all, 1956 was a year of change for OMMC. Even their name got changed.

    Here’s a plate I found on Fleabay. Someone’s already modified it to the ’57 specs!


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    olcah
    US Member - 1 Year
    Replies: 248
    Topics: 58
    #52439

    Richard here is a photo of my RDE-18 base plate. The hole is indeed 2 inches dia.

    RICHARD A. WHITE
    RICHARD A. WHITE
    US Member - 1 Year
    Replies: 1464
    Topics: 113
    #52441

    Mine is from an RDE_17, and clearly is not that size, so I am thinking I should cut that out…

    http://www.richardsoutboardtools.com


    olcah
    US Member - 1 Year
    Replies: 248
    Topics: 58
    #52442

    Hoping that Garry in Tampa or FrankR will set us straight.

    For RDE-18:
    Baffle plate in or out?
    Small plate blank or with two tubes?

    For RDE-17:
    Open bottom pan hole to 2 inches?

    Thank you Billy J for bringing this up!


    frankr
    US Member - 1 Year
    Replies: 3850
    Topics: 43
    #52447

    Sorry, I just don’t remember the details of the plate. I’d say toss it. It really isn’t doing any good in there. Yes, open the pan to 2 inches.


    billy-j

    Replies: 80
    Topics: 7
    #52450

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    Richard A. White
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    Re: RDE-18 powerhead to exhaust plate/gaskets

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    Unread postby Richard A. White ยป Sun Feb 05, 2017 9:45 am
    You mean open this hole up to 2 inches???
    Anyone got a copy of tat service bulletin?

    55 Bigtwin.jpg
    55 Bigtwin.jpg (314.56 KiB) Viewed 16 times

    Richard A. White
    AOMCI Member
    MOB Chapter President
    Yes I mean that hole you marked in your picture. I would just take a 2” hole saw and use the existing 1/4” hole as the guide and make the hole 2”. Nice green paint job ! My father bought my 1955 Johnson 25 HP. brand new and although I thought it was a good running engine it never had close to the top end the 1956 30 HP. had. I thought that was just the way it was until one of my relatives mentioned to me that the engine used to be pretty fast but it seemed that over time it got slower but still ran good. What I think happened was the new design cowl when new did not seat as tight on the rubber seal as it did when everything sat together in place for a long period of time and then the engine was starved air to the carburetor when it ran wide open. Then it just ran as fast as the air it got would let it never reaching it’s full speed potential. You can see the 2” hole in the picture of the 1956 pan author posted. Bill,

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