RDE-18 powerhead to exhaust plate/gaskets

[immulmen]

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]

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.

[immulmen]

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]

Oh, I see. That’s a good idea.

[billy-j]

Re: RDE-18 powerhead to exhaust plate/gaskets

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Postby t2stroke » 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,

[chris-p]

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]

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

[garry-in-michigan]

Somebody told me an Elephant was a Race Horse designed by a committee. . . 😆

[olcah]

More than one way to skin a cat. 😆

[billy-j]

Re: RDE-18 powerhead to exhaust plate/gaskets

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Postby Chris_P » 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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