Re: RDE-18 powerhead to exhaust plate/gaskets

[William Jatzek]

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,