1956 Johnson RD-18 vs RD-18C 30hp

[fry cook]

What is the difference between the RD-18 and the RD-18C? I need an air silencer p/n 277584 for my RD-18, however, the only one I can find is for an RD-18C which has a different part number; are the interchangeable?

 

[82rude]

try looking through this pdf.Lists all the common parts though model numbers for Johnson.http://www.joeoutboard.com/resources...tsRef-1964.pdf don't let the 1964 date fool you this lists common parts etc for many years.

 

[fry cook]

Thanks! This will be useful going forward, however, the air silencer is not referenced. I can find plenty of them for RD-18c's but not for an RD-18. Do you know what the C means? My parts catalog has a special note stating that the RD-18 calls for an air silencer that has a unique part number I can't seem to locate. I did buy one with a different P/N and I had to return it... It was too large to fit into the cavity.

 

[F_R]

The C means there is a mid-year engineering change (or changes) somewhere in the motor. Probably the silencer. I'm looking through my stuff, but haven't found it yet.

 

[F_R]

Ah, looking at the parts list for her Evinrude brother, I see a major mid-year change, and a few minor ones. The major one was that they returned to the carbon seal on the crankshaft, after the catastrophic change in the previous year. Minor changes were starter handle, idle stop screw, fuel filter, and yes the silencer. But I see no reason for why the silencers shouldn't interchange.

 

[fry cook]

Good information! Mine being the RD-18 do you think it has the inferior seal on the crankshaft? What model is the her Evinrude brother? I'll do some cross referencing.

Thanks.

 

[HighTrim]

Boy, I cannot think of seeing the silencer any different. I cannot see why it would not work? Now im curious as to what changes were made to the silencer, have not encountered that one yet!

Im going downstairs to dig through parts ill see what I can find out.

 

[F_R]

Good information! Mine being the RD-18 do you think it has the inferior seal on the crankshaft? What model is the her Evinrude brother? I'll do some cross referencing.

Thanks.

The Evinrude Big Twin identifies the change by serial number instead of by model number.

If you have or can find a Johnson parts book, it should show which model number has the inferior seal. That would be a part $303804 seal. The better one is a carbon seal with several washers, spring, snap ring, etc. You can put the carbon seal in place of the 303804, but you have to split the crankcase and replace the bearing to do so. If the 303804 has not failed and destroyed the powerhead (water intrusion), you might as well just replace the seal with a new one and run it.

 

[HighTrim]

Ok so I had 2 RD18s to restore for others. I took some pictures of the silencer on the RD18. I also took pictures to show what Frank was referring to, about the bottom crank seal. Both motors I tore down today for restoration had blown out seals, and rusty drive shaft splines. As he stated, they were junk, and need to be replaced, to ensure the motor does not self destruct. In fact, one of the motors powerhead has a blown lower cylinder, due to this blown out seal.

 

[HighTrim]

Sorry, this forum does not seem to like my pictures, ill put them on photobucket and post them.

 

[HighTrim]

Here is the bottom crank seal, missing chunks. Top of drive shaft showing rusty water on it.

 

[HighTrim]

Here is an RD18 silencer. Please post pics of yours, im curious to see what they changed.

 

[HighTrim]

Here is what your bottom piston and cylinder will look like, when that seal is gone. Not good.

 

[F_R]

Ok, I've done some digging, and maybe I've come up with something. At least some assumptions.

1955 and early 1956 used a 203575 (front) silencer half, and a 203576 (rear) silencer half. Both of those parts were to be replaced by a 277584 silencer assembly.

Late 1956, the part numbers were changed to 203916 (front) and 203917 (rear) halves. These two halves continued into 1958, at least on the Gale 25hp versions.

The Johnson service manual for 1956 makes a big deal of making sure the gasket between the silencer and carburetor is perfectly aligned, with a note that if it is not aligned, eddy currents develop, which affect the mixture between the two cylinders. Pictures also show that vertical divider in the silencer bore.

I have not found a picture of the early 203575/203576 halves, since they were to be replaced with a new assembly. But I have a strong suspicion that the change somehow involves the airflow. I also have a strong suspicion that the early one did not have that vertical divider that separates the left and right sides (??)

In other words, it seems the change was all about airflow.

BTW, 1955 had a funky rubber radiator hose elbow on the silencer intake. But I doubt that has anything to do with anything.

So, my conclusion for fry cook is that the later style silencer, whatever it is, will directly replace the early style. And even if you were to find an early one, you probably shouldn't use it for the same reason that the mfr changed the design.

Lastly, I must say that in all my years of service work back in the day, I've never experienced any of the so-called problems.

 

[HighTrim]

Very interesting thanks Frank. Learned something new today. ill see if I can find any without the divider.

 

[fry cook]

Wow, You guy's are the best! My parts book calls for the air silencer P/N 277584 for my RD-18, however, the only one's available are for an RD-18C's which I am going to order today. When I purchased the boat along with the Johnson motor, the air silencer was missing and I don't know exactly what it looked like. Also, my book shows that my Top seal is the dreaded 303804 and the bottom seal is 302538 will I be OK with this? High Trim, I really a appreciate the photos. I did a compression test and both cylinders were a tad over 100lbs. Again, Much thanks.

 

[HighTrim]

If you have the 302538 carbon seal on the bottom of the crank, that is good! They make it easy to change the o ring in them.

YOur compression is good. Compression test wont show a faulty crank seal however, you may know this but FYI> I like to change the crank shaft seals on motors I go through for peace of mind and better idle.