Johnson 30 HP RD18 gearcase

[olcah]

I am looking at using a gearcase from a 1954 Evinrude 25HP on my 1956 Johnson 30 HP RD18. I notice that at the top of the Evinrude gearcase just below the shaft seal is a bushing that apparently steadies the driveshaft. The bushing is fixed in the 1954 gearcase, no movement. It is part of the gearcase and it has it has no separate part number in the parts list. There is a similar bushing on top of the gearcase for the 1956 Johnson 30 HP but that bushing is not fixed in the case. It is free to rotate and it can move axially below the seal. It has a separate part number 304084. So design changed.

Can I still use the 1954 25 HP gearcase on my 1956 30HP motor?
Why this design change? Was factory trying to equalize wear on the bushing by letting it move? Is wear on that bushing a problem?
Thank you.

[frankr]

The free floating bushing was a design change. Since the bushing can rotate, the surface speeds / wear are divided between the ID and OD. HOWEVER, the ’54 gearcase won’t bolt up to the ’56.

[olcah]

Frank,
Thanks for explaining the change and maybe from an embarrassing moment…

[1946zephyr]

quote OLCAH:

I am looking at using a gearcase from a 1954 Evinrude 25HP on my 1956 Johnson 30 HP RD18. I notice that at the top of the Evinrude gearcase just below the shaft seal is a bushing that apparently steadies the driveshaft. The bushing is fixed in the 1954 gearcase, no movement. It is part of the gearcase and it has it has no separate part number in the parts list. There is a similar bushing on top of the gearcase for the 1956 Johnson 30 HP but that bushing is not fixed in the case. It is free to rotate and it can move axially below the seal. It has a separate part number 304084. So design changed.

Can I still use the 1954 25 HP gearcase on my 1956 30HP motor?
Why this design change? Was factory trying to equalize wear on the bushing by letting it move? Is wear on that bushing a problem?
Thank you.

Are you in need of a gearcase assembly?

[olcah]

Yes, I could use a complete assembly with the internals and drive shaft.

[1946zephyr]

Shoot me a pm

[Mumbles]

quote OLCAH:

It is free to rotate and it can move axially below the seal.

I’ve changed a few of these drive shaft bushings and they were all pressed in. If they could rotate freely, they would chew out the aluminum housing, wouldn’t they?

[olcah]

I believe that they were attached on RDs prior to 1955 and floating in 1955 and 1956. That is what I have seen on several motors. It is reflected in the parts lists too. In 1955-1956 the bushings are numbered and shown separate. They do not appear in earlier lists as they are part of the gearcase assembly.
Why did you change the bushings? Could you see wear?

[olcah]

The RD18s I have all have the moving bushing. They have not chewed the housing.

[Mumbles]

You’re right, those bushings are full floating. I was sure I had pressed them out before but it might have been later needle style bearings I did. There is a ’57 Big Twin here waiting for a water pump so I’ll check it when I drop the gearcase.

[Author]

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