Johnson HD-20 and HD-25 Parts Compatibility

[jsonova99]

Hi everybody, I’m new to the forum, I just bought an old HD-20 to restore. It’s been sitting for a long time. My first of many questions to come is how compatible are parts between HD-20s and HD-25s? For example, I wanted to replace the lower unit on my HD-20 because of a broken fin, the HD-25 lower units seem to pretty common, I’m assuming they are interchangeable, but would like to be sure before I commit to anything. Another part would be the carburetor and also the engine block and internals. Again, I’m assuming they are interchangeable but would like to be sure. I’m not sure about the ignition system.

Thanks in advance, I’ve brought older outboards back to life before, but mostly 70s and 80s vintage, this is the first one that I’m hoping to actually restore and the first of this vintage that I’ve worked on. It’s an ambitious project, this was left for dead along time ago, but it turns over freely and seems to have good compression which is why I gambled on it (got it very cheap).

[cajuncook1]

Welcome to the forum!

I believe the HD-20 had the starter recoil and the HS-20 had the standard rope sheave. As far a interchangeability of parts, I believe there should be a fair amount of common parts and lower unit should be one of them. I am not 100% certain of this, but we do have a number members that have a lot experience with the Johnson motors.

[1946zephyr]

The only difference I can see, is color. HD-20 is a silver pre-war motor and the HD-25 is a green post-war motor. I actually have a restored HD-20 myself.

[jsonova99]

Regarding color, somebody sprayed everything but the tank a dark green, peeling pretty badly so it should be easy to remove. Were the HD-20s painted silver or was it just polished aluminum?

Glad to hear the parts are pretty interchangeable because HD-25 stuff seems to be pretty easy to find. I’m hoping to start tearing it down next week. Will take lots of pics as I go.

[Steve A W]

Cajon
jsonova was asking about the HD20 vs the HD25.
I’d like to know the same. I’ve got an HD20 that came from a relative and I’d like
to have it running but it only has 25lbs compression and a dead coil.
I’ve picked up three HD25 parts motors, unknown condition, to check out.
Hopefully I can get the HD20 running and maybe an HD25 going out of the pile.

Steve A W

Member of the MOB chapter.
I live in Northwest Indiana

[kevinf]

I picked up some HD-20s a couple of years ago and I’ve built a couple of running ones from the parts. There’s still at least one in the left overs if I get bored. The pre-war HD was painted dull silver not polished. It looks original. If you run into parts that aren’t interchangeable between the 20 and 25 it shouldn’t be a problem getting parts there’s lots and lots around.

Kev

[1946zephyr]

They built the HD-20 1941 and 1942. The HD-25 was like 1946 – 1951. Did you know that a 1966 JW tank will bolt right on to the HD-20?

[cajuncook1]

quote Steve A W:

Cajon
jsonova was asking about the HD20 vs the HD25.
I’d like to know the same. I’ve got an HD20 that came from a relative and I’d like
to have it running but it only has 25lbs compression and a dead coil.
I’ve picked up three HD25 parts motors, unknown condition, to check out.
Hopefully I can get the HD20 running and maybe an HD25 going out of the pile.

Steve A W

Steve AW and jsonova, I am not Johnson guru. There are plenty of distinguished members that hold that title.

I just did a little digging and reading and found that HS and HD 20-25 shared a lot of common parts. The diagram that I copied and pasted together above showed that the power heads were all rated for 2.5hp, cylinders bores and strokes were the same. (Pistons, conrods, and crankshafts, and power heads).

It would seem that they also share common magneto parts as well. Here is a diagram that displays the make up of the HD and HS magneto.

It would seem that they also share common lower unit parts as well. Here is a diagram that displays the make up of the HD and HS lower units.

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