1956 Johnson RD-18 30hp Pistons

Home Forum Ask A Member 1956 Johnson RD-18 30hp Pistons

Currently, there are 0 users and 1 guest visiting this topic.
Viewing 2 posts - 1 through 2 (of 2 total)

  • OB1

    Replies: 4
    Topics: 4

    Many years ago (like 20) I had bought various parts required for rebuilding my old 1956 30hp Johnson. This included a full gasket set, rings and a complete (alleged) 30hp powerhead. I knew one of the pistons was scored on my motor, hence the purchase of the extra powerhead. Somewhere between there and now I completely got side-tracked while indulging in other pursuits. A recent Craigslist old outboard impulse purchase (’59 18hp) and years later I’m bitten by the bug again and ready to dive in to finish what I started.

    My first step was to get the pistons out of the core powerhead in preparation for the transplant. After pulling the pistons from the core motor I noticed that the wrist bearings had plain bearings – not rollers. Given this, it appears I really have a 25hp core.

    So, the question is… Will they work? According to service information both have a 2 3/4″ bore & 2 7/8″ stroke. However, in the parts manuals they have different numbers 376794 for the 30hp and 375781 for a 25hp. Do any old outboard gurus out there know if these can interchange without major carnage or loss of power?

    • This topic was modified 1 week, 3 days ago by OB1.


    US Member - 2 Years
    Replies: 51
    Topics: 37

    I believe the pistons were designed differently for the 25 and 30hp powerheads, hence the two different part numbers. As far as I know, the location of the wrist pin was changed slightly to play a role in the port timing to give the 30hp those 5 more horses than a 25. But I have no proof of that, only what I’ve heard.

    In theory, the 25 pistons should work, and not hit anything throughout the piston stroke. The unknown is what horsepower loss would occur using the 25hp pistons in a 30hp block. I’m pretty sure the block part numbers are different for 25 and 30hps but I would think that the worst that could happen using the 25 pistons would be that you make your 30 into a 25hp.

    Measure and compare the dimensions of the porting and wrist pin location between the two powerheads you have, that should give you the answers you’re looking for

    1 user thanked author for this post.
Viewing 2 posts - 1 through 2 (of 2 total)
  • You must be logged in to reply to this topic.