303977 cylinder head evinrude 25hp

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  • October 7, 2020 at 2:25 pm #218049
    fleetwin
    Participant

      US Member

      Yes, I have never seen that crazy looking casting design either…Doesn’t mean it is different on the other side where it counts though…..

      October 7, 2020 at 2:31 pm #218050
      ede
      Participant

        US MEMBER PAY BY CHECK

        The casting number can be the same. Machined differently it will get its own identity, a p/n. Unless we had the engineering specifications, we would not know the differences. I would not make the change you are suggesting. Find the right part to do the right job, Ed

        October 7, 2020 at 2:59 pm #218057
        The red boat
        Participant

          I agree – if I could find a duplicate to that cylinder head I’d rather replace it in-kind.
          That’s been my challenge- trying to properly identify that cylinder head and replace it. But as I’ve discovered it seems to be a bit of a unicorn .

          October 7, 2020 at 3:30 pm #218061
          frankr
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            US MEMBER PAY BY CHECK

            That is really weird that we have here to heads with the same casting numbers but obviously different castings (fins). I’m going to stick my neck out and guess they made a revision to the mold. All those fins do look problematic as far as successful casting goes.

            October 8, 2020 at 2:59 pm #218152
            trex014
            Participant

              Okay I think I’ve figured out the confusion on the head for this motor. From the picture you have a 55 Johnson with an Evinrude powerhead. The block you have was in use for Johnson/ Evinrude between 51-56, rd11 though rd18, which started as a 25hp block and I am unsure what changes were made but the same block was used on the 56 30hp.

              Since the Evinrude block you have has a Welsh Plug, that powerhead came from a 55 Evinrude 25, the first year 25hps had that Welsh plug with the serial number stamped in it.

              Now, I have four 54 Johnson RDE-16’s which has a very similar head as the one you stripped.

              I have a 53 Johnson rd-15 which has another head that is pictured below

              I have four of the 55 Johnson RDE-17’s which has the same head as the online picture you found and the one on Outboard Nut’s 55 25hp.

              I have 2 54 Evinrude’s with the same head you stripped.

              Thus between 53-56 there were three different heads heads in use, why? Maybe due to casting difficulty or better design but notably look at the castings where the cowling attaches, they changed to accept the new flip up hoods in 55 though still had the casting bosses to be tapped for a pre 55 cover.

              According to my books:
              1953 rd-15 head part #: 375779
              1954 rd-16 head part #: 376336
              1955 rd-17 head part #: 376336
              1956 rd-18 head part #: 277419

              I have the head you’re looking for but it has a stripped hole as well. Send me a PM and I’ll keep digging, I know I have the one you’re looking for somewhere. Hopefully this helped.

              P.S. if you can’t tell I like the 25hp rd’s 🙂

              • This reply was modified 4 years, 9 months ago by trex014.
              October 8, 2020 at 6:07 pm #218165
              The red boat
              Participant

                wow!!
                great information!

                I have a NOS 55 RD-17 cylinder head (like yours) in the box I can use for this Evinrude. It is a 25920 motor which is a 55 25hp evinrude.
                will it work or is there anything it will mess up?

                That Evinrude block is indeed on my 55 Johnson. I got it cheap and it has great compression so I’m using it until I get time to tear down and rebuild the original Johnson block that came with it.
                Good eye!

                October 8, 2020 at 11:23 pm #218190
                trex014
                Participant

                  If I was in your situation I would bolt up the NOS head you have and run it. In my mind, a 55 cylinder head and a 55 cylinder block should give you the results you want.

                  When you put it together with the new cylinder head, spin it over slowly by hand with no plugs in and feel for any resistance or things hitting where they shouldn’t. Then put one plug in and repeat. Then take that plug out, put the other one in and repeat. That will give you the peace of mind that nothing is hitting.

                  All of those heads mechanically should interchange, the only difference would be what cowling mounts they have

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