18 -> 20hp…a simple conversion?

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  • December 1, 2015 at 4:51 am #3089
    johnyrude200
    Participant

      Well today I spent time replacing a broken driveshaft on what I assumed was a ’73 25hp motor, only to realize after completely reassembling it that the motor was really a 20hp and I had inadvertently installed the wrong crank and head (let’s just say, the compression numbers were awfully high…as in 160PSI…which was a pretty obvious red flag).

      2nd crank swap out, I was searching through my parts bins for an 18hp crank and head, and came back to this topic. Taking a few minutes to review parts diagrams, it appears the 18 & 20 are the same motor, with the only difference being the leaf plates. The carb is the same (including high speed orifice), the head is the same, rods, pistons, and crank are all the same.

      20hp motors seem to be the less common out of the 18-20-25 family, have others noticed this, and are there any other things differentiating these motors…other than the cowel?

      December 1, 2015 at 11:33 am #27882
      wiscoboater
      Participant

        I have a 75 20hp as well. According to Frank the difference between the 20 and the 25 was a bit more compression and some exhaust tuning in the leg. Mine is an odd ball motor with a triple fin gear case and an electrical plug on the port side. The serial number does not come up on marineengin, and the tag shows it to be Canadian built.

        December 1, 2015 at 1:00 pm #27885
        frankr
        Participant

          US MEMBER PAY BY CHECK

          The differences in 18-20-25 (22 cubic inch) motors will confuse the best of us. They all are based on the original 18hp, then went crazy from there with different combinations of pistons, cylinder heads, carbs, etc. For years, Evinrude dealers sold the 18hp, while the Johnson dealer down the street had 20hp motors.Your best defense is simply go by the parts books. Trouble starts when you try mixing parts.

          December 17, 2015 at 3:37 pm #28717
          johnyrude200
          Participant

            I’ve been meaning to follow up on this post, here are a couple of pics I took showing the differences in the cylinder heads if folks are trying to identify the 18/20 heads vs the 25hp heads. Note that if you accidentally install a 25 head on a 18/20 motor, I had compression readings between 180-200 PSI (normally about 115-130 with the right head). Can’t imagine it’d take long to blow up a motor that way.

            Forget about the parts #’s, just look at the inside of the cylinder heads. Seems that later parts actually had the number 20 stamped near the inside of the plug hole (cleaned off the one on the left to reveal the stamped number better for the picture…these were off of dead motors). I tried looking up part numbers before realizing it’s easier to just look at the inside cupping. Pretty easy from there on.


            December 18, 2015 at 11:30 am #28752
            76-j
            Participant

              Access to a surface plate and height gauge to measure the distance from intake and exhaust ports to the c/l , or , where the cylinder block and crankcase joins, will yield significant "differences" between many these motors, over ~ 20 year period.

              just sayin’ !

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