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Should I rebuild a Mercruiser 3.7L 4 cylinder 165 hp motor?

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  • Should I rebuild a Mercruiser 3.7L 4 cylinder 165 hp motor?

    Should I rebuild a Mercruiser 3.7L 4 cylinder 165 hp motor? I've heard good things and bad things, I want to know your thoughts on the matter. I want to know how much fun it will be, lots of torque, but then again maybe 165 hp won't push the 19 foot boat very fast. Thoughts...
    Galaxy Full Resto (Splashed 2017): https://forums.iboats.com/forum/boat...-and-pre-alpha

    Regal Sebring Resto:https://forums.iboats.com/forum/boat...-195-vbr-build

  • #2
    Rebuilding is not the problem. Depending on what needs to be done and the extent of the 'rebuild', parts can be an issue.

    When running, 3.7L engines were quite good. For their size, yes lots of torque, and quite good on fuel. But they have their problems.

    From the photo in your resto thread, that engine still has the stator-based alternator. That would need to be replaced with an external alternator. The other issues could be dealt with as you do the rebuild. Have you done a compression test yet? The results of that would determine if a rebuild is called for.

    Chris.....
    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    The world takes on a whole new perspective when viewed from 100’ below.
    1972 Bertram ‘Bahia Mar’ 20
    2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
    (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


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    • #3
      I would source a 4.3 and abandon the 3.7
      Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

      1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

      Past Boats
      1970 Wooster Hellion - Merc 9.8
      2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
      1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

      What’s behind you doesn’t matter.Enzo Ferrari

      Comment


      • #4
        Originally posted by achris View Post
        Rebuilding is not the problem. Depending on what needs to be done and the extent of the 'rebuild', parts can be an issue.

        When running, 3.7L engines were quite good. For their size, yes lots of torque, and quite good on fuel. But they have their problems.

        From the photo in your resto thread, that engine still has the stator-based alternator. That would need to be replaced with an external alternator. The other issues could be dealt with as you do the rebuild. Have you done a compression test yet? The results of that would determine if a rebuild is called for.

        Chris.....
        Engine has already been machined and ready for new parts. I have a new cam and lifters sitting in the garage. Just not completely sure if I want to rebuild it. Yea I am considering the alternator but finding a bracket is a challenge, and the cost would continue to climb. Timing components seem to be hard to source but what I have is still good. New chain would be great but not a deal breaker.

        I would want to replace a Mercruiser 120 with this. That's my main desire for this engine. But this engine has power steering and the destination boar doesn't. Wonder how hard it would be to transfer power steering from donor boat to current.
        Galaxy Full Resto (Splashed 2017): https://forums.iboats.com/forum/boat...-and-pre-alpha

        Regal Sebring Resto:https://forums.iboats.com/forum/boat...-195-vbr-build

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        • #5
          If you change it out go with the 4.3l. You have to redo things anyway and going to a 120 is going to be a dog. The drive ratio will be off for a 120 also.
          83 Sunrunner 16.5 120 merc Alpha 1 drive SOLD 8/1/13

          1996 Crownline 202BR
          5.7l EFI 0f791070
          Bravo I drive

          GMC Yukon tow rig

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          • #6
            Originally posted by thumpar View Post
            If you change it out go with the 4.3l. You have to redo things anyway and going to a 120 is going to be a dog. The drive ratio will be off for a 120 also.
            I have everything from the donor boat, transom assembly, lower unit, everything. I broke the lower swivel pin ear on the 120 transom assembly so I need to take that motor out and replace the assembly. So I was thinking why not put the bigger motor in as an upgrade. Already have everything, just needs rebuilt. 4.3 is not on my radar. How long does a cam last on these motors 3.7L?
            Galaxy Full Resto (Splashed 2017): https://forums.iboats.com/forum/boat...-and-pre-alpha

            Regal Sebring Resto:https://forums.iboats.com/forum/boat...-195-vbr-build

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            • #7
              As long as the cam was run-in properly when it was installed, and good oil is used, and changed regularly, there's no reason for the cam not to last a long time.

              Changing from a 120 to a 165 is a great upgrade. You could use either the 120 drive, or the 165 drive, as the ratios aren't that far apart (1.98:1 to 1.84:1). You may need a slightly larger (pitch) prop with the 1.98:1...

              Chris......
              xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
              The world takes on a whole new perspective when viewed from 100’ below.
              1972 Bertram ‘Bahia Mar’ 20
              2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
              (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


              Comment


              • #8
                Originally posted by devildogae View Post
                .... Yea I am considering the alternator but finding a bracket is a challenge, ....
                Most likely you'd need to fabricate the brackets yourself (it's not that difficult)...
                That said, there are a few companies around making them. stonyloam would be the man to ask.

                Chris..........
                xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                The world takes on a whole new perspective when viewed from 100’ below.
                1972 Bertram ‘Bahia Mar’ 20
                2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
                (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


                Comment


                • #9
                  if you are intent on the 3.7, get rid of the cast iron head, get an edelbrock aluminum head for the ford FE engines and drill the large hole in the end. this will help save the head gasket the next time the coolant temp creeps up a bit. I would also put a bore stabilizing spider in the deck of the block, and tig weld it in. however that needs to be done prior to boring/honing.

                  if the cam is new, have the nose of the cam flame-hardened for the water pump seal. as chris stated, the cam will last decades. however the snout where the water pump seal rides does wear quickly (or simply install the speedy sleeve from day one)
                  Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

                  1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

                  Past Boats
                  1970 Wooster Hellion - Merc 9.8
                  2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
                  1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

                  What’s behind you doesn’t matter.Enzo Ferrari

                  Comment


                  • #10
                    Many have persisted with these engines and its to be admired....but my feelings are that they demand too much to be worthwhile.
                    A friend went through 3 of these engines in a few years. He eventually replaced it with a 3.0. The 3.0 drops straight in using the same pattern for engine mounts etc. He didn’t even need to change the prop from memory.
                    The 4.3 will need the engine mounts changed and likely at least a prop change.

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                    • #11
                      I just rebuilt mine and so far I’m pretty happy with it.

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                      • #12
                        Rebuild it! It is a vast improvement over the 120. You have the motor and all the stuff, go for it. Mine is a 87 and runs great. Make sure you have new cam seals in it as part of the rebuild. Make sure your timing chain tensioner is in decent shape. If the head has not had hardened exhaust valve seats installed you might consider having that done, not too expensive. The problem with the stator/rotor alternator is not the alternator itself, but the water cooled regulator. If it works, leave it for now. Should push a 19’ boat into the low 40s. You should be able to just transplant the whole power steering system into the new boat. Good luck.
                        Terry
                        87, 4WINNS H 190
                        470/Alpha I

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                        • #13
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                          Be sure to have the camshaft bores checked that they are not out of round, if they are, have the bores line bored for bearings.
                          Kenner, Louisiana 1980 Donzi 18'. 2+3 6.2 MPI Alpha 1

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