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Thread: 3.7 to 5.7

  1. #1

    Default 3.7 to 5.7

    I've been having issues with the 3.7, starting and now the mag failed and not charging. I have a great running 5.7 that I'm wanting to replace the 3.7 with. Does anyone know the difference in weight? I currently have both engines on the ground and have test ran the 5.7. I have measured and have the space need to build the front mounts. Both engines are MerCruiser Alpha 1 gen 1 so thinking I shouldn't have issues. Just curious if the boat will be a little heaver with the 5.7? I know though that the 3.7 (488) is a very heavy itself.

  2. #2
    Join Date
    Dec 2006
    Greenfield Me. USA

    Default Re: 3.7 to 5.7

    Ayuh,...... A fully dressed 5.7l is atad over 500 lbs,.....

    You'll need a matchin' drive too,..... the gear ratios are very different,.....
    Any Grease is Better,... Than No grease at All....

  3. #3

    Default Re: 3.7 to 5.7

    Thanks, looks like 5.7 has a 1.47 and the 3.7 has a 1.62 I was told the 5.7's drive was new but that could be just new to the boat.. Looks new and u-joints look new. I've rebuilt the gimbal, bearing, new senders, hoses and bellows. I'll put a new pump in it and use the 1.47. Cleaned the engine bay today and will to a test fit to see where I need to install the front mounts I plan on using oak blocks glassed in.

  4. #4
    Join Date
    May 2010
    DC Burbs/Maryland Eastern Shore

    Default Re: 3.7 to 5.7

    The wood used in boat construction for stringers etc is not structural, it's merely a cheap form to lay fiberglass roving over to form a beam or stiffener. Often you see dimensional lumber like 2x10 used for this, as it is easy to work and doesn't require high levels of skill to cut it to shape. Cheap or poorly designed boats you will see these wood parts barely covered, not covered at all and just tabbed to the hull, or laid up so poorly that they rot and get waterlogged.
    When you build the new engine mounts you might want to consider eliminating the wood to avoid possible problems. That is, you're going to encapsulate the wood, then turn around and poke 4 holes in there to put your lag bolts in. I would recommend a non wood core like HMMW plastic (King Starboard) Coosa, or even a poured thickened epoxy (use one for this purpose as layup epoxy will cook at a high temperature and ruin everything) into a mold made up from cardboard taped in place. You cover this with a few layers of heavy roving and you'll have good worry-free engine beds.
    If you want to go the extra mile, set the engine in place, align it, then mark the holes for the mounts. Move the engine out again so that you can drill holes. You'll drill the holes 1/4 inch oversize, then using thickened epoxy, glue in 316 stainless steel threaded rod, as deep as it'll go.
    Practicing the ancient art of ren-ching

  5. #5

    Default Re: 3.7 to 5.7

    Thanks for the info, not allot of information even on YouTube building engine mounts.

    I'm going to set the engine in today to get the location and rough height for the mounts. I will be using a 1 inch pipe so that I can see what direction I need to go.
    I'll use the alignment tool only at final install.

  6. #6

    Default Re: 3.7 to 5.7

    Looks like I will build flush with the top of the side boxes. once glassed it will put the mounts in the middle of the adjustment.

    I noticed the cooling hose from the gimbal is larger on the 5.7, should I install and reducer, not sure how the fitting comes off the old gimbal?

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