How to repair motor mount pads?

Discussion in 'Materials' started by Sprint 18.5, Oct 4, 2021.

  1. Sprint 18.5
    Joined: Aug 2021
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    Sprint 18.5 Junior Member

    Had to hole saw a broken lag bolt out which held down the rear fixed motor mount and need to plug it. I made two different new treated dowels, ones tight and one sloppy. Any choice and advice on a glue or epoxy etc.
     

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  2. gonzo
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    gonzo Senior Member

    Epoxy is the best choice.
     
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  3. fallguy
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    fallguy Senior Member

    Do NOT glue in a dowel. That is a major no-no if any bolt goes back in. Because..,you will be going into end grain which is the weakest way to hold and it won't.

    To repair the hole, the best way is to get an empty caulk tube and mix up a batch of epoxy and fumed silica to peanut bitter thickness and inject it into the bottom od the hole and slowly back out while filling. Make sure to keep it cool. Any holes filled bigger than say 1/2" can get really hot and crack down the center. Large holes need to be filled a little at a time to avoid cracks, smoke, fire.

    If you are just filling; fine to use dowels.
     
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  4. Sprint 18.5
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    Sprint 18.5 Junior Member

    Just asking but I have a un opened $50 tube of 3m Panel-bond for bodywork applications. 3 packages of Epoxy is about same price.
     
  5. fallguy
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    fallguy Senior Member

    Maybe okay; not sure about the thickness.
     
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  6. Coastal Ogre
    Joined: Jun 2019
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    Coastal Ogre Junior Member

    • What size engine and how much does it weigh?
    • Where do you sail/motor... inland waterways or blue water/open oceans?

    On my catamaran I would make sure the repair is structural, if only to prevent the motor from moving in rough weather. I routinely see seas in excess of 5' which probably means my Yanmars (in the transoms) experience zero-g when falling off waves. Since I do not live in a static environment I would not use any type of thickened epoxy or a plug to secure a ~300 lb motor.

    I would recommend the Gougeon Brother's free document called: Fiberglass Boat Repair & Maintenance (2014), Appendix D; repairing machined holes in fiberglass laminate

    Good luck!
     
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  7. Sprint 18.5
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    Sprint 18.5 Junior Member

    Ok I’ll pick up some Epoxy at Autozone and get mixing. So I guessing my procedure is to mix 3-4 different batches and add each batch before getting cold/hard so they won’t bond in different layers but to adhere as one.
    Then with motor mounts attached to block and free, align the motor vert/horizontal with the tool, then mark and make pilot hole bolting down, re checking for resistance and maybe fine tune again.
     
  8. Sprint 18.5
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    Sprint 18.5 Junior Member

    Not sure on engine weight but it’s a GM 4.3 V-6 Wellcraft deep v 19” ft inboard/outboard operating in a smooth river inland occasionally crossings other boat wakes.
     
    Last edited: Oct 5, 2021
  9. fallguy
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    fallguy Senior Member

    Just so you know, I prefer to prefill the hole about halfway in old wood with plain epoxy. Then I use something like an old rag to push out the epoxy mostly before adding my filler or just add pb epoxy to the bottom and let the epoxy come up. It is messy, but works to deal with any anomalies in the hole. It does add to the mess factor, but I always feel like the hole repair is a bit better if it is gooey. I have no empirical data to suggest I am correct, but a few overbored holes I filled and redrilled as evidence that hole filling blind holes is pretty easy to do poorly, and in your case, it'd be easy to leave voids and fill the top part of the hole. Pure epoxy prefilled about halfway prevents that. Especially helpful in repairing okd threads because the epoxy travels into all voids.

    People may come on here and criticize the approach, but I have plenty of experience on this business.
     
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  10. hoytedow
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    hoytedow Carbon Based Life Form

    You can also cut a plug cutting across the grain which you can epoxy into position so that the new piece has the grain of the plug aligned with the original wood. It will be as strong as the original and require much less epoxy, thereby producing less heat and lessening the potential for cracking.
     
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  11. fallguy
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    fallguy Senior Member

    Yes, certainly, if you have the capability, but sometimes it is really hard to cut a plug this way unless you buy the plug cutter.
     
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  12. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    He still has the hole saw. That will produce a suitable plug. The centering drill bit will make a hole in the wrong place, if used, but presumably Sprint took it out to drill out the broken bolt.
    Put it in a drill press, cut the plug. ¼ fill the hole with epoxy and push in the plug. All the exposed wood will absorb a lot of epoxy, so it will need topping up in half an hour.
     
  13. fallguy
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    fallguy Senior Member

    I forgot he had the hole saw. A great idea.
     
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  14. Sprint 18.5
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    Sprint 18.5 Junior Member

    I might scratch the idea of the plug I cut with the grain instead of against but order some extra slow curing hardener with resin and filler from Gougeon Brothers which they recommend two pours. It was a total of $103. delivered but still haven’t decided which route to take.
     

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  15. fallguy
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    fallguy Senior Member

    D0920F93-6AA8-4B61-9A85-BEE2DB9ED16D.jpeg Dude! That is an end grain plug!!! What u did is same as dowel.

    nooooo...won't hold anything

    cross the grains only. More grains are better than less if you want to be a pro.

    these grains are going the wrong way
     
    Last edited: Oct 6, 2021
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