Rusted Motor Mount Lag Bolts

[MikeD]

Hi All,

While checking on things in the bilge last weekend, I noticed the engine seemed to be vibrating excessively. When I checked the lag bolt securing the motor mount to the engine bed, the rusty nub just came right out. This weekend, I tried to replace them with the same sized stainless bolts (just to get me through the season), but they would just turn in place and not bite into the previous hole.

Here are my temporary fix ideas so far:

1. Try longer/bigger bolts. I'm a little nervous about the amount of material down there before I get to the hull.
2. Shims. (If that's the correct terminology.) Could I place toothpick-like bits of wood into the holes, then screw in the new lag bolts - give them something to grip?
3. Epoxy. Drizzle or inject epoxy into the holes and either redrill or simply sink the bolts in before the epoxy sets.

I'm real open to suggestions. I was hoping for a couple of seasons before lavishing attention on the Yanmar, but it looks as though it's got other ideas...

Anyway, here's a picture with the lag bolt removed.



I have a similar issue with the bolt in the same location on the port side as well.
Pictures of everything here.

Thanks as always!

[Figment]

The bolts are half the problem, the beds are the other half. (What exactly is going on with that bolt on the left?) I wouldn't be surprised to learn of significant rot in the beds. If so, then toothpick shimming won't last more than a few hours of running-time, I'm afraid.

I'm a huge fan of potting fasteners in epoxy, but this is one application in which I'd want to be ABSOLUTELY sure that I could either get a torch or a grinder onto the bolthead just in case the wax doesn't release. Of course, for it to be a long-term fix you'd need to excavate the rotten area of the bed first.

Not a great thing to discover a week before labor day, you have my sympathy.

[MikeD]

I'm a huge fan of potting fasteners in epoxy, but this is one application in which I'd want to be ABSOLUTELY sure that I could either get a torch or a grinder onto the bolthead just in case the wax doesn't release.

Access to these bolts is pretty good. So, how does one "pot fasteners in epoxy"? Is this wax readily available? Car wax?

[Tim]

Wow...those are rusty, nasty screws. There has been a lot of moisture in there for a long time.

I agree that there's an underlying concern here with the condition of the wooden beds themselves. I would look at anything you do as a generally temporary measure, with a known need for more detailed inspection and repair later.

The hardest part of your repair at this point is proper access to the holes with the engine in place. It will make the job more difficult, but still possible.

A good repair can be made by bonding the fasteners in epoxy.

Bonding fasteners and hardware

Installing screws and other threaded fasteners with WEST SYSTEM epoxy dramatically improves load carrying capacity by spreading the fastener's load into a greater area of the substrate. There are several methods or levels of hardware bonding depending on the loads on the hardware.

Basic fastener bonding

For improved pullout strength and waterproof connections, the easiest method is to simply wet out stripped fastener holes and new pilot holes before installing the screws. Epoxy penetrates the fiber around the hole, effectively increasing the fastener diameter. Epoxy also provides a stronger interface with the fastener threads than wood fiber and keeps out water.



1. Wet out a standard-size pilot hole. Work the mixture well into the hole with a pipe cleaner or syringe (Figure 15). Thicken a second coat of epoxy as necessary for stripped or oversized holes.
2. Insert the fastener in the hole and allow the epoxy to cure.

Advanced fastener bonding

For greater strength and stability, drill oversized holes to increase the exposed substrate area and the amount of epoxy around the fastener. If the fastener/hardware can be clamped by other means, the oversized hole can be extended to the end of the fastener.



1. Drill oversized holes 2/3-3/4 the depth of the fastener. The diameter may be up to twice the fastener diameter (Figure 16a).
2. Drill a normal sized pilot hole at the bottom of the oversized hole to the full length of the fastener. The normal sized pilot hole serves to hold or clamp the hardware in position until the epoxy cures.
3. Wet out the holes and the fastener with epoxy. Allow the epoxy to thoroughly soak into the exposed end grain of the wood.
4. Fill the hole with thickened epoxy/adhesive filler. Use 404 High-density (preferred) or 406 Colloidal Silica.
5. Install the fasteners with just enough force to hold the hardware in place. Allow the epoxy to cure thoroughly before applying load to the hardware (Figure 16b).

More information is available in West System publication 002-550 Fiberglass Boat Repair & Maintenance, which you surely have in your library. (What do you mean you don't have this?)

Or, for really detailed information, check out The Gougeon Brothers on Boat Construction: a great reference for all things epoxy, regardless of what brand of epoxy you like. Chapter 14 covers fastener bonding in extensive detail.

If you want to be able to remove the fastener, you can coat it in ordinary paste wax first. The epoxy won't stick to the wax, enabling you to remove the fastener later. Figment's point was that the wax doesn't always guarantee easy removal, so be sure you can get in there with some other means if you have to. (There's always a way...)

[MikeD]

Thanks Tim. I tried my version of the "basic fastener bonding" above. FWIW, here's what I did:

I poked and prodded and gouged around in the holes with various screw drivers. Then I inject acetone into the holes, dried out with a paper towel, and then puttered around while I waited for it to evaporate. Then, I mixed a little batch of epoxy and injected some into each hole. Next, I put a couple of slivers of a mixing stick into each hole to give my new lag screws some bite, coated the threads with Turtle wax, then screwed them in. Then, I injected some more epoxy into each hole just for good measure.

I'll probably wait until Tuesday, then go down after work and see if they set well enough to hold the engine.

Note to self: Yogurt containers do NOT like acetone.

[Figment]

If the bolts don't release, a bit of heat on the bolt will melt a thin layer of the epoxy on the surface of the threads, and they should be fairly easy to break free from there.

(I've had spotty results with turtle wax. I've found that pure carnuba wax is more reliable, beeswax a close second)

[MikeD]

If the bolts don't release, a bit of heat on the bolt will melt a thin layer of the epoxy on the surface of the threads, and they should be fairly easy to break free from there.

(I've had spotty results with turtle wax. I've found that pure carnuba wax is more reliable, beeswax a close second)

Thanks Mike, and duly noted for future "potting of fasteners"...

[Quetzalsailor]

Been there, myself, on other applications. The engine beds in our '72 Morgan 27 were close to sodden mush within the fiberglass wrapping so I replaced the whole works.

I've done the thread creation/replacement in epoxy on several applications. Adjustment screws on walnut harpsichord jacks: I potted the oversize holes, drilled and tapped for the tiny screws. Several plastic or wooden objects with knurled or threaded attachments: cleaned out the mess, used epoxy filled with West's microfibers, cleaned the attachment with lacquer thinner or sprayed the threads with Teflon slip spray. Structural attachment by bolt or screw to fiberglass. Depending upon thickness and size of thread: oversize hole or root-size hole, filled epoxy, drilled and threaded, or Teflon slip spray. Screw attachment to wood, repair: Wet out the wood with epoxy, filled epoxy, match sticks, screws. Small screws can be removed, either with brute-ish force or a soldering iron to heat the interface. Too much heat ruins the embedment; West says 200 degrees is plenty, more is too much.

Always important for things you want to get apart: use smoothly threaded fastenings. Hot-dip galvanized lag bolts, or deck screws, for example would bond by mechanical engagement to the rough surface, despite the lubrication.

As for dissassembly early in the curing process: you're never gonna reliably be there at the right time: curing rate depends upon many variables including mass of adhesive. What you see happening on the surface may/will be happening at a different time down amongst the threads.

About drying sodden wood or core: isn't it alcohol that folks use? Acetone does not mix with water and I doubt it carries the water out when it evaporates. I doubt alcohol works either. Heat, low humidity, vacuum, applied over time is what I read about for drying laminates or core. A piece of green wood dries in air about 1" in from the faces a year; a sodden, oil-soaked, polyester encapsulated chunk of wood would take longer. I've read that vinyl-ester is more tolerant of excessive water in the substrate but don't know the parameters or whether it's appropriate for bonding hardware.

[Duncan]

I was surprised the first time I heard of lag bolts to secure engines to their beds, since they seemed so crude.
I get the impression this practice isn't uncommon, though.
I can't help thinking that a stud with a nut, washer(s), shims, etc., would be a better setup in the long run?

We used to use lag bolts on some types of construction, but only where you wanted something quick 'n dirty.

[Ceasar Choppy]

A couple of thoughts after seeing the pictures:

1. It appears that there is a shim underneath the mount that is off center a bit, which would explain why you can't get a new bolt in the same hole. I bet the engine is out of alignment and will need to be aligned when you do this fix. Those holes are for a 3/8 bolt, no?

2. Not a fan of lag bolts either, but when all you have to bolt into is wood, its probably your best bet at this point. I'd be willing to bet the wood is rotten which is why those bolts rusted up and broke.

3. Acetone is best for removing water because of its fast evaportation rate. The water molocules get carried away by the evaporting acetone... or so I've been told. But it will still take you a long time to dry out a stringer that is saturated. And you will have to make sure the wood isn't completely saturated/rotten. An epoxy plug won't do you much good there.

4. I've used PAM (the cooking spray) with great success to prevent metal bonding the epoxy. Any good silicone spray would work as well (McLube?). Carnuba wax works well too, although you really have to make sure you coat it well. Needs to be almost liquid to do that esecially for a threaded bolt.

[Quetzalsailor]

The Morgan 27 had a lovely bronze lagbolt bronze washers and a pair of opposing, toothed cast iron wedges. The engine beds were plain old fir encased in polyester-'glass, mostly oil soaked and rotten.

I sawed the whole mess out and replaced it in pressure treated wood but used enough new epoxy-'glass encasement so that the real structural work would be done by the glass (I do not know the adequacy of bonding to pressure treated wood). I used stainless bolts and engine mounts. I arranged the nuts to be accessible in prepared transverse holes in the sides of the beds. The aftermost beds were bolted through about a 1/4" layup of 'glass, assembeld by taping the nut into the wrench. The engine mounts required that I build the new beds about 2" lower.

You might find it easier, and maybe even possible to leave the rotting wood in place and engineer a fiberglass repair/patch/assembly which would do the work without depending on the wood.

[MikeD]

Yes, lag bolts certainly don't seem like the ideal choice for this application. Another Ariel owner suggested I use hanger bolts. I got his email after I returned from working on the boat for the day. Kind of a "duh" moment. Assuming they set up properly, you could tighten down the mounts with a nut. Just like the original configuration for the A4 (I'm assuming) which can be seen in a couple of the pictures.

1. It appears that there is a shim underneath the mount that is off center a bit, which would explain why you can't get a new bolt in the same hole. I bet the engine is out of alignment and will need to be aligned when you do this fix.

There is a shim, but the holes for the bolts are still accessible, only stripped / rotted out.