Engine mount lag bolts

[ChrisOB]

I had saw the front engine mounts wobbling against the fiberglass the other day.  I torqued down the bolts a bit, but they didn't bite that well.  Checked today after a short motor sail and one of the bolts had jumped out into the space below the oil pan.  I'm a little surprised that these mounts aren't through Bolted or bolted into some kind of sleeeve.  I've seen some people taking about shoving toothpicks in the hole but that sounds pretty iffy to me.  Is this an epoxy drill and fill situation? I'm in the Bahamas at the moment so I don't mind a quick fix for the time being but I'd prefer to just do it correctly for another 10 years

[KWKloeber]

Chris

You assessed it correctly. The temporary toothpick fill isn't the long term solution -- you're not repairing a stripped-out hole in a piece of furniture.  Ideal (IMO) is to replace the lags with machine-screw-threaded fasteners.  One way is to affix (epoxy) a stainless coupling nut into the stringer. 

I have done this in another application where I needed a blind fastener underneath a deck where I mounted a crossover block so the owner could remove and service it in the future without a major job.  I sunk a nut into a hunk of G-10 and epoxied everything to the underdeck above the headliner, to reinforce the area so it took the strain of a crossover block.  The same could be done in an engine stringer.

Alternately, when my time comes, I plan to sink studs in the stringers.  Essentially an upside-down stainless hex bolt with an extra nut or two threaded partway down for added grab in the epoxy to resist the upward pull.

Unlike wood that has fibers that compress when cut into by the lag threads and body, epoxy is iffy.  By its nature, we don't want it to be compressible.  So you need to get a precise hole diameter for a given screw thread -- I don't know how many times I've snapped off a tapping screw because the pilot hole was too small.

-ken

[Stu Jackson]

Chris, I'm the bozo who wrote about the toothpicks.  Happy to report that after many, many years, they are still holding.  My logic was simple:  those bolts do NOT hold the engine down, the gravity thing does.  So the lag bolts are in shear most of the time.  Therefore, my goal was to keep them in place, hence the toothpicks and silicone sealant.

Last year the forward lag bolt on the port aft mount backed out.  I had never had this one "go off" before, only the forward port mount.  That lag bolt is a BEAR to get to:  you can either see it or touch it, but not both!!!

I ended up buying a set of four ratcheting box wrenches (only a few dollars more than for just one at another store) that also tilt.  I was able to get a new lag bolt in and couldn't do it without that tool.

It's still in from April 2018.

Your boat, your choice.