Undoubtedly, Ross's advice is the sure way to go. However, for what it's worth, I've had 3+ year success with doing the epoxy down the hole method. This way I avoided having to move the engine to gain access for more invasive surgery.
A few years ago this topic came up on another forum. I was a newbie to boat repair (and still consider myself one), but I gathered my courage and posted the following description of my fix. This pertains to large lag bolts (wood screws with hex heads). It is necessary that the bottom of the hole is still sealed -- or any epoxy injected into the hole will run out the bottom. This also assumes that the bore hole is only stripped, and the wood still is good shape, not rotten. If you do go this route, inspect the result frequently. Put a socket on the bolts from time-to-time to verify the bolts are still tight. If not, then its to plan B.
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I have a 1980 Cherubini 36. So same vintage as your 1981 37C. My Yanmar 2QM20 is attached to the mounting bed also by stainless steel lag screws. When I bought the boat last year, the surveyor noticed during the sea trial that two of the engine mounts were moving around a bit. (I negotiated a modest repair allowance to cover in case I needed a professional repair.) When I delved into the issue after the purchase, two of the four bolts did cinch down OK. But I could sense that if I gave them some real torque, they would strip. The other two bolts continued to turn with only light torque, so their holes were stripped. The motor mounts holes themselves wouldn't accommodate the next larger diameter bolt. And I didn't think that an extra 1/2 - 1" longer lag bolt would add much extra purchasing power to otherwise stripped holes. Also I didn't feel comfortable drilling deeper. (And in any case, I couldn't get my drill in a position that would drill down vertical into some of the holes without removing the engine.)
As a first go-around, decided to take a shot at doing a repair on my own:
-Cleaned the lag bolts. Dulled the sharp edges of the threads just a bit with a Dremel tool grinding attachment. Then wrapped the bolts tightly with plumber's Teflon tape from tip to hex head, so that no metal was showing. Tape was wrapped in the direction so that when the bolts are tightened, the tape seam stays in place. When this was done, the tape was stretched tightly over the threads, but not torn, and also stretched on to the central core. I also left a wad of Teflon tape hanging past the lag screw tip. Two wraps for good measure. (After I started this solution, I mentioned it at the local chandlery. A couple of people said they had known mechanics who used similar remedies. They also heard that coating the bolts with a solvent resistant wax/grease - brand not remembered - would also prevent epoxy from sticking to the bolts.)
I flushed out the mounting holes with acetone in case oil had worked its way in. - With a syringe to which I attached a small tube that would reach the bottom of the holes, I filled the holes mostly/partially with West Systems epoxy. Had to judge how much to put in because I didn't want it over-flowing out of the hole and around the engine mount.
- With the epoxy still wet, I inserted the bolts in the holes and made the first turns by hand so I could feel that the threads were channeling in the existing hole threads (rather than cross-threading them). Then tightened them all the way down with the socket wrench, but not too tight.
- Epoxy allowed to cure fully.
- The Teflon tape did keep the epoxy from adhering to the bolts. They were easy to unscrew. They came out actually with the Teflon tape still mostly wrapped around the threads. My thought was that the wad of Teflon tape on each bolt tip would leave a void space so that there would be room at the bottom for the bolt to be fully tightened. As already mentioned above, I could not in my case get a drill bit down into the hole to drill it deeper. Another idea would be to grind off say 3/8" from the end of the bolt after it is withdrawn from the hardened epoxy hole. Or maybe just not worry about it.
- Put the bolts back in (caefully not cross-threading) and torqued firmly. They are still holding firm after about 70 hours of motor time.
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(Update: Last time I checked about six months ago, the bolts are still holding firm after 275 hours and 250+ outings).
