Hi Tim,
Do a search on this site for chainplates and another for chain plates, don't specify boat model as a common way of securing chainplates was used on all models from early 1970's to the mid 1990's, certainly including the M376. The weakness of the design is the little stainless sealing plates screwed down around the chainplate penetration to prevent water ingress. The issue is that the sealant used under the plate tends to dry up over the years then crack with the inevitable working of the chainplate admitting water which then penetrates both the cored deck and, worse still the marine ply part bulkhead to which the chainplate itself is bolted. If this goes undetected for years, it can result in the bolts tearing out of the soggy marine ply. All this is hidden behind the beautiful cabinets below, so the owner can be in blissful ignorance.
Those owners who know of this, lift and rebed the sealing plates every 5 years or so using a more modern sealant which will remain flexible for longer, and some of us have fitted hatches in the backs of cupboards / bookshelves to permit inspection of the chainplates and their bolting onto the part bulkheads, but this is prevention, not cure.
If the part bulkhead is going rotten, the only cure is to remove the cabinets and replace the part bulkhead, or at least the part which is water damaged. While the part bulkhead is out, you can cut away the lower grp layer of the deck to dig out any water damaged balsa, replace it with marine ply then remake the deck underside in grp, ensuring that the side of the slot for the chainplate is sealed over with grp.
If it is not possible to inspect the chainplates / part bulkheads on the M376 you are going to look at take a straight edge with you and use it to find any bulging up of the side deck around the chainplates - this will only occur if the top part of the part bulkhead is no longer fully fixed to the lower part so that the top part is held in place by bearing on the deck.
Peter.
