We just finished replacing all the shroud chainplates on our 29 year old C37 yawl. We did this with the mast up and the boat in the water in Mexico. A few tips follow. If anyone wants more details feel free to email us at: firstname.lastname@example.org
We had cracks in two shroud chainplates that started with crevice corrosion on the sides against the hull and migrated across the edges to the fronts of the chainplates. You could feel the cracks with a fingernail. We pulled the lowers on one side and replaced them then did the opposite side then the uppers.
Yes, you can pull a chainplate out using the shroud it's attached to as mentioned by Sue. Just tighten it up and lean on the wire. We didn't need a sledge and didn't loosen them all at the same time.
All of our chainplates went under the rubbing strakes. Cleaning out the 5200 was a chore made possible with a Fein sander with narrow saw blade attached and Debond Marine Formula. It really does dissolve 5200. Since we went with 5/16" 316 instead of the original 1/4" thick material the Fein tool allowed enlarging the pockets in the rubbing strakes for a perfect fit.
We didn't want square holes in the chainplates and didn't like the idea of the original square shouldered carriage bolts in round holes either. Both lead to stress risers in the chainplates (confirmed by Don Kohlman). Our preference was to use button head, socket cap screws but the threads run all the way to the head on these and we didn't want threads bearing on the chainplate holes either. We had some carriage bolts modified by a machine shop to take the square shoulder off. We then put two flats on the heads so they could be held with a wrench.
We had the new chainplates made locally in Mexico with a water jet cutting system. Very nice work and very precise with no saw, drill or heat induced stresses in the material. I told them to make 1/2" holes +.003 over and they were all dead on. They were all buffed to a high gloss and bent like the original ones.
We used Lifecaulk instead of 5200 and tighted the bolts in two passes waiting about 10 days between initial and final tightenings. The caulk cures slowly when it's under a chainplate and we didn't want to squeeze the glue line down too thinly.
The Fein tool with saw also allowed us to cut the interior teak slats (we do have the cabinetry and book bins installed) so short sections could be removed.
We removed, cleaned, polished and inspected the four mizzen shroud chainplates and the two backstay plates (split backstay). Using the same modified carriage bolts as for the main shrouds, we had some SS sleeves made to press into the original backstay chainplates that were 1/2" ID. This got rid of the square shoulder in a round hole as originally built. We used the original mizzen shroud bolts.
We didn't mess with the stemhead fitting yet but there are no rust stains so we figure that can wait a while.
Hope this is of some use to someone taking on the job.
C37 #22 (1980)