Thanks for the reply. I have made all of the changes recommended by Westerbeke. These include; the entire exhaust system, air box, thermostat and re-routing water hoses for better circulation. This was all done within the first year after after my boat was delivered. I expect that is why my engine has lasted as long as it has, but apparently these modifications were not enough to prevent engine failure. The failure mode was still the same.
You mensioned that you have a Yanmar 54, was this the replacement for the 42B or original equipment? Have you heard that Westerbeke replaced/rebuilt the 42B's at their expense or the owners?