The bronze one will not cake up like the cast one. We have had 16 years with a lot of engine hours on ours. It has never had an issue and has been checked a number of times.

The heat exchanger, after 14 years needed to the ultrasonically cleaned (no other way works right for a bunch of reasons). That model Yanmar is a great engine. I have the 85 hp turbocharged model which is the same engine with some tweaks.

Steve is correct about the time if you have the part, your tools in place, and the engine is accessible (and someone has prepped the area for a bit of work). In my experience.. Nothing takes 30 minutes on their clock ;^))). Seriously, your engine is in near the galley in front of the companionway in the cabin. Then the mechanic should bring the engine up to operating temperatures after doing the work. He should be putting down some "protection" on the sole and then cleaning up a little. Assuming they don't charge you for ordering,, walking to and from the boat, or "talking" time, then I'd guess you'll have an hour or and hour and a half of billable time.