Every few hundred hours I've pulled the exhaust elbows from my
2GM20F's to chip the carbon away. The starboard engine in particular
has a tight turn in the exhaust hose, and since it gets used some at
low rpm for charging, it tends to get some buildup.

I've been thinking that its time to replace the elbows this year.
Here's my question: The mixing elbow, which is made of some light
alloy and thus is wearing down, is screwed onto to the exhaust elbow
(cast iron?) with a connecting nipple. Since the iron parts seem to
be holding up well, can I simple replace mixing elbows, or should I do
the whole system. This would be an easy decision if I though I could
get them apart without destroying them, but I've assumed this would
involve a large bench vise, oversized pipe wrench, and a breaker bar!
Also, the cost is not trivial - the mixing elbows are about $100,
but the exhaust elbows plus the nipples are another $125. (All times
2, of course!)

Plus a follow up question - does anyone know a good source for parts
like this? I tend to use Marine Diesel Direct (Torreson) and I've
also seen OK prices at shipstore.com, but does anyone know of low cost
parts supplier, perhaps in the Boston area? The major distributors,
like Mastry tend to quote list price.

Here's two pics I took a year ago - obviously it hasn't gotten better!
http://www.sv-loki.com/img_0793.jpg
http://www.sv-loki.com/img_0794.jpg