Jeff wrote:
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

The build up in the elbo is not carbon. I have replaced three of these
on my Yanmar 3GMD which looks just like yours. I inspect the elbo every
500 hours by removeng the exhaust hose and running my finger up pass
the devider to feel the opening. It is the high temperature of the
water that is causing the build up. Only the cast iron mixing elbo
needs to be replaced. Remove the assembly as in your picture and take
it to a shop with a large vise. The elbo and the stainless union are
standard right hand threads. If the elbo does not break lose use a
torch to cut away some of the elbo materal staying away from the union
threads and it will expand and come lose.
Your engine overheat alarm will not sound until the water exiting the
engine heat exchanger exceeds 160 degrees and these deposits are more
likly to form at temperatures above normal engine temperature but below
160 degree exit water temperature. Most Yanmar owners replace the raw
water pump impeller and forget to replace the impeller cam when it is
worn.

From the author of four books on boat refrigeration
http://www.kollomann-marine.com