MasterTemp 400 chamber pot

[TXWill]

So the chamber is rusted through, but the sheet coil is not outwardly leaking.
I'm assuming a small leak at the manifold rusted the chamber.
I'm considering gettting a new chamber and reusing the coil.

thoughts?

just a homeowner with mechanical skills looking for advice and watching YouTube videos on this subject

 

[JamesW]

How old is the heater?

 

[RDspaguy]

You assume, which means you will be doing more work on it soon. Be sure where it is leaking. If it has been leaking long enough to rust out the combustion chamber it should be fairly apparent where it leaked from. The o-rings are a weak spot, obviously, but deterioration of the exchanger tube ends can also cause a leak there, and would necessitate replacing the coil as well. Pinhole leaks in the copper tube can be hard to find.
I would not want you to throw part after part at it until you regret not buying a new one. Or needlessly damage your new parts by not accurately addressing the cause. My advice is to be very sure of everything it needs before buying anything. Look for other potential problems also. Is the blower loud or corroded? Is the ignitor good? Etc... These should be a factor in your decision. As should warranty. New heaters installed by authorized installers usually carry a longer warranty than a diy replacement, which has a longer warranty than replacement parts. It is cheaper to fix it in the short-term, but buying new might be a better long-term investment. Whichever way you go, know what you are getting into.
Best of luck.