Regarding replacement of a Smith Cast Iron Boiler thermocouple, boiler
model GB 100, natural gas, residential submodel W.  I have been to the
Smith web (search Smith Cast Iron boilers) site and can't find the
answer to my problem.

I need to replace the thermocouple.  Normally a simple repair on any
furnace, this one lacks reasonable access to the combustion area in
order to disconnect the sensor end of the thermocouple.  I have a
"knothole" and a slot I can get my fingers in, that's all.

The pilot light and the sensor are fastened together on a plate with a
hook notch.  Apparently when everything is seated correctly, the hook
notch sits inside the combustion chamber in a precise spot that I
can't see.  Wiggling the pilot feed tube and the thermocouple causes
the plate to work loose and drop down, but I cannot draw it out of the
combustion chamber as it doesn't fit between the burner tubes and the
various corrugations in the body of the unit.  Thus, I cannot
disconnect the thermocouple to replace it, and it will be challenging
to guide it back in and replace it.

Greater access is conceivable, but not without EXTENSIVE dismantling
of the unit, including disconnection and removal of junction boxes,
plates, sensors and wiring.  The unit was professionally installed. 
The thermocouple has been changed in the past without taking apart the
whole unit, but I don't know how.

Please tell me precisely how to remove and replace the thermocouple
and how to re-seat the thermocouple-pilot assembly inside the
combustion chamber.  Thank you.

* No penalties for comments, follow-ups, sincere attempts, etc.  I am
a regular user and know the system.