It seems to be working.

[Alan Drover]

A couple of weeks ago after the antifreeze was changed and the 1 piece thermostat/bypass hose housing fitted I put the old 2 piece thermostat housing in a very strong solution of kettle descaler. I had a look today and the build up of limescale is almost gone. Hopefully a few more weeks and it'll all be gone. A partial success and a part saved I hope.
The bypass hose housing was too far gone to save though.

[Alan Drover]

This is a timely reminder to change the antifreeze regularly to avoid corrosion. The antifreeze will stay protective but the corrosion inhibitors don't as proven by the state of the cast iron housings and now that the replacement is aluminium the antifreeze will be changed in 2 years. Due to various circumstances, some beyond my control, others not, it was not done for too long.
I wonder how long this OAT stuff lasts. I know there's waterless coolant available at extortionate cost but if  something arises which needs work on the cooling system then that can be very expensive.
I'll stick to the 2 year replacement.
How I miss that very useful drain tap on the block of the 3mb 2.25.  With the introduction of the 5mb and metrication it was replaced by a bolt and the original tap won't fit the later engines nor the 2.5.

[Alan Drover]

I removed the thermostat housing from the descaler solution. There was still a little scale left which I removed with my narrow belt sander then gave the housing a coat of black paint.
Now I have 2 of the 2.5 early thermostat housing part number ERC 7642 which has now been superseded by the superior single casting.
I suppose they might be useful to someone with an early 2.5 who wants to keep it original.1 is NOS from Craddock's and the other is the cleaned up one.
I disposed of the descaler down the toilet which needed descaling anyway.
Edit. It descaled  the toilet too.