How to remove calcium in Lister D water jacket

Hi All,
Just a very quick question!
Renovation going very well on two D’s, after repairing frost damage, and removed rust from water jacket, but I seem to have a very heavy deposit of calcium in one engine, that is very stubborn to remove.
Does anyone have a good remedy to dissolve calcium while engine is in bits!
Many thanks in advance
Mark

I've never had to descale any of my Lister D's but a good descaler should do the job, are you sure its scale as the hoppers have rusty metal that breaks off and blocks the holes and channels.
I've used Kilrock when an engine is running and boiling.

Maryalice

Citric acid descaler from Wilko is cheap and effective, especially when warmed up - don't breath in !

Thanks guys, definitely calcium! I shall give your suggestions a try once it’s up n running
Many thanks
Mark

Calcium Carbonate I hope! Mixing water and calcium could be entertaining