Dissolving rust in a heat exchanger.

[Turbo]

Having used hot citric acid on brake calipers I can attest that it eats away at cast iron surprisingly quickly so it should do your job.

 

[W.olly]

Tried the ultrasonic cleaner four attempts and nothing tried an acid I got from work and nothing. Poked prodded around and nothing.

So surgery it was.

applied masking tape to prevent the JB weld blocking it again

The original was knackered. Would of been to much a fagg to fix that