I've done this on about 4 blocks and heads and not had any issue with
it. The acid may have been fairly weak but you could see the activity
due to the bubbling at the water passages. This eventually died down
and I rinsed thoroughly. Considering the water pump and thermostat
housing were aluminium alloy they were not noticably damaged by the
process as I would have expected, they were however sacrificial.

Rex B wrote:

On Thu, 09 Sep 2004 23:03:13 +0100, David Billington
wrote:

||I have used battery acid from old car batteries to clean cast iron BMC A
||series engine block water passages. Bolted up an old water pump to seal
||that apperture and covered the top of the block with vaseline, filled up
||the water passages to the top and left it for a day, then flushed out
||the whole thing with lots of water. Left it nice and clean. I can't
||comment on sodium hydroxide though.

You are a brave soul, or one who has plenty of spare blocks
Texas Parts Guy