I don't think it will work. KOH is about like NaOH, in other words a
strong base. If you heat it, I'd think it would behave like a hot
caustic cleaning solution. Very tough on grease but won't touch rust.
The thing about rust is that if you put rusty steel into an acid
solution the acid attacks the rust much faster than it attacks the
steel. So if you put it in acid for the right length of time then
it will dissolve away all the rust but leave just about all the base
If I had to clean a car engine I'd certainly start with a grease-removing
soak followed by very hot water. Scrub it to make sure it's clean and
repeat the hot soak/rinse/scrub cycle until it is.
You know the problem with an acid soak on an engine block? How can you
guarantee that all the acid gets neutralized and rinsed away? I might
use a weak acid and longer soak followed by really serious rinsing.
The last rinse cycle should be in water hot enough so the whole block
will self-dry within 60 seconds.
Hot caustic baths are what commercial engine rebuilders use to "hot-tank"
engine blocks. Local services often offer the same thing, for small-time
rebuilders. There are commercial hot-tanking solutions but they are
basically lye, if you'll forgive the pun.
They often advertise it as a way to remove rust. But it doesn't strip rust.
What it does do is clean out the oil and some other crud trapped in cooling
passages, as well as varnish and oil in the oil galleries. Some of the flaky
rust in an old engine is trapped there in mungy old oil that got into the
cooling system, so some rust does, incidentally, come out when you hot-tank
To remove attached rust you either need to remove it mechanically or with
acid, as you say. I'd be wary of using acid in a cast-iron engine because it
will be hard to remove. Iron has a lot of pores where graphite flakes out.
Maybe someone has experience with it and knows how it works out in practice.
I'd just be darned sure to flush it out with a mild alkaline solution if I
were going to try it.
Oxalic acid would probably do a much better job. That was the principal
ingredient in the two-part powdered radiator flush kits and it was really
good at cleaning the cooling passages in cast iron blocks.
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.
||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
No, it's rubbish at it.
If you really need to remove rust from a cast iron block (rather than
oil, varnish, sludge, grot and the usual stuff) then look into
NB - Make sure this block is a block, not an engine, and that's it's
all made of cast iron. A few bits of steel are OK, but one aluminium
casting left in place really isn't likely to survive most of the
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:
Try mollasses, mixed one part mollasses to four parts water. It wont hurt
the good metal it you keep it completely submurged, but it is slow. You can
get mollasses cheap in bulk at the produce store, they feed it to horses to
keep them shiney!