Tuesday, July 5, 2011

Cleaning the block in a caustic bath

Most of the rebuild books will tell you to take the block into an engine shop to be "hot tanked". Well, that's no fun. Here's an alternative to that.

A caustic solution will dissolve the oil and grease deposits in the block, most importantly within the oil galleries where it's not possible to clean effectively otherwise. It will also remove or at least effectively loosen up the old layer of paint in the block. After the bath, the block will be slimey from the solution and needs to be hosed down with a lot of water to mechanically flush away the caustic.

The caustic will not remove rust, no matter what concentration is used. That's chemistry, you need an acid to convert rust..

(Beware: the solution is dangerous to human beings so be careful with it)

My tank rests on two blocks of metal to take it off the ground, under it but not visible is the heating plate. I used the engine hoist to handle the block, this was very convenient!


After experimenting with smaller objects, I decided to use a 3% solution. The temperature of the solution was around 70 degrees Celsius (an estimate based on the fact that I could not hold my hand to the drum for more than a few seconds). The overall time in the solution was 48 hours. This treatment was enough to take out all of the oil deposits in the block and a great majority of the paint outside. Using a 10% solution would surely have been more effective on the paint but that's just a bit extreme to me... The caustic is dangerous enough in 3% concentration...

As I mentioned, the block will be slimey from the caustic after the bath. I hoisted it outside and gave it a 10 minute shower from a garden hose.


Most of the paint peeled off at this point. There was not much oil left, it ends up as a light brown substance on the bottom of the drum. I drained the drum into the sewer for two reasons: 1) the caustic I used is sold as drain cleaner so it's supposed to go there and 2) this sewer had a oil separator.

Immediately after showering the block I thoroughly wiped clean the cylinder walls and gave them a thick coating of engine oil to protect against even the smallest amount of rust. This is important, the cylinder walls are virgin cast iron and will begin to rust in seconds, yes seconds, when not immersed in water anymore.






With the engine back in it's stand, I started my compressor and blasted the block inside and out to dry it. There's a limit to how dry it can get this way, since the compressed air has moisture in it and you can't blow water off the blocks microscopic-sized pores anyway. To accomplish that, I placed the block next to an electric heater (2000 Watts) and left for lunch & errands. The heat will quickly soak into the block and the rest of the water will evaporate.

The result is a bone-dry engine block (for except the cylinder walls which were coated with oil):


No oil, grease, or water! This is what I had been waiting for, to have the right tools and the right process to do this properly. The block does not look that clean because of the flash rust, but that's a layer just a few micrometers deep. It's easy to handle, more on that next time.

2 comments:

  1. An acid is caustic, just as a base is caustic.

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