Here is what I've learned about CLR, engine block crud and time...
If you need to remove your water pump and use CLR in your block to remove crud at the back it will work. At first I left the CLR in there 2 days before draining and neutralizing it. It removed quite a lot, but it was obvious there was more. Then I tried vinegar (strong) which also worked but seemed to lose its punch after a couple of days and thus was going to be more expensive of a process. Then I poured CLR in there and let it sit for 7 full days. Then I drained it, got a lot more out and neutralized it. I learned something interesting about neutralizing with baking soda and water: if you dissolve about 1/4 box of soda in a gallon of water and use this, you will cause a lot more of the crud to come out. Watching it as well as I could, I can tell the CLR soaked deep into the crud. The baking soda causes the same bubbling reaction that baking soda and vinegar (or any acid) cause and this bubbling action helps remove more junk. Be careful not to pour too much in too quickly. At first it will react and bubble rather violently. I think the reaction causes the crud to break off and into smaller bits where it will flush out easily. The secret seems to be that you must leave the soda/water mix in there for hours, change it and then add some more soda/water. At the end I wasn't getting any more crud out of there, so I'm going to stop, put it back together and try oxalic acid with the engine running later. I would conservatively say I washed enough junk out of the back of that block to fill a beer can. And it could have been one and a half or two beer cans. The vast majority at the end was fine, like black beach sand or volcanic sand and it came out in a slurry.
Another hint: buy a nylon barb fitting (threaded) and attach a rubber tube to this. Stick a bolt in the end to plug it. This will make your work easier.
Hope it helps somebody.
Jon
