Kroil is an extremely effective penetrating lubricant. Almost every professional machine shop I've been in has a bottle of this sitting prominently beside the workbench. I first saw it about 8 years ago, and asked the mechanic why he used it. His words are the same I now say to those who ask me: It will unstick ANYTHING.
I frequently take apart antique machinery or general equipment. There is almost always rust, grime, burned grease, metal shavings, and the wear of decades that prevent me from separating bolts from nuts, pins from holes, or keeping sliding surfaces from doing anything BUT sliding. I've used every possible penetrating lubricant on the market. Some worked OK, but nothing really was "magic" until I found Kroil. Not many products make me laugh with glee. But the satisfying twist of an otherwise impossible-to-remove bolt or the turn of a shaft that was rusted solid now make me smile because of this little orange can.
Kroil doesn't work instantly. It takes between a few minutes and a few days (for extremely large bearing surfaces) to work its magic. I once let it sit for a week on a 300 pound flywheel that was being very stubborn, and it came right off.
Kroil is not for general lubrication purposes. It's very thin (which is part of how it works) and is not very sticky. But that's not the reason I use it; I use it to get things apart. Kroil has a weird creeping capability, it finds its way up and across metal surfaces like some sort of strange science fiction amoeba. After I use Kroil to separate things, I'll typically clean them completely (dip in mineral spirits) air-blast to remove residue, and then re-oil with a more permanent lubricant. The Kroil won't hurt anything if it stays, but I like to get a thicker material in everywhere to avoid having to fix the problem again in a few years.
It's somewhat hard to find in a retail setting. I've never seen it in a hardware store, but that doesn't mean some don't carry it. (The label on my bottle says "For industrial use only - not for retail sale" which is somewhat antiquated.) I typically get it directly from kanolabs.com
, though eBay also might have some good deals. There are now several variants of Kroil including graphite and silicone, but I stick with the old-fashioned stuff since I haven't read the data enough on the other mixtures to figure out if it's worth changing.
If someone asked me what critical items I'd want for my toolbox, this would be among them. It comes at an even higher value than general-purpose sprays like WD-40. Simply put, Kroil is the most useful lubricant I know of.
A recent example of when I have used Kroil came when I bought an Ideal #3 Stencil machine on eBay, which is used for cutting out cardboard or paper letters and numbers for making paint stencils. I purchased the machine for $40, which is about 1/5th the normal price, because the machine was rusty and jammed.
I took the risk because I knew Kroil would work. Indeed, when I opened up the box, the rust was pretty severe. All of the vertical punch letters were rusted in place, and the dial didn't even spin at all to change letters. I liberally dosed all of the moving component interface areas I could see with Kroil, and then started to take it apart. After an hour or so of time, I was able to get all of the moving components back into fully operational condition after slowly working them through a few gritty and then progressively smoother cycles with the Kroil finding its way into the nooks and crannies.
Even the central shaft which was frozen solid with several hundred pounds of turning force, after two hours or so I was able to feel a little movement, and after another hour and some huffing and puffing I was able to get the assembly off the shaft.
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