If a steel crankshaft has been polished to a "10-10", what exactly does that mean?

Thanks

It would be next to impossible to polish a crank shaft down that much and keep it round and with out any taper.  It's Probably been ground down to a .010 under-size on the main journals  and .010 under-size on the rod journals. This is considered to be the first under-size grind. the next would be .020 then .030 and so on. If done properly their shouldn't be anything to be concerned about. In doing this only .005 metal is removed. It would be wise to get it checked for size, taper, run-out and checked for cracks since steel cranks are prone to crack.

Both the rod journal and the main journals ground .010 under size.

Best to verify it.

If only .001 wouldn't it be   1-1? And the only time that a small amount would be removed is for some specialty application.

Sorry. Dicer is right  I added an extra 0 in all of my figures. It should have been .010,.020 etc. instead of .0010 &.0020  etc. Good catch Dicer!

 I went back and edited my previous post so it would make sense. We often do polish off a small amount and then use .001 under size bearings but as I said before it would be impossible to polish as much as .010 off and keep the journals round and without taper.

When one is told a crank is "10-10" or "10-20", or whatever undersize, which journals are read first? I think it's always mains, then rods, is that correct?

That's the way I've always read them but that depends on the source so it's always important to check any crank before placing it into service. I've found cranks that are stamped wrong and others that have been ground for a second time and not re-stamped. Even new cranks need to be checked. Don't trust any one else's work. Especially something that is as critical as a crank. Always check them for taper and run-out on every journal. Be sure to check at several places around the journal and especially as close to the radius as possible. If a grinder doesn't keep the stone dressed you'll find this area to be a larger diameter than the center of the journal. Make sure that the radius is what it's supposed to be. This is where the strength of a crank is. The larger the radius, the stronger the crank. This is because a larger radius makes the crank less prone to cracking. Some grinders and even manufacturers of new cranks fudge on this. As an example one popular company advertises the radius on their stock replacement shafts as a .094 radius and try to make it seen like this is an improvement over a stock crank when the stock radius specification for the same application of the one being replaced is .109.