As I've said many times before.....
All the answers so far are correct. You do not need a Visa for hour-building.
However - there is no guarantee that the immigration officer who sees you on the day you arrive will know the rules as well as you do. Even if he does know the rules, he quite likely won't know enough about aviation to understand the difference between hour-building and training. (E.g. you don't need a Visa to do a BFR, or get a checkout with an instructor, but you try explaining to someone with no aviation background why that doesn't count as "instruction".)
I still have yet to hear of anyone not being allowed into the US to hour-build. But I'd still recommend a Visa if you have to the time to arrange one, just to be on the safe side. If the immigration officer gets it wrong and decides not to let you in, you have absolutely no right of appeal - you will be on the first plane back home, and you will have to tick "Yes" if you are asked if you've ever been refused entry to the US on any future visits. Take whatever steps you can to make sure you're not the first unfortunate person to fall foul of someone else not knowing the rules!
FFF
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