Found this small hairline crack just before having them rebored. It it likely to cause loss of compression?
Not much but looking at your photo the crack is actually in the bore itself, I think I would be tempted to get another cylinder if possible, or get that one repaired by a specialist with another liner.
Some engineers might weld it up and then sleeve it, personally I would look for another barrel.
I'm looking for another barrel but they are a bit like rocking horse doo doo