These Harley tappet blocks like to crack right on the slot's sharp corner. Doesn't really matter if they're aluminum or cast iron - they both crack. Sometimes there's a little casting mark(like this one) and that's where they crack. You can weld 'em or braze 'em . . . however, it may be best to find a different block. Maybe relieve that edge just a bit with a tiny Dremel? If that piece breaks off, it's not gonna be good.
I have possession of the "original Noot's" bead hone for tappet blocks. Works for many brake master cylinders too. It appears about worn out, but still works great. I run it in reverse and it works the opposite side of the honing beads, then back clockwise to smooth up the bore. The wire ends leave micro scratches to help hold oil in the bores. I slow the rpm way down and use long strokes to get the crosshatch running with the bore. Michael gave me 2 pairs of blocks, so I checked fitment with a lifter . . I'll use the best for a set.