Many older blocks have been "successfully" bored +.125", in that the engine did not seize immediately.
However, when up to operating temperature (oil, water, and metal) the cylinders are no longer round, or the same diameter at all points of the stroke length.
Bill Jenkins explained all this for the SBC 350 block in 1973. Boring beyond +.030" added 5 inches but ruined the ring seal.
