While working on your traditional transport, mainly during a restoration, a bolt will certainly break off, the remainder of which will stay in the engine or other part and you will not get the part in question (anymore).
Time for a left tap (set)! First carefully drill a suitable hole in the remainder (and do not damage the existing thread!) And then with great courage, policy and faithfully screw in the tapered, left tap. With a bit of luck you will get the remainder out; with a bit of bad luck, everything must first be heated. In short, a left tap is an indispensable tool!