If timing chain in a 4.3 liter Chevy engine broke will valves be damaged?

Yes, if the timing chain breaks in a 4.3L Chevy engine (or most interference engines), it's very likely that the valves will be damaged. The pistons and valves are designed to operate with precise timing. When the chain breaks, this timing is lost, and the pistons can strike the valves, bending or breaking them.