Stovebolt valves have a really nasty habit of sticking in the guides. it's possible the guide broke instead of letting the valve slide through it freely when the engine was first fired up. Once it broke, it would just be a matter of time until pieces found their way into the cylinder, or back into the intake manifold where they could go into any, or all the cylinders. Make sure the guides have enough clearance when the head is reassembled. I'd rather deal with a slight amount of oil consumption due to sloppy guide clearance than the possiblilty of stuck valves and/or parts breakage!
Jerry
