I don't like the idea of peening the edges. The stress and pressure from peening the valve seats may also make the seats "out of round". I ran the head lines in an engine plant and know the tolerances. The roundness for the valve seats is held to 13 microns ( or 0.013 mm). This is a VERY TIGHT tolerance to hold and we had the machine operators for the valve seats check them every 10 - 15 minutes. 1 mm is equal to .040", so take .040" and cut it into 1000 equal pieces and then take 30 of those. It is thinner than the diameter of your hair. Peening them could throw them out of roundness unless you remachine the seats afterwards.
The only reason that the seats would fall out is if they did not get their press fit correct which is determined by the hole size. The hole size tolerance is so tight, you cannot just use a drill and keep it in spec., you have to use a reamer which can hold a tighter tolerance. This is also monitored in the machining line with load cells to monitor the press in load for the seats. They set the limits of the load cells by testing sample parts to determine the acceptable load for a good part. If it allows a bad (not enough press fit/interference) part through, they did not properly establish their limits, or someone opened up the limits on the press in operation for the load cells.
I was involved in a change to the metal that the seats were made of and had to test the new parts to make sure that the limits were acceptable for the new parts vs the old ones. There was a difference in changing the hardness for the seats and we had to establish new load limits.
Bottom line, if they had proper control of their process like they are supposed to, this wouldn't happen. It is the responsibility of the head machining plant, which in this case is in Mexico for the 5.7 L engine, to keep their process under control.
The engine plant that I worked in literally made 1 million heads per year and we never had a problem with the seats falling out.
