Dave: I have installed many valve seats in much larger engines. The head has to be heated up to 450 degrees. The seat is on dry ice ...very cold. We drive the seat in and when the head shrinks down and seat expands....its there for good. Is your prcedure similar. If a seat becomes loose when head is at normal temp...the bore where the seat goes in was machined too large. A manufacturer defect. Capt,n