As of 1945, the vast majority of R-3350 flight hours were in B-29's so it makes sense that you hear of more engine fires on B-29's. I worked with a fellow who had worked on refurbishing B-29's for the Korean War. He mentioned that the B-29 was the last big airplane to use hose clamps on fuel and oil lines and they had a lot of trouble with leaks during acceptance flights on refurbished aircraft. He said that they tested different clamps and found that the radial style hose clamps had less tendency to loosen then the tangential style. I think that people get carried away criticizing the use of magnesium as P & W and many other companies also used magnesium without serious problems. Solid magnesium will not burn; it has to be in a liquid state to ignite. If you have an engine installation that is prone to fires, the magnesium will add fuel to the fire but it's not the reason that you have a fire problem. A former instructor pilot mentioned that they had lots of brake fires.