The motormounts are in cast-aluminum pairs under each header. Two bolts hold the upper mount to the block and there's a long bolt holding the pairs together. A 1/4" rubber vibration separator is between the castings. What happens in half the cases is, age slowly collapses the rubber on the block side, putting a bending strain on the block bolts, which snap off. Some owners, after butchering the block drilling out snapped bolts, tap to the next larger size & report no further problems from long-term nrglect of the rubber isolators. A temprary fix is to loosen the block bolts and rotate the rubber 180 degrees every 5 years or so, to spread out the load. Substituting urethane for rubber does NOT work: heat from the headers will melt the urethane, requiring you to chisel things apart before you can remove the mounts!
The second problem is the long bolt & nut holding the two castings together. The bolt is supposed to be dropped in from the top, but during engine changes, its often forgotten till the block is back in. So it gets put in from the bottom. The nut is supposed to be retained by a cotter pin, but thats impossible when the bolt is in backwards, so it gets left out. Then when age collapses the rubber a little, the nut loosens and the bolt drops onto the road somewhere.
