The earlier 740 crossmember is prone to stress fractures that can become catastrophic. Check yours soon! The front crossmember is shaped like an inverted hat cross-section. Look closely at the areas on either side of the oil pan, where the side upper "wing" makes a 90* bend downward to run underneath the oil pan in the center. Between the oil pan and the mounting bolts for the steering rack. On mine, the right side bend developed a crack that was displaced and getting worse. The left side had a crack that had not displaced but had started to "Y" in two new directions.
I drilled some 1/8" holes at the extreme ends of the cracks, in an effort to halt the cracks growth until I could get them welded up. It helped.
Right side crack, from below. Oil pan is to the left, steering rack mounting bolt at the right margin
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Right side crack, from below, shows displacement
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Right side crack, from above
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Right side crack, after grinding out a small valley along the crack, preparing to accept weld
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Right side crack, after welding, bottom side
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Top side weld(very tight access!)
Left side, before welding
Left side, before welding, closeup
Left side crack, prepping for weld
Left side crack, welded
After welding, I cleaned up the burnt paint flakes and sprayed some fresh paint for protection, but on this car, the leaking oil would have provided its own protection soon enough!
If these cracks were much worse, I would have probably wanted to weld some gussets in place.
Originally I had thought these cracks resulted from the diesel engine's extra weight and vibration, but apparently the earlier gasser 740s develop these cracks also. Probably from rough roads or freeway expansion joints.