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#1
03-02-2013, 12:40 AM
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740 Front Crossmember Cracks
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 rbefore.jpg Right side crack, from below, shows displacement rbeforedispl1.jpg Right side crack, from above rbeforetopclose.jpg Right side crack, after grinding out a small valley along the crack, preparing to accept weld rbeforetopprep.jpg Right side crack, after welding, bottom side rafter1.jpg 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.
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1985 744 gle d24t 1985 745 gle d24t Last edited by ngoma; 09-30-2017 at 10:50 PM. Reason: Fixed broken photobucks link 
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#2
03-02-2013, 02:49 PM
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Check at your lower control arm pivots on the frame too, that's where mine were worst. Look by the xmember to car mount bolts too.
I used a later frame that is a much better design details; http://www.brickboard.com/RWD/index.htm?id=957777
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#3
03-03-2013, 12:24 PM
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FWIW, model year 1986 appears to have been the changeover point to the newer crossmember design. 1983-1985 700 series diesels will be affected by this problem, probably on "not if, but when" basis....
For diesel owners looking for replacement crossmembers, 1989-1991 740 and 940 GLE models having the 16-valve B234F gasoline 4-cylinder are the only other models that use a welded-on lower mount bracket on the left side like the diesel models have, and they were manufactured after the change to the updated crossmember design, IOW, they use the same desirable crossmember that the diesels only got for their final model year in 1986. In cases where an original crossmember is too far gone to be repaired by welding as detailed above, this may be the easiest way to find a replacement.
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#4
03-03-2013, 09:22 PM
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More...
I recently bought another 740 D24T, the lichen-covered white wagon. Took advantage of good weather today to inspect the front X-member, which is quite a bit worse, almost a candidate for replacement but we will see what my welder friend says.
Left side was worse on this one, cracked in several different planes, and along 90* (factory) welds. I was not impressed with these factory welds. Left side, I could wiggle the upper left portion easily with my finger!  Closeup  Front side, from below looking up.  Right side, engine mount extension  Under the grime, shows the extent of the crack  We'll see how well the welder tip can get into some of these places! Maybe time for some gussets as well.
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1985 744 gle d24t 1985 745 gle d24t Last edited by ngoma; 06-03-2013 at 12:13 AM. Reason: Fixed broken photo link
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#5
03-03-2013, 09:41 PM
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Just change it out, it doesn't take long, and you are no longer stuck with the flawed design.
I knew mine was bed when my front tires would leave black streaks from toe in in reverse. Last edited by 745 TurboGreasel; 03-03-2013 at 09:43 PM.
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#6
03-04-2013, 10:38 PM
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Wow! That one is pretty far gone. Never knew they could get that bad. Surprising it did not have symptoms already like those 745TG described on his, I imagine those parts were moving around a good bit. Almost every pre-'86 one I have looked at had at least a little bit of a crack forming on at least one side, but never saw one fail so completely as this. Wonder what caused it to be so especially bad in this car?
I agree, looks like a replacement candidate, especially with the bolt holes elongated in addition to the disintegrating of the x-member...
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