Following up this topic. I went to John Bailey's gathering of the middle Atlantic GT lunatics this Saturday and had the GT Guys do some work on my car. 30Kmi service, spring adjusters and checking it over. I also asked them to check the tranny axle bolts. Yup, mine were still original.
I asked Rich to measure the release torque on the bolts and it was about 40 ft-lbs on all 4 bolts (2 per side), and THEY WERE ORIGINAL BOLTS. No problems, not broken, not cracked (that I can see with a plain magnifying glass), perfectly good. I wanted them replaced anyway, because they already were in there, and they had the kit to do it. BUT, if you have more than say 10,000 miles on the car, and the bolts haven't failed by now, I can now make a pretty good mechanical engineering/materials argument to leave them alone.
One of the things Mech Engineers learn somewhere back in machine design and strength of materials courses is that bolts almost always fail in tightening, and if they don't fail in tightening, then they almost never will fail in service. The reason is that once the bolt is brought up to proper torque, it is always in static tension after that, and when you release the torque wrench, the bolt actually turns back a tiny bit, which relaxes the torsion, so then you have only tension on the bolt (see my other discussion about how bolts do not see cyclic stress if they are properly designed and installed). ANd yes, we also learn that "bolts" are fastened with nuts, and when they are screwed into a machine part, they are technically called "machine screws," so let's not debate that fine point.
The reason our original bolts failed quickly is that a batch, or several batches, of them got hydrogen embrittled, which caused cracking under the head of the bolt. However, not all the batches were thus affected. So, if you have put substantial miles on the car (and it's now at least 9 years old), you almost certainly don't have the bad bolts. If you already put in or paid the labor to pull the half-axles off for some other reason, and it makes you feel better, replace the OEM bolts/plate with the Ford replacement or ARP parts. Otherwise, leave 'em alone. Don't go lookin' for trouble, or as someone else once put it, "If it ain't broke, don't fix it."
