hmmm... things have changed in sparks. My CV jointed shaft made there was not bad at all. Granted that was a Jeep and back in hmmm. 1998??? I see they are still there. This is whom I used way back then
http://drivelineandgear.com/
My guess would be that Jeep shaft is just a different animal. On smaller vehicles, there are a lot of parts and options out there to cobble together something. With these, there is only 1 medium duty truck on the road for what, every 10,000 consumer vehicles sold? So there are going to be far fewer sources to build from. I'm not saying it's impossible, but Pointman's response is exactly what I expected to hear. I was anticipating that we would either need to find a very specialized driveshaft maker accustomed to the atypical needs (e.g. one that makes driveshafts for some obscure niche market like monster trucks or extreme farming / construction equipment), or design the components ourselves (sounds like another awesome project, but one of many on my "list").
I think, as the Army has decided on and found to be largely true, that you can get decent results from the regular driveshaft. In fact, those of us having issues are essentialy re-experiencing something that the Army found and "fixed" long ago. So in essence we are finding a new variant of the problem, where the "fixed" heavier-duty driveshafts eventually are able to cause similarly destructive vibration scenarios to the original smaller shafts. It's probably the sum of several stacked effects, over the course of many years, such as spline wear leading to lateral movement ("hinging"), u-joints wearing and allowing a little play, other components such as the pinion bearings wearing and adding a little more play to the mix, etc. You add all these things up, over 10-20 years, and you get an elusive vibration that manages to cause a small vibration... that vibration is then enough to start affecting things, such as increasing the shaft runout and imbalance, loosening a u-joint bolt, wallowing out a u-joint cap hole in the yoke, beating on the bearings in the u-joint, and the vibration increases. It's an upward spiral, until it reaches a point that it breaks something.
We know anecdotally and from the military reports that the FMTV is sensitive to vibration issues. To put it another way, the safety margin is small. The original FMTVs had issues, and the Army's fix pushed it back in the black, but by how far? You add up decades of tiny wear and tear on parts, and you creep back across the line into the danger zone.
The logical takeaways to me are:
1.) If you bought a new FMTV, take the driveshafts in to be balanced, have the u-joints replaced, and have the cardboard in the rear shaft removed (or replaced). Use thread locking adhesive (e.g. LocTite blue) and install safety tab washers on all the u-joint bolts, while torquing them to 17-24 ft*lb. I would recommend thread locking adhesive on the alternator, starter, and air compressor bolts too, since once they loosen it beats itself apart.
2.) If you break an extremity engine accessory (e.g. water pump, alternator bracket, starter mount, air compressor / front engine cover, fan clutch / blade, etc.) in a mechanically violent way (e.g. it cracks or breaks off, as opposed to wearing through, rusting, or losing a rubber seal / hose), keep a close eye on it, and you are wise to have your driveshafts balanced as soon as possible.
3.) If you break multiple extremity engine accessories within a very short amount of time (e.g. a tank of gas = 250 miles), stop all driving over 30 MPH until the driveshafts can be balanced.
At worst, you're out $200 for driveshaft balancing to gain the peace of mind that nothing is wrong with them and it was just coincidental broken things. At best, you save yourself the cost of replacing an engine and/or transmission, which with labor is roughly the cost of the whole truck (my total replacement was in the neighborhood of $7000 out of pocket, on a $10k truck).
