Originally Posted By: Dualie
well the truck is currently getting a new set of spider gears after the bottom one shead 3 teeth off making a lazy right hand turn in a parking lot empty. Thankfully the ring gear and pinion are fine. Having new bearings and races put in the hubs at the same time.
I have the 75W 90 in stock so im going to use that. but it still makes me wonder why ford changed the spec's I just looked and my 1994 F-Superduty (F-450) with the dana 80 still specs the same fluid.
Most likely, the tooth was ready to go and damaged from another incident or extreme wear, then picked that moment to let go. Lot's of towing or tire spinning (either burnouts or spinning up in mud or snow) will hasten side/spider failure.
What does everything look like? Is it all dark, black or burned looking? Was the gear oil evil smelling? If so, that's low lube or heat from towing. I have had a rear axle temp gauge on my towing rig for some time now and it isn't hard to reach 250 degrees. It's a cmbo of high loads and high speed that makes heat. Also, the lower the gear ratio, the hotter the axle will run (more teeth and friction surfaces to generate heat). I have found that at 55 with a given load, the temp is 180. I increase to 70... it goes up to 225.
It would probably be worth putting in a temp gauge, at least. Autometer makes a nice one and they also sell a bung you can weld into the steel cover, or you can buy an aftermarket voer that has a built-in port (I use the Mag Hytec)
The Dana 80 had a so-so reputation in towing circles... at least in the extreme end of it). It's used as a medium duty axle, but it isn't really one. It has a high torque rating but a relatively low oil capacity and tend to get hot.That's one reason why Ford started putting the finned covers on them. They die like flies in motor homes due to cooked oil. The aforementioned temp gauge and an expanded capacity cover will help a lot. If nothing else go to 15K oil changes.
The temp gauge will tell you if you need Grade 90 or 140 (odds are good it will need 140). If it consistently runs over 210 degrees, it should be 140. At 250 degrees the 90 has thinned a bunch.... depending on the oil, it could be to something around7-8 cSt (about a 75W). At 250, the 140 may have thinned to below a Grade 90 viscosity. Remember, at 210 degrees,viscosity will be right at graded viscosity. Above, it gets thinner.