FYI Stopped over to Ford picked up the Motorcraft PTFE LUBRICANT XG-8. 3oz for $10.00 Amazon wants 15. This is a polytetraflourethylene base lubricant. Designed for the slip yoke.

 

Interesting. I just used valvoline crimson grease that I had already, worked great

 

So I set aside some time this weekend to grease the slip joint. It took about an hour to pull the drive shaft, add the grease, and reinstall the shaft.

Not only did this fix the clunking at stop and start, but it smoothed out the shifting in all the gears, especially the hard TC lock up in 4th gear.

This has been the most gratifying maintenance/repair I have done in my life. Instant results!!!

 

How does it smooth out the shifts?

 

I pulled the rear drive shaft out this weekend as well. I had to drive the slip yoke off with a brass bar it was on pretty good. I buggered up the blue friction coating so I removed it with a torch and wire wheel. Swapped out the u-joints greased the slip splines, installed and went for a run. Wow no clunk and I also noticed smoother shift getting on it on the highway for the last shift. Next weekend is the front shaft and the door seal modification.

 

I'm not an engineer. I have no clue. All I can say is that it shifts smoother now.

 

I dont see how it smooths out shifts. Can someone please explain this to me.

 

I'll take a stab at a theory! Tell me if this makes sense to you.

Ok, so I'm assuming that the drive shaft has a slip joint because the shaft is connected to the differential which is in can move separately from the transmission due to the fact that it is attached to the suspension system. The slip joint allows for any "slop" as the rear axle bounces up and down, and also forwards and backwards due to mechanical tolerances. If it didn't have this "slop" then those movements would be sent back to the transmission, and I can only assume that it wouldn't be good for it.

So, on to why I think it smooths out shifting. I believe that the "clunk" that I was hearing before was either the rotating of the wheels catching up to the transmission, or the transmission catching up to the rotation of the wheels. Since the slip joint was dry, when it accommodates for this slop it binds up a bit and then pops into place when there is enough force added.

When shifting between gears there is a split second where the transmission and rear wheels are not rotating at the same speed. So when one or the other catches up, it feels like a rough shift because the slip joint is dry and it releases the force with a slight pop. When it's grease, there is no pop because the joint is functioning smoothly.

That's my guess. I could be completely wrong!

 

Well thats a pretty guess and I'll believe it if someone else chimes in and says yes thats it, but that is a pretty good theory u wrote.