strange problem

[NC325iC]

alright, when i took out my subframe to do bushings i noticed 3 sets of marks on my driveshaft, one set that i made to line it up right another set that is lined up when mine are lined up and a third set that is about 1/2in away from being lined up when the other 2 are lined up. the problem here is that i dont know which set is correct.

i get a vibration through the car above 60mph but i am running urethane rsms and tabs, do you thing i should try the other set of marks since ill be doing a clutch job soon anyway?

[NC325iC]

anyone wanna chime in thoughts/opinions

[DJB]

Quick summary: visually line it up with the other U-joint, moving one spline in either direction to verify it is exact. My write-up on aligning the e30 drivershaft:
Aligning the drive shaft isn't nearly as difficult as people make it out to be. It's only an issue when people don't know that the drive shaft has a specific mating.

Theory of operation

Drive shafts typically use U-joints, as they are simpler and more robust than other types. A U-joint is a non-constant-velocity joint: any angle misalignment causes the output to speed up and slow down relative to the input.

But there is an easy way around this problem in one special case. If you use a pair of U-joints and the input and output shafts are parallel, the change in intermediate shaft speed is canceled out if the U-joints are assembled to be aligned with each other. Again, this cancellation only happens when the joints are aligned with each other. The worst case is a 90 degrees mis-alignment where the speed change is doubled.
A U-joint assembly misalignment means a constant wheel speed will cause the transmission end of the drive shaft to speed up and slow down with each wheel revolution. In reality doesn't result in the engine noticeably changing speeds. Instead the "cogging" vibration is taken up by the flex joint, transmission/differential gear lash and clutch springs (or torque converter). The result is more than just throbbing and vibrations, along the way this "cogging" is putting higher peak loads on the gears and fatiguing springs and rubber.
You might not notice a drive shaft misalignment, or be able to identify it as such. If the input, intermediate and output shafts are all in line, the U-joint alignment doesn't matter. And the e30 drive train design puts them nominally in line. (For good reason: even well-paired U-joints will be changing the speed of the intermediate shaft. This is also a reason to keep the drive shafts as light as possible.)
My experience
I had a misaligned drive shaft from clutch work done just before I bought my e30. I had a vibration that was most pronounced when cold. I initially attributed it to bad wheel balance, then flat-spotted tires. I though I had tracked it down when I discovered one brake rotor (ATE brand!) had non-parallel hat surfaces. After I replaced the rotor I then moved on to blaming the vibration on the resulting out-of-round wear on the tires.
I never considered the drive shaft, and the heat shields would have prevented checking it anyway. I only noticed it while doing some transmission work, and even then had to ponder if I was really seeing what I thought. The paint dots finally convinced me that the shaft was about 30 degrees misaligned, and that I should trust my first impression and realign it.
Realigning the U-joints turned out to be easy, once I worked out the right technique. The goal is to have the yokes on the intermediate shaft in the same plane. The splines are fine enough that the paint dots only get the setting approximately correct.
I found it easiest to rotate a rear wheel until the rear U joint was exactly vertical and horizontal. You can use a level, but you might have to visually line it up if you can't reach the sides of the rear U joint. Using the level on the front U-joint yoke, or just lining up with the car body, insert the front half of the drive shaft.
Visually compare the alignment, looking down the driveshaft and using the level. Now comes the key procedure: the check. Carefully disconnect the shaft and rotate only one spline in each direction. (I found moving a single spline was only possible when a helper held the rear shaft steady while I very gently wiggled the front free. Otherwise the splines bound up and released suddenly, causing me to lose my position.) Check that you can tell that the alignment is incorrect.