[NEVER ENOUGH]

Phasing is a term that describes the alignment of the single-cardan joints on opposite ends of the drive shaft. As discussed above, a single-cardan (or u-) joint does not rotate at a constant velocity if the operating angle is non-zero. The drive shaft speeds up and slows down slightly as it rotates due to the nature of the joint. One way to reduce this is to make sure the joints at each end of the drive shaft are aligned properly. If the yokes on each end of the shaft line up with each other, as seen in the figure below:

If the shaft is ever taken apart, it is important to get it re-aligned properly when it is re-assembled. One way to do this is to mark both sides of the slip yoke. However, you should check that the joints really do align properly, don't assume they are. The reason for the phasing is that the speed variation of the joint is related to its operating angle and its angle of rotation. In order to get the most effective cancellation, the joint yokes *must* be aligned exactly with each other and the operating angles must be identical. Any variation in either angle will show up as un-cancelled vibration, which will get worse at higher speeds