Originally posted by biglar
Techinspector1, I'm unsure as to why it's necessary to line the engine trans with the centerline of the car. Seems like aiming it at the diff would give less angle to the u-joints. I'm not arguing, and haven't thought about that before, but I'm curious. There's a real possiblilty I'll run into just that problem with my Model A.

Lar.
The included angle of the two joints (front and rear combined) must be at zero to prevent vibration and premature failure.

If you have, for instance, zero lateral angle on the front joint and 5 degrees lateral angle on the rear joint, then the two joints are out of phase. If, again, for instance, you have the front joint at a lateral angle of 2 1/2 degrees and the rear joint at a lateral, but opposite 2 1/2 degrees, then you have a zero included angle and everything goes down the road happy.