Originally Posted by andrewakessler
Perhaps this is a strange question:
I own a 48 Chevy Pickup with a G-Body clip. As I come closer to installing my engine and transmission onto the frame, I'm wondering if there is a preferred method for laying out the drivetrain.
I currently have the rear axle position fixed. Should I use this as a general starting point, or should I also set the engine to the desired placement within the engine bay and worry about the driveshaft length at the end of the process?
Obviously the engine only has a certain amount of play within the engine bay, and I know there's a certain drivetrain geometry that must be followed, but I wasn't sure if there is a time-tested smart way to do this.
First, position the truck at the attitude that it will have when running down the road. (level, slight rake, down in the back a little or whatever). If you are planning to lower the truck later, remove the tires and block the whole mess up at the exact attitude it will have when completed. Level the whole mess left to right and front to rear. Assuming using a carburetor, bolt the intake manifold to the motor. Position the motor in the truck with the carburetor mounting pad exactly level left to right and front to back. This will angle the transmission down at an approximate 3 degree angle as related to the ground. You can move the motor/trans forward, rearward, left or right within the engine bay to position it in relation to other components that you want to miss (power brake booster can, steering, etc.) I recall that some OEM mopars had the motor moved 3 inches toward the passenger side from center in order for the valve cover to miss the brake booster can. If you move the motor/trans around like that to find the ideal nesting place, pay attention to what the transmission output shaft casting is doing. Is it up against the floor or a crossmember? Just pay attention.
Once you have nailed the position of the motor/trans and have insured that the centerline of the crankshaft is 90 degrees from the centerline of the differential, you can worry about the attitude of the differential. Since the motor/trans is installed at ~3 degrees down-bubble, the differential pinion shaft should be angled at ~3 degrees up-bubble in order to align the two. I usually back off a little on the differential bubble, like positioning it at ~1 or ~2 degrees up bubble. It is my feeling that when toddling down the road, the torque from the motor will make the pinion climb the ring gear a little and make up the difference. You'll get quite a bit of differential rotation with leaf springs, not so much with a 4-link or 4-bar and very little with ladder bars. This routine assumes a daily driver vehicle, not a hard-launching drag race truck. If that's the case, then you need to begin thinking down-bubble on the pinion.
Don't even think about a driveshaft until the whole mess is done and sitting on the ground on it's own. Then, with the truck sittin on it's own, at the attitude you have finalized, take a yoke and push it into the splines of the transmission output shaft. This operation is best done on a drive-on lift (yeah, you have to push the truck onto the lift) where you have room to move around under the truck. Push the yoke onto the output shaft as far as it will go, then pull it out 3/4". Now, measure between the centerline of the trans yoke and the centerline of the pinion yoke. That will be the dimension you give the shop to make your driveshaft.