Questions about shortening a drive shaft

[Reno Speedster]

I am working on my 26 speedster project and the only thing i need to do before assembling the rear end is shorten the drive shaft (I am running a Muncie auxiliary transmission and special drop brackets that move the axle behind the rear spring)

I have been doing a lot of thinking about how to do this so the center of the universal joint is aligned to the pivot point formed by the ball joint at the back of the transmission. I was planning on assembling the rear end, properly setting the lash with the ring gear and then somehow marking the point where the shaft sticks out of the drive shaft bushing. This mark would be used to cut the shaft down to the correct length (planning to take it to the driveline shop to be cut down), drill the pin hole, mount the U joint and be done.

But I keep thinking that this might not actually put the pivot point of the universal joint in line with the pivot point of the ball joint. At the same time, I keep wondering how important this really is. It seems like properly setting the ring gear lash and any variations in the length of the drive shaft bushing or the propeller shaft itself would lead to a misalignment between these to pivot points.

So, how important is it to get these pivot points aligned? Am I over thinking this? Is there a better way to do this?

[Mark Gregush]

Thinking out loud: Guess what you could do is once the drive shaft is cut and machined, assemble and install the rearend before drilling the hole for the rivet then rotate the driveshaft mounted to transmission, letting it find its center, mark where the hole needs to be. Drill and install the rivet and call it good.
Or you could just measure an unmolested driveshaft and drill the hole in the same place on the shortened one. Thats what I did and seemed to work fine.

[RajoRacer]

According to some "older" published material on accessory transmissions, for the Muncie, the d.s. should be shortened by 13 & 1/4".

[Reno Speedster]

The problem is that I have the Muncie and the lowering brackets, which means that the standard Muncie shortening length will not work.

[speedytinc]

Build the D.S. first. (if using stock parts) You will fit the gear clearance to the D.S. as a built unit moving the carrier center for proper gear mesh.
If a bit tight, a shim gasket can be added to the D.S. spool @ the rear end housing.
The pinion gear is a fixed entity. Proper gear mesh is achieved by moving the carrier/ring gear into or away from the pinion gear.

The U-joint pin should be centered in the holes provided for pinning.
Expect to remove some material from the front bushing To get the pin near centered.
Slide your U-joint into position & drill thru it into the new, modified shaft. I would then disassemble to drill the pin thru hole in a mill.
Reassemble & pin. If the D.S. turns freely, set the pin. If not, slightly face the bushing till it does.
You see the U-joint has a certain position as confirmed by the holes for pinning.

[Allan]

John's advice re setting the pinion gear mesh is correct, but contrary to accepted recommendations. Setting the backlash between the ring gear and pinion by shimming the driveshaft in or out will not necessarily get the correct full face engagement between the gears. That is done by shifting the diff centre within the housing , as advised. The backlash takes care of itself when the mesh is set correctly.

Allan from down under.