I bought one of these Magnetic Bore Laser Sight to help with alignment of the transmission to the differential. Still pondering on the options for how best to solve my issue.

Edited Wednesday at 12:30 AM3 days by inline6

22 hours ago, inline6 said:

I backed off the bolts securing the transmission mount (Techno Toys Tuning) to the body to see what kind of change in angle would result. Picture of the mount I have (for use with the 240SX transmission) in the early Z:

Do you have a Nissan factory transmission mount to try? Maybe you're fcoused on the wrong member. The T3 mount is the only "non-factory" part.

Good thought. I do. I will have a look at it to compare.

Ok, I took a measurement on the factory transmission crossmember. The crossmember has a "drop" of 5/8". So, when the crossmember is bolted into place, the top surface that the transmission isolator is 5/8" lower than the ends. The Techno Toys Tuning has the same amount of drop. The difference is that the hole for the isolator is shifted back also - it is not exactly in line with the transmission crossmember mounting holes like the stock one.

It seems that all is well up front. It is possible that the 240SX transmission locates in a slightly different position vertically than the stock 4 speed. However, as I discovered when loosening the transmission crossmember bolts enough to drop the transmission about a 1/4" or so, the angle of the output shaft of the transmission didn't change much. It went from about 89 degrees to about 88.5.

Turning my attention to the rear differential isolator, it is easy to see that there is an angle "built into it". While somewhat difficult to see in this picture while installed in the car, if you look closely at the vertical plate of the isolator and compare to the straight edge, you might be able to tell that the top of the vertical plate is closer to the straight edge than the bottom.

Removing the isolator from the car, I confirmed that part number of 55415-E4102. This one was replaced new by the prior owner of the car and remained unused other than supporting the diff in the car for about 28 years.

Outside the car, it is easier to see the amount of incline built into the part. I measured the angle on the cardboard surface and got .3. So, 3.6 minus .3 is 3.3 degrees.

Ok... Why? One would have to assume that there is something different about my car. Too much angle on the rear section of the driveshaft which is causing a driveline vibration - it would not be a possibility that the engineers got this wrong, would it?

And, now that I have been through all of this, I am fairly certain that I have been here and done this with my track car before. I believe I ended up modifying the differential isolator to reduce this angle for that car as well.

I am very interested to see if this issue (too much upward incline at the front of the differential relative to the output of the transmission) is present in unmodified pre-72 model year cars.

Edited 8 hours ago8 hr by inline6

I'm not saying you're wrong about the vibration source, but many years ago, a friend had a '56 Ford pickup that someone had slopped a 302 ci engine into. The engine was visibly out of alignment both vertically and horizontally, and it ate u-joints every six months, but it never vibrated at speed.

I hear ya. I guess since I now know I have something that isn't right, I feel I need to fix it.

Working on it today, I found some interesting things. One, I was thinking would be a problem, and it is. The front yoke of my driveshaft had some corrosion on it. I tried to clean it up as best I could and was hoping that when I sent the driveshaft off to be shortened and balanced, that the part where the seal rides would be ok. This black goo in the first pic is the rear transmission seal wearing at an accelerated rate. In the second picture, I have removed the yoke shield, and you can see where the seal was contacting at the two points (front and rear edges of the seal) on the shaft. In the third picture, you see where the yoke has been touching the transmission tail shaft inner bushing. That should not be, and could be indicative of another problem. Or, it could be evidence that the driveshaft is not happen and vibrating around because of the improper u joint angularity. This is after 500 miles.

I received a new yoke this week from ZCarDepot. I am going to transfer the yoke shield over to the new one (two spot welds) and then take the parts to a local driveshaft shop. Given where the "wear" marks are from the tail shaft bushing on the old yoke input shaft (kind of centered front to back), and that I didn't have any problems with rubbing, I think the propeller shaft assembly length is ok. But, it barely fits between the transmission and the differential. When installing it, I have to bottom it out inside the transmission and rotate it to a particular position before the rear will go onto the front pinion flange. Once there it pulls back just a bit and the two mating surfaces contact.

As far as how I am going to deal with the excessive angle at the pinion flange, I am open to ideas. The front of the differential points upwards noticeably when mounted in the car. I think I will need to focus my efforts on modifying the front differential/suspension crossmember, and/or front isolator. I have a spare front differential/suspension crossmember, and a spare isolator (55415-N4301). I could flip it around 180 degrees and cut and weld on it to remove the incline...

And, I could cut the top surface of the differential/suspension crossmember where the isolator sits to lower it a bit.

Going to do other stuff for now and wait for opinions and think on it some more.

Edited 4 hours ago4 hr by inline6

You could look at the RT mount and customize the diff angle