I tacked up some engine mounts last week (although one weld subsequently snapped - so I am back to square one) but before it broke I did the following...

1.Mounted the diff.
2. Chopped a piece of wood to act as a dummy prop shaft
3. Used a t square to mark off the angle the prop 'should' have followed if it were perfectly aligned from the diff input flange and the gearbox output flange
4. Compared to the actual centreline that the 'dummy prop' was following

Without a super sharp pencil - and trying to hold 3 things at once - it looked as though the alignment was out at both ends - and in both the vertical and horizontal planes.

Horizontally - i.e. left to right in the car - it was roughly 3 degrees at each end - the gearbox effectively pointing more towards the o/s side rear wheel than it was towards the diff - by my calculation the prop would be about 20mm off centre at one end if it were a rigid tube.

Vertically - i.e. up and down in the car - it was roughly 2 degrees out at each end.

The diff input flange is vertically square - as is the gearbox output shaft - as far as I can tell

How does this compare to anyone elses setup? I know the uj's are there to take out this discrepancy - but I want to be starting from the right ballpark...

[Edited on 11/12/13 by jps]

I assume the piece of wood was straight to start with?

It's hard to picture what you've got from your description, but ideally:

* the engine and gearbox should be parallel to the fore-and-aft centre-line of the car, but not necessarily exactly over it.
* the face of the diff should be at 90 degrees to that centre-line.
* the centre-line of the gearbox driveshaft should not be exactly in line with the centre-line of the diff flange - it should be two or three inches offset (the shape of the diff will usually facilitate this). Having them exactly in line is likely to lead to excessive wear of the UJs (called 'brinelling', if I've spelt that correctly!).

Maybe the following image will help...



[Edited on 11/12/13 by David Jenkins]

quote:

---

Originally posted by daveb666
I assume the piece of wood was straight to start with?

---

quote:

---

Originally posted by David Jenkins
It's hard to picture what you've got from your description, but ideally:

* the engine and gearbox should be parallel to the fore-and-aft centre-line of the car, but not necessarily exactly over it.
* the face of the diff should be at 90 degrees to that centre-line.
* the centre-line of the gearbox driveshaft should not be exactly in line with the centre-line of the diff flange - it should be two or three inches offset (the shape of the diff will usually facilitate this). Having them exactly in line is likely to lead to excessive wear of the UJs (called 'brinelling', if I've spelt that correctly!).

Maybe the following image will help...



[Edited on 11/12/13 by David Jenkins]

---

why do we all still use UJs instead of CVs for props, given this problem can be tricky to avoid or eliminate completely?

quote:

---

Originally posted by Grimsdale
why do we all still use UJs instead of CVs for props, given this problem can be tricky to avoid or eliminate completely?

---

I think i've had a 'light bulb' moment - thanks to your post David - and because of the limits to how far apart I can actually get the components.

Just to check - I need to worry less about the actual alignment between the output/input (apart from making sure it isn't 100% aligned!).

If I ensure both diff input and gearbox output are parallel to a centre line in the horizontal and perpendicular in the vertical - the alignment will be OK?

quote:

---

Originally posted by jps
If I ensure both diff input and gearbox output are parallel to a centre line in the horizontal and perpendicular in the vertical - the alignment will be OK?

---

quote:

---

Originally posted by Grimsdale
why do we all still use UJs instead of CVs for props, given this problem can be tricky to avoid or eliminate completely?

---

The diff flange and gearbox output shaft should be parallel (hard to measure but a digital angle finder on the bellhousing joint is a good place) with each other, which you have. But in my understanding either the horizontal or vertical (but not both) should line up. If both are out of alignment (which you have) the prop shaft will move in and out of the tail shaft on each revolution. Leading to vibration, tail shaft oil seal and UJ failure. Now I may be wrong and talking a load of BS as it's only what I've picked up on the interweb so hopefully someone else will have a more expert opinion.

Doesn't matter, it's only a single angle from the driveshafts POV.

Something that is equally as important as lining the flanges up is to ensure that the yokes are in the same plain at either end. Just make sure that the sliding part hasn't been removed at sometime and put back at 90 degrees....As for the comment about using CV joints U/Js have been used since day one and as a commercial vehicle engineer I have seen U/J s do hundreds of thousands of miles without failure.....