Diff Mounting and Knocking

[Davidb]

I thought I would re-open this thread rather than start another (and to prove that I did actually do some research!)

I have had the occasional clunk from the back of my S2 Elan since I rebuilt it five years or so ago-not serious until last Saturday when I was returning from a Lotus Club of BC run (having demoralised a couple of Elises ) when I started to experience "driveline shunt" I think it is called. Quite severe jerking in the drive when on and off the throttle.

I just spent some time under the car and have concluded that the differential mount centre steel bushes are too long! It is impossible to tighten the two vertical bolts sufficiently to compress the rubber mounts. I have played with washers top and bottom but have realised that Rohan's comment that sometimes replacement mounts are simply too long is the problem that I am encountering.

The steel bushes in these mounts are 1.6125" long-can anyone give me the measurement for an original or a known, working replacement? Many thanks.

[oldelanman]

The steel bushes in these mounts are 1.6125" long-can anyone give me the measurement for an original or a known, working replacement?

Just dug out the old original mounts from my S4 and the inner steel bush measures 1.620" so I don't think that's your problem. It could just be that your mounts need replacing. If the lower end of your steel bush protrudes beyond the face of the rubber it may be an indication that the mounts have started to fail.

[Davidb]

Many thanks for the quick response Oldelanman.

The mounts look very good- I wonder if it is something else?

The large washers that go under the mounts and above the diff housing-do they have the same size hole as the bolt or is it larger to accommodate the bushing? If it were larger it would take up the extra length it appears to me.
(I do have a diff brace in place).

[Davidb]

I should add some more info:

I have had very little clearance under the diff since I rebuilt the car and this has always concerned me. I get the occasional clonk but it is not a significant.

I run the ride height lower than stock so if I can raise the diff it would be an advantage. I have sliding spline driveshafts fitted during the cars racing career in the sixties.

I accept Rohan's statement that the length of the sleeve inside the diff mount determines the height of the diff.

However, the bolts tighten up with no compression of the rubber mounts-the sleeves are taking up all the available space-the rubber mounts still look like new...

[oldelanman]

The large washers that go under the mounts and above the diff housing-do they have the same size hole as the bolt or is it larger to accommodate the bushing? If it were larger it would take up the extra length it appears to me.

All of the large spacer washers which sandwich the mount are identical and fit the shank of the bolt so the inner steel bush should be clamped between the upper and lower washers.

If your diff has been contacting the chassis there are a number of possible causes ....

Too many spacer washers between the diff and the mount - with a diff brace fitted you may not need a washer at all.
Deformation of the chassis under the diff due to careless jacking.
Loose tie rods and/or failed bushes allowing the diff to rock under accel/decel
Failed diff mounts - over time the rubber compresses due to the weight of the diff and allows the diff to drop, this will show up as a gap between the bottom rubber face of the mount and the large spacer washer. The rubber of the mount may appear to be in good condition but, as said previously, the inner steel bush will protrude at the bottom. This may be what people observe as the inner steel bush being too long perhaps?
Photos show one of my old mounts.

Hope this helps.

[Davidb]

Thanks again Oldelanman for your quick response.

The chassis under the diff is mostly horizontal but the flat section is crowned very slightly at the closest point-old jacking damage as you say. The chassis is straight however so I can live with that. The chassis also has the so-called 26R mods so is quite heavily reinforced.

The torque rod bushings are all good and tight and the forward torque rod mounts are heavily reinforced-very bad design as they were originally!

I have shortened the sleeve in the one upper mount from 1.6125" to 1.540". This was easy to do with a bench grinder without touching the surrounding rubber "ring". I feel I have nothing to lose and this is an experiment. I will fit new ones if this doesn't work!

I will now do the other side and will report back after I have test driven the car.