Differential Wear Query

[AlfaLofa]

I've obtained a spare 3.9:1 diff unit (nose only).

(Using the Osprey book "How to Restore Gearboxes and Axles" as my guide) the diff appears to be in good condition with no marks that I can see on any of the teeth.

What it has got though, is slight movement of the pinion before rotational movement is transmitted to the crown wheel.

I would estimate that at the maximum movement at the perimeter of the diff flange is perhaps 1mm before the crown wheel turns.

So my question is - is this normal?

And am I correct in thinking that a slight tightening of the pre-load will eliminate the movement by removing float at the bearings?

I make this assumption (which is probably wrong) after reading this article I found on the web:
http://bernardembden.com/xjs/diff/index.htm

It's about a Jag diff but the techniques are relevant.

[types26/36]

If I understand your problem you want to adjust the backlash of the crown wheel to pinion.
By tightening the pinion nut and increasing the preload on the pinion bearings it will not reduce the backlash of the crown wheel, it will only preload the pinion bearings more and probably cause them to fail.
To reduce the backlash, the crown wheel would have to be moved deeper into mesh with the pinion by adjusting the crown wheel via the carrier side adjusting rings.
I don't think backlash can be determined by just rotating the pinion by hand, a DTI needs to be attached and the backlash measured on the crown wheel.

[Quart Meg Miles]

Correct, Brian, and RTFM (Read The Freaking Manual),

Your link to the Jag item shows what has to be done to the pinion and it is easier on the bench. I could PM or Email pictures and descriptions of how I did it though I had the crown-wheel out and, be assured, that pinion nut takes a lot of wrenching and in only tiny movements.

The backlash sounds wide at 1mm at the flange and you'll need two dial indicators and some other stuff to do the job as described in the manual. But sort the pinion preload first.

Specialists charge ?350 to set up a diff. If you are going to move things about perhaps you should replace the bearings? I did, but I had more than a whine, probably due to overtightening the pinion 20 years previously when I wasn't so careful.

[AlfaLofa]

Brian, Meg,

Thanks for the feedback - but I have no intention of attempting anything myself.

I was simply trying to find out what was normal or abnormal - or perhaps what is correct, acceptable and unacceptable.

As the diff is a spare it can wait.

[john.p.clegg]

Steve

Shame because with a couple of spanners and a dial gauge,bearing and gasket set and the section from the Lotus manual you could do it yourself,it's not hard,even I did my last one (fitted a quaife LSD).and it's nice and quiet and satisfying..

John

[AlfaLofa]

John - perhaps you have given me the inspiration I need

I've obviously got the manual and the spanners - and probably every other tool I need except for a dial gauge.

Can you advise me re the gauges and mounts that I would need?

My knowledge and use of dial gauges is zero!

[john.p.clegg]

Steve
Don't remember much about it,just that i followed the manual/instructions I got with the bearing/gasket set and was/am quite pleased with the result..

John

[Quart Meg Miles]

PM on its way to you, Steve.

Edit: PS Are you sure your new pinion flange is the same as your existing one? They did change at some point after my car was made but don't know whether all 3.9s were the old pattern. I think the later ones were bigger diameter as well as different hole spacing; someone will know.

[AlfaLofa]

The flange has the early PCD of 2.75" - so will fit my car.