Rear end whine

[Fingolfin]

Hi all,
Earlier this month I experienced the joy of a snapped halfshaft for the first time! Luckily I had a spare. I had been running a 4.55 differential, so I took this "opportunity" to change the diff for the 3.9 in preparation for the new engine (1275, which is not yet fitted - still running the 948 for the moment).

All assembled, it functions perfectly well, but now I have a fairly loud whine from the back, only under acceleration (it stops instantly as soon as the pedal is released) and only audible above about 25-30 miles per hour (louder in proportion to speed).

Am I correct in guessing the machine shop didn't get the diff pinion-to-gear clearance correct, and the whine is from the new differential? What else could it be?

[philthehill]

Did the machine shop completely overhaul the diff assy or just change the diff?

It is imperative that the crown wheel and pinion teeth are correctly meshed otherwise the assy will whine.

If the replacement diff was S/H it may be that the diff was already worn and whining before you purchased/fitted it.

[Declan_Burns]

Fin,
You have a problem which is not easily rectifiable by a home based mechanic. There are new 3.9 crown wheel and pinions on the market at the moment which were commissioned by a company called Bastuck here in Germany a couple of years back. They are distributed all over and this could be linked to your problem. They were made in Turkey and they got their tolerances wrong. I fitted a set on my car and could not achieve the required backlash between the crown wheel and pinion. I had to shift all the shims on carrier away from the ring gear to get it to fit. The backlash is only about 1°. I really need to take it back out and turn it down a couple of thou so that the backlash be increased. It does whine slightly when warm on acceleration but getting better as it beds in.
If you want to tackle it yourself read through this.
It's the Bible when setting up differentials.
http://www.pirate4x4.com/tech/billavista/Gear_Setup/
Regards
Declan

[Fingolfin]

Thanks for your replies, gents. The 3.9 came to me from eBay, and had been used and not taken care of after its service life - it arrived with rusty crunchy bearings, some rust on crown gear, etc. The machine shop changed the bearings and crush spacer and seal, and I believe they also changed the "cross pin" (no. 24 at Moss, https://mossmotors.com/mg-midget-austin ... rear-axles), because something very similar in appearance was in the box when it was returned to me. They're a supposed British car specialist, but who knows in this part of the world. I do not believe the pinion and crown gear were changed, but, like Phil said, they could have been in poor shape to begin with..

[Declan_Burns]

Fin,
Either the CWP was beyond it's service life as Phil has mentioned or the machine shop set it up wrong-or both.
You could take it out and check the tooth pattern as described in the link above. I find artists yellow oil paint better than engineer's blue for this purpose. It should give an indication of what is wrong although under no load conditions on the bench.
The cost of the Moss CWP in your link is over the moon. A new CWP here costs 247€.
http://shop.bastuck2.de/index.php?&cat= ... =1&lang=en

But as I mentioned above I was not happy with their tolerances and the set up mathematics (and stampings on the diff) as given in the workshop manual no longer apply. I would think that Moss are selling the same CWP's.
Regards
Declan

[philthehill]

Whilst the garage/engineers who undertook the differential overhaul may be British car specialists they may not be transmission specialists.
The diff overhaul should have been placed in the hands of transmission specialists who are familiar with the 'A' Series diff.
Whilst the crown wheel and pinion can be set up using tooth marking to get the interface correct it does take someone with years of experience of doing the job to get it right.
The person overhauling the diff also needs access to shims to enable the pinion to be placed correctly fore and aft in the diff housing.
Whilst the crush spacer seems to be the only spacer currently available my preferred spacer is the solid non crushable spacer with the requisite/appropriate shims.
The crush spacer can be easily overtightened which can then place excess loading on the pinion bearings which can also lead to bearing whine.
To sum up - in most cases it is better to buy a good serviceable unit in the first place even if it costs more than to have a cheaper unit which requires professional work being carried out on it to bring it back to serviceable standards..
Phil

[Fingolfin]

Phil, I was assured by the supposed specialists that a differential was entirely within their capability - and in fact I had them replace the pinion seal on my 4.55 diff years ago, and it's been perfectly fine. Remember I'm in the United States. "Good serviceable units" are rare here and very few can tell the difference between them, though I probably had better check with the American MG forums to be sure...

Declan, your link was a very interesting read! I only used Moss for their parts diagram - I don't, as a rule, purchase from them. A supplier here in the States has a new gear-and-pinion set (http://www.spridgetmania.com/part/BTA12 ... te--Midget) but I haven't got $500 to spend, and it may have been sourced from the Turkish place anyhow. (And Mania isn't a supplier I like to buy from either - horribly overpriced.)

Probably I will reinstall the 4.55 for the time being. Not ideal for the 1275, but I may not get it in before winter anyway.

[philthehill]

Replacing a diff pinion oil seal and completely overhauling a diff assy is as chalk is to cheese.
If few can tell the difference then it is all the more important to either get a good used but serviceable unit or a use a reputable transmission engineering company.
Do you have a 4.22 diff you can use as an interim solution?
As regards 'A' Series spare especially for the 1275cc Midget/Sprite - many of the 'e' bay listings are in North America so there does appear to a surplus of used spares over with you.
They may be a bit more expensive than over here but the variety is envious.
One of my last NOS purchases for a 1275cc 'A' Series came from the USA as the part is no longer available over here.
Phil