Clunking Differential - play/ backlash?

[greyV8pete]

Re: Clunking Differential - play/ backlash?

Update: Had a look today and the oil is low!

Looking for a source of a leak I found the rubber boots around the cv joints are full of oil (should just be grease, right?) and there are a couple of tears where it can seep out.

The paperwork I have shows the diff oil was done in 2016 which was only a couple of hundred miles ago.

I’ve ordered a Sierra Haynes.

Does anyone think if I replace the seals going to the cv joint (not sure how feasible that is), then I might just be lucky enough to not have to source a new diff?

There is no path for diff oil to reach the inside of the cv boots. The diff oil seal runs against the outside of the metal “can” that contains the inner tripode joint. The rubber boot is clipped around the can and the drive shaft so it completely and separately contains the grease. CV grease is black and runny so may look like oil. Diff hypoid oil has a distinctive smell. How did you check the oil level? If using an improvised dipstick (bent wire or finger) what colour was the oil? Were there any small shiny metal flakes in it? If so that means something is seriously worn inside.

Sorry to say that in my opinion, with both the clonk and the scraping noise you mentioned earlier, the diff is u/s. As you need to remove it to replace it I would take it out and get the back cover off and all will be revealed. Fit all four new rubber boots while the drive shafts are out. Search this forum for how to do that without trying to dismantle them. Peter.

[Peterx]

Re: Clunking Differential - play/ backlash?

Totally agree with Peter's analysis above but if you really have got oil (instead of grease) in your CV joints then that could explain the 'scraping' noise.

[charlh94]

Re: Clunking Differential - play/ backlash?

Ah I didn’t realise there wasn’t a path! Can’t see any other source of a leak so can only presume it wasn’t filled properly or the oil came out years ago.

I first tried the check the level with an Allen key, but the oil was lower than it could reach. So in the end I got a long screwdriver in at an angle that touched the oil so hard to workout where the level is other than lower than it should be!

The magnet on the plug definitely had some filings in the sludge: https://drive.google.com/file/d/1ZC8...w?usp=drivesdk

[kahawi]

Re: Clunking Differential - play/ backlash?

Given that there is no drain plug, what is the collective opinion on top-up vs drain and refill? Our diff’s probably haven’t done many miles, but the oil is going to be at least twenty years old.

What’s the best option:-

1. In the car with some kind of vacuum:
2. Loosen the end plate and drain:
3. or remove the whole diff assembly?

Cheers, Colin B

[greyV8pete]

Re: Clunking Differential - play/ backlash?

Given that there is no drain plug, what is the collective opinion on top-up vs drain and refill? Our diff’s probably haven’t done many miles, but the oil is going to be at least twenty years old.

What’s the best option:-

1. A) In the car with some kind of vacuum:
2. B) Loosen the end plate and drain:
3. C) or remove the whole diff assembly?

Cheers, Colin B

Option 1. I bought a jumbo syringe thing on ebay years ago. Use a long piece of clear plastic tubing so you can get right to the bottom. You will be able to replace about 90% of the oil like that. You can always repeat the process after a few hundred miles to further purge the old stuff.

Option 2. Not an option on the Cabrio. There isn't enough space unless you take the fuel tank out. Don't forget that the crownwheel protrudes beyond the joint face, so you have to get the cover past that. Also having oil still dripping out as you try and remake the seal isnt a good idea.

Option 3. Not unless you really need to for other reasons.
Peter.

[charlh94]

Re: Clunking Differential - play/ backlash?

Hi all, finally found the time to look back into this diff issue! Currently I have the exhaust off which gave me enough room to get the cover plate off so I now know it’s a 3.38 ratio.
I’m now ready to get a new diff. But struggling to find one, seems to mainly be 3.62.
Any suggestions for where to look?

[milliemarlin]

Re: Clunking Differential - play/ backlash?

Will it make much difference?

With your current 3.38 the engine revs at 30 will be 1,456 and 2,912 at 60mph (depending on wheels and tyres). For comparison with a 3.62 diff, 30 will be 1,560 and 3,119rpm at 60. Only about 200rpm higher at 60 mph. Slightly less noisy but the power of the engine should be fine in the light weight car.

I have been told a 3.9 is best for my Cabrio although I am quite happy with the 3.62.
Paul

[chris_cussen]

Re: Clunking Differential - play/ backlash?

If you are thinking of a refurbished diff then Rally Design do 3.9 and 3.6 diffs. https://www.rallydesign.co.uk/index.php?cPath=1783 £495+VAT

I've a 3.6 diff with a bit of backlash lying by my shed. You are welcome to have that and get it refurbished yourself. I did have a quote from a gearbox company of £150 + parts for the job.

[charlh94]

Re: Clunking Differential - play/ backlash?

I have thought the same, just wanted to play it safe.

It’s a 3.5L RV8 with a bw65 box (Can’t remember tyre size right now) so not sure if anyone knows what diff ratio works well with this?

I just worry that it will be revving high when cruising, but I suppose the added acceleration would be enjoyable ahah!

[greyV8pete]

Re: Clunking Differential - play/ backlash?

I have a 3.38 diff fitted now in my 3.9i V8 Hunter with 205/55 x16 Toyo tyres. It gives good cruising revs but plenty of acceleration when needed. 1st gear is only needed on uphill starts. There is so much torque low down in a V8 you just don’t need to rev them. If you start changing diff ratios you then have issues matching the speedo to read correctly!

[milliemarlin]

Re: Clunking Differential - play/ backlash?

...It’s a 3.5L RV8...

With that engine I’m not even sure you need a gearbox!

With a 3.9 diff 60mph will give 3,361rpm in fourth. But I don’t think you need the extra oomph. The 3.9 recommendation was given to me to match the Pinto engine I had at the time. Pinto output was 66 kW (89 hp) and 140 Nm (103 lb⋅ft). At cruising speed I find the engine is mostly drowned out by the wind noise.

The Sierra diff has a good reputation for being strong so should not be problem in any ratio. Other V8 owners may be able confirm this and recommend ratio choice.
Paul

[charlh94]

Re: Clunking Differential - play/ backlash?

Thanks Chris! I’ll have a bit more of a look and bare this in mind. Do you know what gearbox company it was?

[charlh94]

Re: Clunking Differential - play/ backlash?

If you start changing diff ratios you then have issues matching the speedo to read correctly!

Good point! Would definitely rather play it safe and keep a 3.38 ratio!

[chris_cussen]

Re: Clunking Differential - play/ backlash?

The place I contacted was TES transmissions http://www.testransmissions.co.uk/ in Westbury.
I used them to rebuild a gearbox for a Fiesta ST

If you have an electronic Speedo then you can re-calibrate it for the new diff ratio.

[dave_kitson]

Re: Clunking Differential - play/ backlash?

Hi - Welcome to the club! There have been a few threads on this over the years. Put 'Cabrio diff' into the advanced search box in the top right corner and see what you can find. Also see FAQs at the top of this forum for comments on wheels/tyres.

My Cabrio runs a tuned Rover 3.5 V8, about 200bhp through a Rover 5-speed and Sierra 3.92 on the 'standard' wheels and tyres. At 70mph in 5th it does an indicated 2800rpm. Not excessive to my mind, but if you want better economy, go for a lower ratio diff. But if you want economy, you shouldn't be running a Rover V8!!

I've just looked on ebay, and there are a few Sierra diffs, no idea if they are any good, but they are supposed to be pretty tough. Mine whines at about 50mph, not uncommon apparently, easily cured by going faster. I would stick to whatever ratio you have, unless you want to recalibrate the speedo. Or unless you are changing wheels/tyres, but see FAQs. I remember that the build manual suggested filling the D-shaped rubber doughnut with Sikaflex 221 to make it stiffer - may even be possible to repair it this way, at least temporarily, if that is the source of your clunk. Use clingfilm to retain the filler till it goes off.

If memory serves, the oil level in the diff should be 5mm below the filler hole - check with Haynes manual. For a dipstick, bend a piece of rod at right angles and put grooves in the dipping end at 5mm intervals, by gripping gently with side cutters and turning. Whatever you do, don't drop it in the diff!
Hope that helps - Dave