8 replies to this topic

[smithy3793]

So last night I found that my diff was F****D!

Would this mean that my gearbox is ruined too?

 

I am in two minds whether to have the diff replaced/the gearbox rebuilt.

Or do I just buy a recon engine and box and replace?

 

Any advice is appreciated

[Dusky]

If the diff broke and the shrapnell went round the engine I could see a full rebuild comming... Strip and inspect is the key. :)

[Spider]

I've seen countless Diffs break (big wheel Mokes eat them), many I've done myself

 

In all but one, the cases cracked on the centre web and many others a hole was punched through the diff cap.

[Retroman]

Got to ask What makes you think your diff has gone...?

 

Having traveled some of the bush roads in Auz its no surprise mokespider has come across plenty...

 

and vandalized a few diffs himself...hhaha

 

As spider says when they go bang its spectacular

[racingbob]

do they seem to worse on mpi Cooper sport as my mates went is it the big wheels òr 2.7 dif. once you get to 70000 I would be getting worried

[nicklouse]

it is the 2.7 FD

[Ethel]

Why would the final drive kill the diff, I'd expect the opposite from the ratio/torque effect? I accept it's more prone to killing itself because of the teeth n' pressure angle.

[Retroman]

The Mini gearbox was never designed to take the forces and stress involved when you get to the higher ratio diffs.

I think you are correct Ethel that you would expect the torque effect to be less, but I think its the fact that with the torque on tap it struggles to pull that ratio and instead of turning things quickly it breaks something...?

  The mechanical leverage makes higher ratios more difficult to turn. So you need more torque to accelerate at the same rate as a lower diff....if you see what I mean...its harder and slower to accelerate in 4th than 3rd, and the final drive is like all the other gears in any gearbox, just a constant ratio.

That's why you get poor acceleration with diff ratios higher than 3:1 and they are more prone to breaking, big wheels make it worse.

   With low ratio diffs the leverage ratio is there to help things accelerate quicker by turning easier, hence more fun, acceleration and wheelspin potential.

The Cooper S diff ratios were an option with both standard and close ratio boxes...

 

970 and 1071 ;  3.76 standard with the options of  3.93  4.13 and 4.27

 

1275 ;  3.44 standard with the options of 3.76  3.93 and 4.13

[Spider]

I have found that the taller Ratios (3.1, 3.44 etc) are much kinder than the lower Ratios.
 
This I feels is for several reasons, but before going down that path, I have also found that there's many types of diff failures and sometimes through different causes.
 
I think a common one for Minis is the Diff Pin wearing, along with the Planet Gears and Thrusts. This is down to a lack of surface hardness of the Pins and a lubrication issue. As the diffs turn over faster and faster, so the Oil is centrifuged out of it and due to it's shape, as is spins, it basically make a cavity through the oil. Some Oil does get in, but not nearly enough.
 
The cruddy thrusts that are fitted to the Output Shafts I can't help but see as anything other than someone idea of a joke.The early cars had Brass Thrusts and these are what I fit when I rebuild them. Having run out of them, I now cut my own. I think the standard fibre ones disintegrate in around 5000 km in a Moke and a little long in a Mini.
 
In most ways, a taller FD Ratio here, helps make them last. A Taller FD will also transmit less torque in to these components.
 
Another type of failure is loosing Teeth off the Crown Wheel and / or Pinion, or even splitting the Pinions in to several pieces. The shorter Ratios (4.X:1) are the worst for doing this due in part to the small amount of material between the internal spline and the root of the teeth, but also due to the large difference in size between the gears, ie, the Crown Wheel want's to 'run over' the Pinion. The general softness in the Gearcase here also doesn't help, along with the increased torque multiplication via the ratios in this range.
 
Lastly, cracked cases, in the centre web is another case. I've seen these go due to one of two reasons, one being that the case cracks due to weakness (particularly with the DAM 2886, 3220 a 4818 series of cases) and / or from a Broken Tooth getting between the CW & P.
 
These last two are almost inter-twined. I've seen cases cracked and the CW & P are fine and likewise, I've seen broken teeth and the case is fine, though, this is rare, but one happening almost certainly leads to the other in very short order.
 
Big sticky wheels are a killer too as are the increased rolling diameter of the Big Wheel Mokes.

In summery here, I've found that taller diffs are far less like to fail than shorter ones.