Thread: Blew my Diff .... :(

And no good knowing the names if you dont know what they look like. If you knew what they look like, you would not be asking IMO.

Google image: Rover diff, P38 diff and Sals 8ha diff. Then compare to what you have in your driveway

Originally Posted by mike_beecham

So a 2010 Defender has a P38 diff? Same as an 02 then?
I am just chasing definative answer - as no good having a diff l can fit!!!!

Mike

If your diff has an inspection plate on the rear of the housing, held on by series of bolts then it's a Salisbury.

If no inspection plate then it's a P38(110) or the Rover(90).

A 2010 diff centre will be a p38(110) or Rover(90), so interchangeable with corresponding model.

Originally Posted by mike_beecham

Hi,

I have a 110 Wagon. My mechanic is fairly sure it is a bearing - but hasnt pulled diff apart.

So P38/Rover are interchangeable..?
I have sourced a diff from a 2010 Defender, 3.5 ratio (l assume that is standard). It is an LSD also - would that be right??

Thanks again,
Mike

If it's a standard factory item it won't be fitted with an LSD.

Originally Posted by Bush65

…. For anyone to suggest that a rover diff has remotely similar strength to a Salisbury diff is ludicrous.


IMO, a suitable match for a Land Rover is somewhere in the middle ground between the rover diff and Salisbury diff.


The P38 diff should have been that, but someone stuffed up the design.


What Ashcroft have done to remedy that, will suit many people. IMHO, given 24 spline half shafts being the weak link, it is not unreasonable to suggest/argue that a P38 style axle assembly upgraded with the Ashcroft diff, will match the Salisbury axle assembly strength…..

John, given that the arguement was based on the diff alone, I still think it a big call to say the new HDP38 stronger than a Sals.

I am in no way saying it a bad or negative thing, this HDP38. I think its great, but I do like fact rather than fiction.

Originally Posted by mike_beecham

So a 2010 Defender has a P38 diff? Same as an 02 then?
I am just chasing definative answer - as no good having a diff l can fit!!!!

Mike

A 2010 110 will have a P38 rear diff. An 02 110 may have a P38 or it may have a Salisbury (can't remember the exact change over timing but it was around tgat time).

As others have said, if your rear diff has a bolt on rear inspection plate then it's a Sals, simple as that.

If your replacement P38 is LSD, then it has an aftermarket centre fitted, possibly an Ashcroft ATB or a Truetrac? Either way it's a good thing.

Thanks all. I definately dont have a Sals rear diff. I realise this. I just wasnt sure if it was P38 or Rover.

P38 it is!

Thank you to all!!!!

Mike

Originally Posted by mike_beecham

Thanks all. I definately dont have a Sals rear diff. I realise this. I just wasnt sure if it was P38 or Rover.

P38 it is!

Thank you to all!!!!

Mike

And for future reference, you have a Rover front diff.

I'd be interested in a few pics of the failed internals if you get the chance.

Originally Posted by uninformed

John, given that the arguement was based on the diff alone, I still think it a big call to say the new HDP38 stronger than a Sals.

I am in no way saying it a bad or negative thing, this HDP38. I think its great, but I do like fact rather than fiction.

If you only compare the two diffs given:

• similar material and heat treatment of the gears (a reasonable assumption)
• similar precision in the gear cutting and assembly (a reasonable assumption)
• similar stiffness/rigidity of the gears and their supports, so as to maintain proper alignment between the mating teeth

then the diff with smaller gears will not be as strong.

With material for gears the surface hardness of the teeth has the most influence on both strength and wear, providing the core material has sufficient toughness to support the hard case. This is why strong shaft materials like 4340 are not good gear materials. High performance gears are made from case hardening steels, which can have much higher surface hardness. Another problem with through hardening steels is distortion during heat treatment. High precision of the tooth profile is critical to gear strength/wear and distortion is unacceptable. The tooth profile of high performance gears are finished in special gear grinding machines after case hardening.

Gear tooth strength is based upon a large number of factors, but the dominant one is, the allowable fatigue stress in the fillet radius at the root of the tooth, vs the actual stress at the root fillet.

The allowable stress is determined from the material, the actual stress is determined from:

• the tooth load
• thickness of the tooth at the root
• the stress raiser caused by the root fillet

The 'tangential tooth load' is a function of torque and gear pitch diameter. 'Real tooth load' also involves pressure angle and helix angle of the tooth, but for this discussion, I'll stay with tangential tooth load.

The general equation for torque is load x radius and from this the tangential tooth load is:

Tooth load = 2 x applied torque / pitch diameter

So clearly increasing the pitch diameter reduces the tooth load proportionally.

Thus for the same torque load, the clearly larger diameter ring gear (AKA crown wheel) and clearly larger diameter side gears used in a Salisbury diff will experience lower tooth loads than those in the P38 or HDP38 diff.

Ignoring the fillet stress raiser, the stress at the root of the gear tooth is a function of the load and the squared thickness of the tooth at the root. Because it is squared the tooth thickness has a great affect on the tooth strength.

The Salisbury diff has larger gear teeth than either the P38 or HDP38, made possible because of their larger diameter.

So even if the tooth loads were the same the larger teeth will have much less stress at the root. But when we account for the lower tooth load for the Salisbury, the stress goes down even further.

Note the Ashcroft HDP38 ring and pinion have larger teeth than the stock P38, and they also adjusted the pressure ratio on either side of the teeth to increase the root thickness even further.

The full strength of gear teeth can only be realised if the load is distributed uniformly across the width of the teeth. However the tooth loads, tangential and 'separating' will lead to deflection and loss of alignment, which is why you usually see teeth broken at one side. This is where the stock P38 design is very poor.

Ashcroft greatly rectified those design deficiencies as well as increasing the tooth strength with the HDP38. I expect time will show this diff will perform well in 110 and 130 Defenders, and the weakness will shift to the strength of the 24 spline half shafts, as it is with a Salisbury.

IMHO the HDP38 diff is a good upgrade for those with a P38 style diff, offering advantages over a Salisbury swap.