[watermouse]

I thought I was on a bit of a roll with my diff tinkering. I have collected all the bits to swap both rear and front of my Gv from 4.8 to 5.1.
This post concerns only the rear.
I found the 5.1 vit rear diff a while back and got around to checking it out about a month ago. The backlash was a little higher than spec so I adjusted it in and checked the pattern. I had a guy check my work and he said the pattern was ok but I should increase the backlash back to what it was, so I did.(about 0.20mm. spec is 0.13 to 0.18)
other than this I didn't touch anything else and it seemed smooth if a little light on the preloads but as I would have expected for a used diff.

I finally got around to throwing it into the car on Tuesday. The swap was straight forward enough (apart from getting the axles out, what a bitch!). Everything went back together fine. The old oil and diff were in very good condition.
I drove it home but noticed pretty quickly a constant ssshhhhhh sound. By the time I got home it was pretty noticable. By the time I got back to work the next day (50km round trip) there was noticeable whine but not pulsing like a chipped tooth. There was no way I was going to live with that so it went back on the hoist.
I dropped the oil out and it was filthy. Full of fine metal "filings" but no chunks. Expletives were issued.
The magnetic diff bung:


inside the housing


I decided to pull the diff down to find what might be grinding away. all the wearing surfaces looked dull and/or scored


Looking at the pinion it looks like the pinion teeth are engaging very low, near the bearing

At this stage I had resolved to replace all the bearings so I cut the cages off to try and find maybe a disintegrated race. They all looked dull (from the filings I presume) and there was a tiny bit of pitting on the pinion bearing inner races but no obvious bearing failure.

So I am a bit stumped. I don't know if I should try and rebuild it if the pinion and crown are going to keep munching on each other. Will the scoring on the pinion and crown wheel be an issue if i do rebuild it?
Does anyone have an idea why they would in the first place as I didn't have this diff apart (like I did with the front). No pinion height was changed or pinion preloads adjusted.

Any feedback welcome as I need to make a decision if I should throw the 4.8 back in or try and fix this one.

[Built4thrashing]

Was the R&P a matching set?

[watermouse]

I can only presume so. I didn't disassemble the diff before I put it in and it didn't look like it had ever been disassembled before.

[watermouse]

Looks like I am going to chicken out on this diff until I can find another one. The only thing that seemed to be wearing and causing the filings was the crown and pinion. I don't understand why. It looked fine before going in but I don't think I can take the risk with it unless someone has an idea.

[watermouse]

Update on this: I had to get the car moving so the 4.8 went back in last week, no hassles there.
I finally spoke to the diff guru today. He had a look at the parts on my bench and was a bit dumbfounded until he asked me what oil I put in the diff "You didn't put synthetic in it, did you?".
I said I put in some Penrite 80W 140 synthetic diff oil. I showed him the container.
He said thats what it looked like, that it wasn't getting lubrication. He explained that if it was running mineral oil before you should run mineral, that synthetics often don't mix well with mineral oils. The original oil residue on the diff would be enough to cause some grief and that you may see some foaming of the oil. (which I did on pulling it out)

Now I don't know this guys history that well other than he sets up diffs for track cars amongst others and has done so for about 40 years, so he claims (he is certainly old enough and he came recommended).
But the skepto-meter was running at about an 8. I had never heard of a synthetic/mineral incompatibility issue.

Is that legit advice?
What about semi synthetic engine oils or using a full synthetic oil after a mineral engine oil?

His final advice was find another diff, this ones rooted.

[shandy92]

Ive heard something similar to that before but thought it was more to do with seals, maybe ring a reputable drive train shop and ask them?

[fordem]

Just my $0.02 - I've replaced the transmission/transfercase/differential fluids of all my vehicles with Mobil One 75W90 synthetic and had no issues whatsoever - I have no idea what came out, but I'm pretty certain that neither Suzuki nor Mitsubishi used synthetics back in the 90's when those vehicles were built.

On one vehicle I went from Mobil One to a conventional 75w90 and then back to the Mobil One - again with no issues - although I can report that it shifts smoother with the synthetic.

I've also switched back & forth between conventional motor oil, full synthetic & synthetic blends on one vehicle trying to figure out if there was a correlation between oil breakdown & consumption rate (the consumption rate appeared to increase dramatically after the oil had been in the engine for around 2500 miles - the apparent increase in consumption turned out to be caused by the shape of the oil pan, which caused the level on the dipstick to drop rapidly). As there appeared to be no advantage to be gained from the additional cost of the synthetic, I went back to a conventional oil with no issues.

It's worth noting that most synthetic oils are not actually synthetic, they simply use a more highly refined mineral base oil - if you want the details on this, google Mobil Castrol suit - there was no lawsuit, but, you'll find the details of why Castrol (and others) can market their mineral based oils as full synthetic.

[watermouse]

Fordem, yes, that was my general understanding of "synthetic" oils too. In any case I don't think the oil I used was the main issue, if at all. I just can't see it being that catastrophic in 50km of light footed motoring.
Shandy, I will take your good advice

The diff must have been on the way out or I made an error adjusting the backlash in (my call) and then back out (on his advice). I should have measured it again before pulling the diff down but it didn't feel noticeably more or less than when I installed it.