Light surface rust on diff internals - problem or not?

[BogMonster]

My 300Tdi Defender is getting a pair of Ashcroft ATB diff centres in the near future. Many moons ago, when the front diff was nearly new, it broke the pin in the centre, and at that point I fitted an ex Camel Trophy 4 pin HD diff which is still there today.

The old front diff, which at the time had probably done less than 10k miles, was sprayed up with WD40 and put under a rag in the back of the garage for storage. My plan for some time has been to dig it out and fit the ATB centre to the original diff, then refit it to the vehicle, as it is nearly new in mileage terms.

I was a bit surprised when I located it the other day to find that there was light surface rust in quite a few places, including on some bits of the crownwheel. However, it seems to have cleaned off OK with more WD40 and some light wire brushing, and though there is some very light pitting on the outer face of the crownwheel, the gear teeth appear to have cleaned up perfectly.

The question is this: am I setting myself up here for a diff that will whine like crazy, or should it be OK? The ATB came with new side bearings already pressed onto it (less to buy them than to get a garage to change over the old ones), so that should be no problem, and the pinion and the pinion bearings appear to be fine with no rust. I can only assume the rag covering it absorbed some moisture over the years and where it was touching the diff surfaces there was some corrosion, as it's only on one part of the crownwheel.

I'd be interested in any thoughts. I think it will probably be fine?

[AMB]

For what my inexperienced opinion is worth, I agree that it will probably be fine. 

A light pitting may even improve issues.  Many years ago, it was practice amongst some engine tuners to leave newly machined parts outside to rust, before lightly rubbing down and fitting.  The theory ran that rusting would remove any rough edges/spikes because they were likely to rust first.  The remaining light pitting would tend to retain oil far better than a smoother surface and thus reduce wear.  On the advice of an experienced tuner, I tried it on my first bike (a D7 Bantam) and a couple of later bikes.  Have no idea whether it extended life, but certainly no related issues whilst I owned them

[cackshifter]

If the CWP teeth and pinion bearings are OK, that's all the moving bits you are using after all, and the diff mounts OK against the crown wheel  I can't see how rust on anything else could affect it.

[FridgeFreezer]

I'd be tempted to fit a magnetic drain or fill plug just to catch any bits, give it the best chance in life. Can attest to the utterly OTT strength of Nige's ones