i took out the plug to check for filings in the rear differential and found some attatched to the magnet. Not too much really, but enough to make the grunge around it partly silver coloured. How much (if any) is too much? i also dipped my finger in as deep as i could go, and the fluid was not discoloured, nor was there any detectable filings down there. i have approximately 38,000 km on the truck. Should i change the fluid? or are some filings normal?

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Black 98 F150 XL
2WD 5spd 4.6L Triton Engine
Reg. Cab Short Box
Air, Sliding Rear Window,
(K&N, Tonneau etc. coming soon)

 

That is why there is a magnet there, some small amount is normal, but no chips of steel that looks like it may at one time have been gears. Get another "pencil type" magnet and insert it further into the bottom and see what is there. I have removed many differentials and almost all had some metal specks, these were for gear ratio replacements

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2000 F-150, XLT, SC, 5.4, 4X4, 355 LS, ORP, Black/Silver, class III, single CD, '93 Harley Fatboy Black & tons of chrome

 

cool, thanks for the info. just one more question. What colour is the fluid supposed to be? and what colour whould indicate a bad sign? (i'm assuming dark, but that's just a guess)

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Black 98 F150 XL Sportgroup
2WD 5spd 4.6L Triton V8
Reg. Cab Short Box 3.08
Air, Sliding Rear Window,
Boxliner,

 

fluid should be dark and make sure it does not smell like it's burnt

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2000 F-150, XLT, SC, 5.4, 4X4, 355 LS, ORP, Black/Silver, class III, single CD, sliding glass, med. gray, keyless, elect. shift, engine date 8-18-99, '93 Harley Fatboy Black & tons of chrome

 

Defacto,
I agree, it's normal, I found the same in my
diff. I would suggest changing the fluid and cleaning the bottom of the diff housing. Any metal can eventually be detremental. Besides, fluid is easily cheaper than a new set of bearings!!

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yeah, that's true. The Haynes Repair Manual says to change it every 24 months or 30,000 miles. Plus, if i change it now, i can find out what is deep down in the dif (hopefully filings only, not chunks!)

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Black 98 F150 XL Sportgroup
2WD 5spd 4.6L Triton V8
Reg. Cab Short Box 3.08
Air, Sliding Rear Window,
Boxliner,

 

When it's brand new, the "filings" can be fairly large. These are usually junk that's leftover from manufacturing process. After the first oil change, the filings should be very fine. This is from normal wear of mating surfaces.