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If I use the synthetic fluid do I still need to add the friction additive from Ford?
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It depends. Probably. But maybe not. Some say that if you buy Royal Purple brand synthetic 75w140, you won't need to add any friction modifier. Some say the same thing about Amsoil. But others disagree.
Mine has the Motorcraft brand, and it definitely needs the friction modifier. And I'm pretty sure the Mobil 1 does too.
When you buy friction modifier, hold your nose and buy the MOPAR brand from the Dodge Boys. That's the good stuff. Even Ford told their techs to use the MOPAR brand of friction modifier a few years ago when Ford was having quality control problems with their own brand. Ford probably has their problems fixed by now, but I'd still buy the MOPAR brand of friction modifier. [img]/ubbthreads/images/graemlins/shrug.gif[/img]
But if it were me, regardless of brand of lube and friction modifier, I'd do it the hard way to be certain. Add the lube but without friction modifier, but leave room to add a few ounces of friction modifier. Then do the figure 8 test. If you get the bump-bump-clatter-clatter-chatter from the rear end (caused by one tire dragging instead of rolling around a sharp turn) then add an ounce of friction modifier, drive about 5 miles to mix it up good, then do the figure 8 test again. Repete as necessary until the clatter goes away. Some say you probably need about 4 ounces. Others say up to 8 ounces. But too much makes your limited slip worthless, so I wouldn't add more than one ounce at a time.
After you are satisfied with the lack of chatter, then top off the diff with lube and button it up.
The figure 8 test is to find a big paved parking lot that has good traction - not covered with sand, gravel, snow, ice, or even wet with water. Then roll down the windows and drive in a figure 8 pattern, listening and feeling for the clatter.
Ford says the diff cover gasket is silicon seal (Permatex high-temp RTV gasket maker = click here
). Note that Permatex makes several different similar products, so get the right one - the one shown in that link. Black, high temp.
To remove the cover, remove all but one bolt at the top. Loosen that bolt but don't remove it. Then use a putty knife or flat-blade screw driver and break the seal at the bottom, then pull the cover off enough that the old lube will drain into your big drain pan.
To reinstall the diff cover, here's the words from the workshop manual:
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CAUTION: Clean both flat surfaces (differential housing and differential cover) with a suitable solvent to remove all traces of oil film.
CAUTION: Install the differential cover within 15 minutes of applying the silicone or it will be necessary to remove and reapply new sealant.
Apply a continuous bead of sealant of the specified thickness to the differential housing cover. (The specified thickness is about 1/8th to 3/16th inch bead. The photo of the Ford 10.5" diff cover in my earlier post shows how to place the bead of RTV on the diff cover.)
Place 2 bolts into the differential housing cover at the 8 o'clock and 2 o'clock positions. Install the differential housing cover on the differential housing. (In other words, don't mess up your nice even bead of RTV when you stick the diff cover onto the diff.
Install the remaining bolts. Tighten the bolts alternately and evenly. Tighten to 47 Nm (35 lb-ft).
Allow a 1 hour cure time before filling the axle with the correct amount of specified lubricant.
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Note: Some old fogies with grease under their fingernails insist that you use a real gasket instead of that newfangled RTV stuff that the Ford techs use. You can buy a diff cover gasket for your Dana 80 at most auto parts stores.