Best Rear Differential Service Method

[7.3 on 40s]

Just noticed a rear diff. cover seal leak when i got home today. In the past on all the trucks i have had when servicing the rear diff. i would pull the cover, clean sealant surfaces, spray out inside of diff. with brake clean, wait for brake clean to dry out, apply gray rtv to diff., press on and bolt down cover, then fill until fluid comes out of fill plug.

I have a 97 PSD and wondering if i should use this same method. I assume the diff. has no gasket just sealant. Whats the fluid type? 75-90 or 140? Synthetic or non synthetic?

[Diesel_brad]

Your service procedure is fine.

Just add one more step.
After the fluid is drained reach into the bottom of the housing. there are IIRC 2 voids that trap dirt and oil. Scoop that out, then rinse and scoop out again.

I use a gasket, but that is just me. Something else to do while you have the cover off is put on a 08+ alumn diff cover. Will not rot out and helps cool the oil

O for fluid, use 75w-140

[7.3 on 40s]

I would rather use a gasket as well. Ford/Napa makes a gasket for it? I was not aware they did. When you say VOID can you explain a little better. I dont understand.

[jermcd]

I bought my fluid on eBay. Saved a lot buying in bulk. Valvoline full synthetic.

[backwoodsboy]

NAPA sells a Fel-Pro gasket for ~$7.
There are two 'valleys' or 'grooves' at the bottom of the diff housing. I believe this is what brad is referring to.
If you have a LS diff make sure to use a friction modifier.

[Diesel_brad]

Originally Posted by 7.3 on 40s

I would rather use a gasket as well. Ford/Napa makes a gasket for it? I was not aware they did. When you say VOID can you explain a little better. I dont understand.

Napa sells a victor gasket
Advance I think sells felpro

As for the void, It is just a low spot that collects oil and dirt. The lip that the cover bolts to will not let the oil run out(kinda like a dam). You cant miss it. Just pull the cover and reach in the housing

[7.3 on 40s]

Ok i understand. Thanks. Ill def be getting the gasket when i do the service. Also it sounds like from what you all are saying 75-140 synthetic gear lube is what fluid i need. And use an LS additive if i have LS. What can i do to check for it just lift the axle and in neutral see if both wheels spin the same direction? Same as any LS diff right?

[Diesel_brad]

no need to use friction modify. All synthetic gear oil already has it built in and that is what these axles call for

[jcorkin]

i just did mine and used 80w-90 and no friction modifier, should i be adding friction modifier or is it already in it? its advance auto parts brand 80w-90, how big of a deal is it to run it without the fm? sorry for the thread jack....

[backwoodsboy]

Originally Posted by Diesel_brad

no need to use friction modify. All synthetic gear oil already has it built in and that is what these axles call for

Where are you getting this from?

Unless the bottle specifically says it has the friction modifier included, you will have to add this separately. Synthetic or dino.