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Rear Differential Fluid
#1
Good Morning Everyone, I have a 73 Mach 1 with the 3.25 standard axle ratio rear differential and am curious on what type of fluid to add an do I have to use any additives with it. Thanks-

I’m not a man of faith, but I have faith in the Coast Guard-
HMS
Bounty Survivor, November 2012
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#2
No additives unless it has traction lock. Standard 80w90 gear oil will get the job done. I prefer Valvoline or Royal Purple.
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#3
RacerX;158506 Wrote:No additives unless it has traction lock. Standard 80w90 gear oil will get the job done. I prefer Valvoline or Royal Purple.

How do I know if I have traction lock? thx

I’m not a man of faith, but I have faith in the Coast Guard-
HMS
Bounty Survivor, November 2012
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#4
Jack up the rear end. If one wheel spins while trans is in gear, no traction lock.
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#5
How do you know it is 'standard axle ratio'?
Marti report, door tag or differential tag?
Standard axle is one wheel driven, no need for
additive. Then again, a previous owner could
have changed the diff. My diff has been changed
three times since I first owned the car so door tags
and Marti out the window.
Like RacerX said, only way to tell for sure.

mike
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#6
I agree with goodnigh. On my old F-150, I swapped my 8.8 open diff for a Eaton Posi. Only way to tell is get the rear end off the ground, and with the parking off, turn the rear wheels. If they turn in opposite directions, or if one wheel turns while the car is in park, then the diff is open.

Regardless, if you go with Royal Purple gear oil, you should be just fine. Just my opinion, mind you.

Ron
Rusty, a 1973 Mach 1, needs a lot of work.
Billy, a 1976 Ford Bronco, also needs a lot of work.
El Guapo, a 1986 F150, frame-off Resto-Mod.
Bubba, my 1994 F150, daily driver
Formerly, a 1973 Ford Mustang Coupe - a work in progress, then a car-b-qued banana.
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#7
I'm a fan of Mobil1 synthetic gear oils. They're easy to find anywhere and there's a marked improvement in fuel economy in a lot of cases.

1971 Mustang fastback: 10.3:1 C90E 408W hydroller - CDAN4 EEC-V w/EDIS8, girdled, lowered and caged
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#8
I had this experience this past fall:


Bill73Ragtop;151118 Wrote:Just posting an experience I recently had with my Trac Loc rear end. Two years ago I had new clutches installed and a basic overall performed on my 3.25 Trac Lok rear end. It had been sitting for about 10 yrs and was slipping when I removed it.

We soaked the clutches in synthetic oil (75W-90) prior to the rebuild and installed the same grade with limited slip additive to fill the rear end. From the very beginning it groaned, moaned, popped and skipped. If I did a series of figure eights in a parking lot it would quiet down and run smooth for about 2 miles. After about 700 miles, it never improved, in fact it was getting worse.

A month ago I pulled out the center section to drain the oil and check for any debris (such as pieces of fiber clutch disk) found it was clean so re-installed using conventional rear end oil (90W - 140W) with 8 oz of limited slip additive. As I backed the car out of the garage, the difference was already apparent. Within one mile the unit was performing perfectly.

I've since learned from several other local folks that they have never had good luck with synthetic in the older style clutch units.

BTW, I also have a 69 vert with an Eaton Tru-Trac which uses the synthetic (75w - 90) with no problems whatsoever.
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#9
Bill73Ragtop;158796 Wrote:I had this experience this past fall:


Bill73Ragtop;151118 Wrote:Just posting an experience I recently had with my Trac Loc rear end. Two years ago I had new clutches installed and a basic overall performed on my 3.25 Trac Lok rear end. It had been sitting for about 10 yrs and was slipping when I removed it.

We soaked the clutches in synthetic oil (75W-90) prior to the rebuild and installed the same grade with limited slip additive to fill the rear end. From the very beginning it groaned, moaned, popped and skipped. If I did a series of figure eights in a parking lot it would quiet down and run smooth for about 2 miles. After about 700 miles, it never improved, in fact it was getting worse.

A month ago I pulled out the center section to drain the oil and check for any debris (such as pieces of fiber clutch disk) found it was clean so re-installed using conventional rear end oil (90W - 140W) with 8 oz of limited slip additive. As I backed the car out of the garage, the difference was already apparent. Within one mile the unit was performing perfectly.

I've since learned from several other local folks that they have never had good luck with synthetic in the older style clutch units.

BTW, I also have a 69 vert with an Eaton Tru-Trac which uses the synthetic (75w - 90) with no problems whatsoever.

I should probably have mentioned that I haven't used a stock style limited slip since 1995. I have a Detroit Trutrac in 2 of my vehicles, but before I did a major rearend upgrade on my '89 5.0 Mustang I had no troubles using Mobil1 75-90LS gear oil for over 10 years. The LS designation has some limited slip additive already in it.

1971 Mustang fastback: 10.3:1 C90E 408W hydroller - CDAN4 EEC-V w/EDIS8, girdled, lowered and caged
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