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Technical 33 dodge rear gear fluid type

Discussion in 'The Hokey Ass Message Board' started by Sactownog, Aug 2, 2019.

  1. Sactownog
    Joined: Jan 19, 2018
    Posts: 249

    from SAN DIEGO

    what type of gear fluid should be used? I rebuilt my 33 dodge rear end that has 4:37 gears. was going to put 85-140 or 80-90 weight. whats best?
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 43,445


    What does the owner's manual say?
    ClayMart likes this.
  3. Sactownog
    Joined: Jan 19, 2018
    Posts: 249

    from SAN DIEGO

    GOING WITH LUCAS 85-140.
  4. abe lugo
    Joined: Nov 8, 2002
    Posts: 2,110

    abe lugo

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  5. Well, you get points for trying. :rolleyes: Probably could've saved as few bucks and used a 80-90 conventional hypoid gear lube. Being a '33 Dodge it might not have even needed to be hypoid rated.
  6. tb33anda3rd
    Joined: Oct 8, 2010
    Posts: 15,110

    1. Connecticut HAMB'ers

    I can't tell you for sure but the last few "older" gear boxes/axles I have filled required GL1 90wt gear MINERAL oil. because, I was told, it is better for the bronze parts. I was told when in doubt this is a safe bet.
    @Blues4U maybe can explain?
  7. Blues4U
    Joined: Oct 1, 2015
    Posts: 3,816

    from So Cal

    Thanks for the tag @tb33anda3rd . So automotive gear oils come in 3 different API service cateroies, GL-1, GL-4 & GL-5. GL-1 is basically straight mineral oil, no additives. GL-4 has additives to protect against wear, rust & corrosion, and reduce foam. GL-5 has additional levels of extreme pressure (EP) additives to provide protection against wear under severe sliding conditions present under heavily loaded hypoid gear sets. Hypoid gear sets in differentials are the ring & pinion gears where the pinion is offset from the center of the ring gear, as in the image below (A = hypoid, C = non-hypoid or spiral bevel):
    So if the differential does not have hypoid gears it can use GL-4 spec oil. Reasons not to use GL-5 is the EP additives can be aggressive against any yellow metals in the differential (if there are any). They also stink, and will coat the interior surfaces of the diff. If there are no yellow metals you can run GL-5 in a non-hypoid differential.

    The next issue is viscosity.

    Common viscosity grades for gear oil are:
    75W, 80W, 85W, 90 & 140. These can be single grades, i.e. SAE 90; or multi-viscosity grades, i.e. 80W-90. The multi-grades have to meet specs for 2 fluids, or rather they have to flow well enough at cold temps to meet the specs of the W grade, and they have to maintain viscosity well enough to meet the specs of the higher number grade.
    So according to this chart, an 80W-90 has to be no thicker than 150,000 centipoise at -26*F. And it has to be at least 13.5 centistokes at 100C. Straight 90 grade only has to be between 13.5 cSt & 24 cSt at 100c, there is no cold temp flow requirements.

    So which viscosity grade should you use in your car's differential? There is a rule of thumb regarding viscosity of lubricants, for slow moving heavily loaded parts use a higher viscosity; for faster moving, lower loads, use a lower viscosity. Also, in consideration of fuel economy benefits, or for maximum efficiency & speed, lower viscosity will provide better results. For most cars, 80W-90 is the most common, that will provide good flow under colder temps while still protecting at high temps. For heavy trucks we often use 85W-140 because of the slower speeds and heavier loads.

    Then there is the question about synthetics that always comes up. Synthetic oils have naturally higher viscosity index, meaning the viscosity doesn't change as much with changes in temperature, so typical synthetic grades are 75W-90 and 80W-140. You get better flow at cold temps, while maintaining good viscosity at elevated temps.

    With all that said, I'd go with a GL-4 gear oil in 80W-90 viscosity grade. Or, to make it easy to find, we often use NAPA as examples around here, so here, you can get this GL-4 75W-90 in liter bottles from NAPA:

    Or a lot of guys here like Driven oils, here's their version of a GL-4 80W90 from Amazon:

    Just look around, it's not too hard to find.
  8. dirty old man
    Joined: Feb 2, 2008
    Posts: 8,193

    dirty old man

    Totally agree with Blues 4U. Just adding that Summit also carries a GL4 once branded as Brad Penn, now Penn Grade.
    Blues4U likes this.

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