I replaced the factory 3.91 ratio '741 gearset with a Richmond 4.10 ratio '742 gearset, but gear oil continues to 'vent'..grrh!

Since the factory calls for 4 pints I used 1.8L, which is 4 pints, which is 2 quarts. But so far having driven around for about 500 miles I keep on watching the axle vent 'percolate' and dump excess gear oil out. It is worse when I put on some highway miles. I do not baby the darn thing, my last time out was an hour long run @ 120 KM/h (75 M/h) which meant the motor was turning a good 4K. The center section temp never went over 75degC (165-170F), so I think I have been OK there.

Given that Richmond recommends changing the original gear oil out at the 500 mile 'break-in' mark I emptied the stuff out with the suction pump. I measured a total of 1.6L, which is about 3.4 pint (1.7 quart), granted this way of draining it is far from precise.

Alright, so any advice just how much I should re-fill with?

It took me some time to wipe down the fuel tank, suspension pieces (shocks mostly)...just a nuisance, but the factory 2 quart fill was too much (that included the SG additive). So I'm thinking I need a tad less, obviously I do not want to go too low (is there a known too-low value?).

Is the gear oil foaming?

If it is hitting the shocks & slinging all over,it sounds like it may be a bad pinion seal.

mopars4ever: I do not think so, when I pulled off of the highway on that last run (straight into the gas station...LOL) I noticed a small puddle under the axle as I went to check up on it, the puddle had no air-bubbles, and let me clarify, it is not a 'geyser' type of a thing either, just a slow but steady weep

Alaskan_TA: nope, pinion seal all good, installed a new seal actually, sealing up nice between the lip and yoke as well as the seal body and carrier housing, everything up-front is nice and dry, the gear oil is definitely coming out of the axle vent

What gear oil did you use? additive brand?

Last time I had a puddle under an 8.75, it was the pumpkin nuts not tight enough. But your deal sounds like it's coming out of the vent, right?
I just fill 'em to the sight plug. Never had a vent spew gear oil.

Ditto on filling to the site plug. You can also extend the vent by adding a hose to it like on a 4 x 4 truck.

Agreed. I always have filled to bottom level of the plug and never had an issue.

I generally put the SG additive in first, then fill to the bottom of the fill plug, as with others, no issues. Let the gear oil jug set in the warm sun for an hour or two. If winter time, get a big pot of very hot water to warm the gear oil jug, it pumps easier that way,

I would fill it when it has sat over night to the bottom of the filler hole and then when you driven for 30+ minutes on the road to get the gear oil hot and then bring it back and remove the filler plug with a catch pan underneath it and let it tell you what it like to have in the pumkin hot
I have ran a hose off the vent hole using a NPT pipe nipple to give the gear oil somewhere to drip back into the housing, I mounted it up near the top of the gas tank
A lot of 4X4 pickups come with a breather hose setup like that

With a typical vent tube you can pop the top off and add the hose that others have mentioned.
It works well that way...

Gear oil: Canadian Tire (equivalent to AdvancedAuto/CarQuest) house brand, 85W140

Additive: purchased from Dr.Diff when I bought the new clutch discs, actually a Ford Motorcraft part 'Additive Friction Modfier', 4 fl oz (118 mL) although I'm not sure of the actual part#.

topside: yeah, the gear oil is visibly coming out of the vent tube, everything else is dry.

Everyone: thanks for the advice, I will take the route of adding the hose on top of the vent as I am very uncomfortable going to a lower level of fluid. I did just as you guys have done, that being, getting my mix ready (gear oil & additive), filling the pumpkin then checking the level, which was about 1/2" below the plug.

A post I read somewhere on-line was making a point about the higher (numerically) gearsets actually having shallower teeth on the ring gear (due to the requirement of having a higher count of the teeth), this means that the ring gear base actually has more metal, which then displaces a significant amount of the gear oil and requires a lower fill volume. No numbers were given though, so I figured I would follow the factory directions first and go from there.

Ultimately, as long as the gear oil was not draining out of the sight plug I should have been OK.

If you where checking it hot your correct, if cold 1/4 to 1/2 inch down works well

Well you guys, I replaced the factory vent with a spare one which does not have that built-in cap, then tossed a 4" long hose on there, secured with a hose clamp. Took the car for a local ride, came back home to check things, nice and dry!!!

I actually stuck a long screwdriver down the hose to see if by any chance I could measure how far into the hose the gear oil shut-up, but it was dry, which was to be expected given that the replacement vent bolt does not have the OEM cap on it and any gear oil that would get normally tossed out would simply drain right back in. So I'm thinking that with the stock vent and high (numerically) gearing the gear oil must be getting churned enough to simply force it's way out past the regular vent.

We were tight on time, Woodward pre-cruise was next on our schedule, so we headed out. Some more highway cruising (higher speed - 75 MPH) and a pile of driving later and it is still all dry under there.

Next time out I will do the HOT gear oil check because all my checking so far has been done during initial fill and latest re-fill, all cold.

Thank you all, great advice!!!

D360, you are mistaken that more gear oil is better for the differential. So rigging up a system to allow yourself to run the gearbox overfull is backwards thinking.

The fill plug was designed into the casting by the guys who designed the differential. They put it at the right place.

Cold, you fill the diff with the pinion level to where you can stick your finger in and just feel the oil below the threaded hole. That'd be about a half inch down. When it gets warm it should just start running into the threads.

That is all the oil that's needed for the gears to live. Adding more oil than the optimum amount may cause foaming and if you drive long enough you'll find out that having foam trying to lubricate hypoid gears isn't a good idea.

Because your gear oil contains an EP additive that sticks to the gear teeth the surfaces will be protected, but it's better to rely on liquid lubrication rather than the surface lube provided by the Extreme Pressure chemicals.

R.

Hmm...??? As best as I can tell nowhere did I say I was looking for a way to put MORE gear oil then the factory spec'ed. In fact, right from the start I was aiming for the factory fill 4 pint mark.



I completely agree with you on this. I do not want the gear oil to start foaming since that will certainly provider much less protection then an oil film. After all, foaming oil is simply an air bubble, and well all know that does not lubricate well.

Thank you for pointing out the flaw in such an approach, but it was never my goal to overfill.

Mopar drag race engineers use to recommend in the old drag race seminars running 1 quart of gear oil in drag race only cars, Dana 60 or 8 3/4
They swore that was all that was needed for lubrication, that the factory ran the additional gear oil in them for noise control