New shoes, but not sure of the look of the spare

[Charles Talbert]

Well, thanks for the explanation. I've been trying to calculate the difference between the two tire sizes in question. The tire calculators I've tried using have trouble with non-metric sizes. But I'm going to be generous in saying they amount to about 100 revolutions per mile difference, max. You are saying that these differentials can't handle that in the short-term? Interesting...

What are you recommending as a gear lube these days? I know you've mentioned your preference in the past, I just don't recall.

C.D.

We use Synergen 80-90 gear lube in the units we rebuild.

Actually I've never relied heavily on calculations in these situations simply because to me seeing is believing, I'm old fashioned, OK. Realistically, I have no way of knowing how much wear was already present in the units we've seen that failed before tires of uneven height became part of the picture. I just know what I saw when the units were torn down & have some insight as to how long uneven tires were run simply based on the owners statements. How long it would take for a unit to fail if it were built to new specs & installed into the uneven tire height situation I don't know. I do know the life of the unit would be significantly reduced. Being realistic once again, most of these trucks never have the diff's gone into unless some issue has warranted such action. This being said, I wouldn't run the risk of doing something that I know will cause high $$$ problems is simply a common sense view in my opinion. With me this risk would be literally stupid, especially if I knew nothing about the real condition of the diff. Diff's are also typically one of the most neglected gear boxes on a vehicle, this statement also based on what I've seen here. Most owners simply do not hear, feel, or see approaching differential issues. I recently talked with a gentleman who has a beautifully restored M37 if you are looking at truck appearance. The subject of differentials came up during the discussion when he mentioned draining diff oil & finding significant water also drained out. I asked if he pulled the units for inspection after finding this, the answer, no we just filled with new oil. I would bet a differential failure is in his near future because of rust damage on gear teeth & bearings if this isn't addressed in the short term. You would be amazed at how many folks never even think about changing diff oil. When we inspect trucks & point out to owners diff issues that are apparent irregular sounds & characteristics of feel in the trucks behavior, most just say I never noticed it or paid it any attention. This boils down to the issues were there when they took ownership of the vehicle & was never paid any attention at all. Not trying to be the differential hot guy here, just trying to relay information that may prompt owners to pay attention to tell tail signs of their truck & not to run foolish risk doing things like running different height tires on the same axle. Such action will never be a good or acceptable practice no matter anyone's calculation result & would have voided your Dodge differential warranty on your new Power Wagon back in the day. I only post here in an effort to help folks opt out of problems instead of into them, posting on this subject is no different.

[Cheyenne Dave]

Thanks for the lube tip! Do you feel the old lube needs to be completely flushed out, or just drained and refilled. The gears in question have fresh 85W90 with complete rebuilds and about 10 miles use. I guess the same question could be asked of my personal truck, which has about 20K on complete rebuilds with lube that has about 5k of use.

So, on the subject of differential failure, all that can be said with any real authority is that people abuse their vehicles both by action and inaction...that is to say they "tear it up" or fail to "tear it down", or just neglect basic maintenance. On this, I agree with you 100%. That is why I believe that every mechanical assembly should be gone through in detail, unless it can be shown that it has already been done. Anything else is a crap-shoot.

There are just too many variables that can happen over the life of vehicles of this vintage, as you have just pointed out. The last differential I tore down had one side gear thrust washer partially destroyed and the other completely disintegrated. The former owner was the Standard Oil Company and the truck never had anything but stock tires on it. Go figure...



C.D.

[Charles Talbert]

Thanks for the lube tip! Do you feel the old lube needs to be completely flushed out, or just drained and refilled. The gears in question have fresh 85W90 with complete rebuilds and about 10 miles use. I guess the same question could be asked of my personal truck, which has about 20K on complete rebuilds with lube that has about 5k of use.

So, on the subject of differential failure, all that can be said with any real authority is that people abuse their vehicles both by action and inaction...that is to say they "tear it up" or fail to "tear it down", or just neglect basic maintenance. On this, I agree with you 100%. That is why I believe that every mechanical assembly should be gone through in detail, unless it can be shown that it has already been done. Anything else is a crap-shoot.

There are just too many variables that can happen over the life of vehicles of this vintage, as you have just pointed out. The last differential I tore down had one side gear thrust washer partially destroyed and the other completely disintegrated. The former owner was the Standard Oil Company and the truck never had anything but stock tires on it. Go figure...



C.D.

Drained & flushed out is pretty much a waste in my mind, pulling the unit for good visual inspection & manually cleaning out anything that's there is the only way to access it right & have piece of mind.

A word about good oil, it can't be purchased at Wal-Mart. A test that works well is as follows: Pour 1/2 pint of your favorite gear lube in a quart size container. Take a household egg beater & have at it. Some oils will have a tendancy to push outward against the wall of the container & sort of run away from the egg beater. An oil with a good strong additive package will cling too & climb upward onto the egg beater. The container will be virtually empty as most of the oil will be clinging to the beaters. You make the judgement call, would you rather be running an oil that virtually runs from the mechanical assembly or one that clings well & goes for the ride with the assembly? Certainly the one that clings is doing a faaaaaaaaaaaaaaar better job of protecting & cooling the assembly. Many oils that claim to be in the better group actually belong in the non-clinging catagory. Top of the line oils such as Synergen, Hydrotex, Royal Purple & a very few others put over the counter stuff to absolute shame. You can try this test yourself.

You are 150% correct in saying the only way to know what you have is to go through it, then maintain it with the best of the best. Keep water & dirt OUT. Don't run uneven tire heights, & apply a great deal of common sense to your driving habits & maintenance procedures.

You noted some thrust washers had failed in one you had torn down. This is exactly the point we are making here, run that differential with a different tire height on each side & it will go down quickly to the point of catastrophic failure. Point is it just isn't smart to do it, really makes no difference how long you might could do it before a not so good practice hits home in a hard & costly way.

[Cheyenne Dave]

I remember an on-the-counter demo gizmo that did the same thing as your egg beater test. It employed "their brand" and "brand X" chambers, side-by-side, each having an array of nylon gears. A hand crank on the side turned the gears in both chambers, showing the differences in cling factor of the two oils. I don't remember what lube was being pushed, but it was a very convincing demonstration. Anyway, I guess I'll look into these lube options locally, I know my NAPA carries the Royal Purple.

Thanks for the input!

C.D.

[74w300uteline]

lucas oil crank thingamajig

my kids are always cranking and knocking it off the counter at my mechanics shop.

[Cheyenne Dave]

my kids are always cranking and knocking it off the counter at my mechanics shop.

Of course, I remember now! Apparently, even the display was tough!

C.D.

[Charles Talbert]

I remember an on-the-counter demo gizmo that did the same thing as your egg beater test. It employed "their brand" and "brand X" chambers, side-by-side, each having an array of nylon gears. A hand crank on the side turned the gears in both chambers, showing the differences in cling factor of the two oils. I don't remember what lube was being pushed, but it was a very convincing demonstration. Anyway, I guess I'll look into these lube options locally, I know my NAPA carries the Royal Purple.

Thanks for the input!

C.D.

Actually for me the egg beater test was more thorough & convincing. I know the 1 you mentioned also, have seen it used for Lucas products & red line. I think Lucas is a good product, have never used it or done the egg beater test with it. Royal Purple is an absolute top of the line product, may well be the very best there is.

[W_A_Watson_II]

The plastic gear demo for oil cling I remember was for Lucas

[Joe Flo]

Charles,

I didn't mean to upset you. Yes, you are correct. You can damage a differential by running tires of different diameters. I have seen this happen and I'm sure the axles you have worked on have had unmatched tires running on them for sustained periods which led to their failure.
However, driving a few miles (let's say around 5 miles) to get out the woods or down the road to a repair shop isn't going to damage or destroy the axle. If it does, there was something already seriously wrong with it. I'm not suggesting that he should finish his trip or off road adventures with the spare on, just to get himself out of a difficult location. As Dave mentioned, he can put the spare on the front axle with hubs unlocked and drive much further. I guess I should have clarified that in my first reply. Is this something we can agree upon?

Joe

[Charles Talbert]

Charles,

I didn't mean to upset you. Yes, you are correct. You can damage a differential by running tires of different diameters. I have seen this happen and I'm sure the axles you have worked on have had unmatched tires running on them for sustained periods which led to their failure.
However, driving a few miles (let's say around 5 miles) to get out the woods or down the road to a repair shop isn't going to damage or destroy the axle. If it does, there was something already seriously wrong with it. I'm not suggesting that he should finish his trip or off road adventures with the spare on, just to get himself out of a difficult location. As Dave mentioned, he can put the spare on the front axle with hubs unlocked and drive much further. I guess I should have clarified that in my first reply. Is this something we can agree upon?

Joe

Oh my Lord, I'm not upset at all. You can run with the brake drum on the ground if you want, it's no problem for me. I'm not here to find agreement in discussions like this as there will always be someone willing to argue. If you noticed I said it is permissible to run a different tire height on an axle with the hub locked out. I'll have to stand by what I said, running tires of differing heights on the same axle isn't smart, can & will cause high $$$ problems. These are facts I've found to be true, simple as that. This has gotten drug out into much more than it should have. Once again, I try to help folks opt out of problems & not into them. That's the only reason I spend time here.

[Charles Talbert]

The plastic gear demo for oil cling I remember was for Lucas

You've missed the whole point, this is demonstrating Lucas oil stabilizer. This is an oil additive & not a gear lube at all. I was refering to differential gear lube oils, not an additive to mix with over the counter cheapies in an effort to make it decent. 2 totally different products here.

[W_A_Watson_II]

Well, actually the last time I used an egg beater was making pizza dough, and the flower and water sure stuck to the beaters, so I guess I should fill my crank case with flour and water.

[HWooldridge]

Pardon me if this is self evident with regard to tire calculators as I mean no offense:

Pi x diameter gives circumference, so if you know the height, you can easily determine distance traveled in one revolution. For example, a 34" tall tire travels almost 107", while a 36" tall tire travels a little over 113" - a 5% difference. In one mile, that equates to 280 feet of distance traveled. I don't want to put words in his mouth but I think Charles is saying that in his experience, this difference will chew up a diff. I don't know what the magic distance and percentage is - 10 miles and 1%, 2% or whatever, but any mismatch can cause damage.