Pitting on Ring Gear discovered during diff drain - replacement advice

[jake2077]

As the title suggests, I am restoring a 1971 Bronco and decided to drain the 9" rear differential oil. Much to my disappointment, there appeared to be rust INSIDE the diff and even on the ring gear. I cleaned as much as I could but there is obvious pitting and deterioration of the metal. The identification of the gear shows C2AW 4210 W F 1. I found a ring/pinion set on FB marketplace but the part number is slightly different C2AW 4210 F W 3. Does this matter? I know nothing about gearing and ratios etc.. Just looking to make it right and maintain what was there originally.

 

[Madgyver]

Just run it. I would drive it hard as long as there is no play in it. it was set at the factory.

 

[DirtDonk]

I'm tempted to say the same thing, but I'm also of the opinion that "since it's already out" you might as well replace it.
Just in case you're thinking it's rust, that red inside the 3rd member casting is a red oxide primer that they used at the factory. However given that your gear is so pitted, I would not be surprised if you were speaking of other rust not visible in the picture, inside the stamped metal housing and tubing.

I'm no expert, so I'll just put it out there and see if you or others can enlighten. Is that a 4-pinion differential? I see two holes 90° apart and both appear to have pinion gears visible.

Also not an expert on the numbers for gears, but since those are standard Ford engineering numbers my take on the last three digits is that the one with the "3" is just a later update.
However, since gears do have their own codes in other areas, I would not want to stake my life on that assumption.
And for our info, are the F and the W indeed flip-flopped, or is that a typo?

Good luck.

Paul

 

[Apogee]

It looks like a diff that was parked for a long time in a humid environment without being spun. The teeth above the oil line are prone to corrosion if they aren't run every so often. It certainly isn't ideal, but it most likely won't cause catastrophic failure...just some added noise and accelerated wear due to the pitting and corrosion. You could fix it now or you could fix it later, possibly much later, without issue either way. I would take the opportunity to add an oil drain to the housing, as you can learn a lot by what your oil looks like and monitor the gears indirectly in that way.

 

[jake2077]

It looks like a diff that was parked for a long time in a humid environment without being spun. The teeth above the oil line are prone to corrosion if they aren't run every so often. It certainly isn't ideal, but it most likely won't cause catastrophic failure...just some added noise and accelerated wear due to the pitting and corrosion. You could fix it now or you could fix it later, possibly much later, without issue either way. I would take the opportunity to add an oil drain to the housing, as you can learn a lot by what your oil looks like and monitor the gears indirectly in that way.

Appreciate the advice. I'm tempted to let it ride as well knowing that I'm only using this as a cruzer to the local classic car meets and occasional driving with the kids. From my research there are measurements and pre-loading required if I disassemble the spine and ring gears that I am not really sure I can accomplish myself

Is there a good reference link to adding a drain plug? That would be great to have.

 

[Rustytruck]

If it were mine I would run it. it will get noisy then its time to replace it make sure you use heavy duty gear oil. I would be replacing it if I planned long trips in it. while its out count the teeth in the pinion and ring gears. You could look for a replacement set up to swap in later if you have the mind to. if they are 3.50 gearing allot of them out there.

 

[jake2077]

If it were mine I would run it. it will get noisy then its time to replace it make sure you use heavy duty gear oil. I would be replacing it if I planned long trips in it. while its out count the teeth in the pinion and ring gears. You could look for a replacement set up to swap in later if you have the mind to. if they are 3.50 gearing allot of them out there.

It looks to be 3.5 from the tag label but I'd have to confirm by counting the teeth. I have a local transmission contact who does gear work and I've asked him what his charge to do the work would be. If I run it as-is I'm afraid there would diff noise and trouble down the road. When I did the main seal replacement I noticed the yolk nut was very easy to remove and so I'm honestly not sure if the pre-load was done right as well.

 

[bronco t]

The chart at the bottom of the link below shows the ring and pinion tooth combinations for both the 9 inch and 8.8 ring and pinion sets. Interesting is that some gear combinations are said to be prone to make noise, including the 3:50.

https://www.diyford.com/ford-axle-ring-pinion-assembly-guide/

 

[Broncobowsher]

I am normally very much of the "just run it" camp.
When you said there was pitting on the gears, I expected a little pitting. Not massive rust.

For a local cruiser, stock power, never far from home. Run it.
But really those gears are toast. They are weaker than stock, but in stock form there is enough over capacity they will be fine. Surface contact patterns have been altered, the gears will wear at a much faster rate. The issue is if you take a long road trip the gears can start coming apart and cascade quickly into a complete failed rear axle. That would be on a long trip, far from home, when the oil is warmed up and thin, lots of sustained highway driving...

If you do run those gears, put in the thickest gear oil you can find. They will need all the help they can get.

Now the other question, what size tires are you (going to be) running? Engine and transmission as well? It is very likely that the gearing needs to be changed to get drivability back. This is very likely the sign that it is time for a better choice in gearing.

 

[blubuckaroo]

Interesting is that some gear combinations are said to be prone to make noise, including the 3:50.
https://www.diyford.com/ford-axle-ring-pinion-assembly-guide/

Some gear noise is part of the trade-off for a 9".
If you compare a 9" with most other differentials, you'll see that the pinion gear drives lower on the ring gear. Most others are nearer the center.
This gives the 9" more strength since the pinion gear has more contact with the ring gear. It sort of resembles a worm gear, compared to other differentials.

 

[1970 Palmer]

I'd be more concerned about the pitting on the bearings/race surfaces?

 

[JB Fab]

I wouldn't run those gears...... Pitting=friction, friction=heat/wear... They are going to need to be replaced anyway, why drive around with a rear dif that is on an accelerated failure curve?

I would fix them while they are out, one less thing to worry about.

I think I have stock 3.50 gears that are in good shape, if you want then for the price of shipping, PM me and I will look for them?

 

[jake2077]

I wouldn't run those gears...... Pitting=friction, friction=heat/wear... They are going to need to be replaced anyway, why drive around with a rear dif that is on an accelerated failure curve?

I would fix them while they are out, one less thing to worry about.

I think I have stock 3.50 gears that are in good shape, if you want then for the price of shipping, PM me and I will look for them?

Appreciate the help. I think I'll order the rebuild kit including spline/gears and just get it done right so there is no worry down the road. So if this is a 9" factory with 3.5 stamped on the ring gear, which replacement set should I get? There are an overwhelming amount on the usual sites.

 

[Madgyver]

Gears are consumables. They will wear out. 1k miles or 100k miles. You wanted original.
You want to do it right? Replace them, It's like replacing any other mechanical part on your rig that wears out. I see more miles on it, it's not a chipped gear or bad wear on the gear mesh. It's a simple task for me to replace it when I need to. A few hours and it will be driven again if I have all the parts. if you're doing a restore then replace them but have a shop with experience of setting it up do it for you.

 

[Broncobowsher]

Before you go ordering up a new set of 3.50 gears, what tires and drivetrain are you going to be running? For most people the stock 3.50 gears are not a good choice. This is the time to decide if a better ratio would be correct.

 

[jake2077]

So it looks like I found the culprit. The wheel hub/axle seal ($7 part) was in two parts allowing water into the axle and therefor the diff causing the rust issue. No telling when this happened but my guess is the previous owner never looked into it and didn't know it was an issue. Glad I caught it!

 

[blubuckaroo]

That'll do it!
I can't tell you how many times I've opened a rear end and found pieces of the seal's inner lip spring in there. Some times people just aren't careful when sliding the axles in and out.

 

[blubuckaroo]

That'll do it!
I can't tell you how many times I've opened a rear end and found pieces of the seal's inner lip spring in there. Sometimes people just aren't careful and drag the axles in and out.

 

[savage]

I'm surprised it didn't leak (like a open faucet)!