Differential ring gear wear question

mkmskm

mkmskm

Senior Member
I'm rebuilding my first rear end, and I know nothing about this differential. I bought a rolling chassis from a hot rodder who stripped off the coupe body and I'm building a speedster. My question is this ring gear which I think looks good except for one tooth which has some pitting as shown. Will this be an issue or is it okay?
 

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hmmm contact surfaces not very smooth either. Tom Endy has rebuilt literally hundreds of A rear ends and is the recognized expert far and wide. He has posted an extensive pdf of exactly how to do the rebuild on his club pages, i think it is santa anitas a's.
Wait a bit and it is all but certain that he will chime in. in the meantime how about a pic of the drive surfaces of the pinion teeth as well?
 
I would say that there is probly metal fragments in the rear end that got between the teeth and took a bit out on your ring gear.
 
Attached are some more photos, including one of the pinion which I have yet to pull or clean up. I have been using Tom's revised 2014 version of his guide and its been a great help so far. I actually got two complete rear ends with this chassis I bought and so far I've been tearing them down to see what is salvagable and what is not. The first unit I tore down had a combination of water and sludge in the rear end and from the looks of the axles the grease seals had failed. The ring gear on that unit has pronounced wear and the axle housings are junk as the spring perches are worn clean through and the spring beat indents into the housing. The second unit had nice clean oil drain out and the bearings looked almost new. The axles were nice and clean and the only thing that looked suspect so far was this gear.
 

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I'd still wait for Tom to weigh in. He will. can always pm him if need be.

The axle housings you mention that are damaged are not necessarily beyond repair. That type of wear is not uncommon. Because they are forgings they can be MIG welded easily. For the perch use a carbon or porcelain rod of the correct diameter (IIRC 3/4 inch) in position and weld over it to build up the worn metal. Carbon is the easiest to use since it conducts electricity. You will have to break the rod out after you are done, tho. Then you can dress down the welds with a 4" right angle grinder and it will be good as new. Same for the wear on the housing itself. Just do not allow weld to sag inside the housing else it can interfere with installing the grease seals later. weld on the edges a little at a time. when done, dress with grinder.
 
From what I can tell from the photos it appears to be a serviceable ring and pinion gear set. There is some wear on the ring gear, but it is not that severe and can be compensated for when you set the backlash. The small amount of pitting on the one tooth does not appear severe.

When I reject an R&P set it is because of teeth broken off either the ring or the pinion gear. I also reject them if 3 or 4 teeth on the ring gear show severe rust pitting. This is caused when a rear axle assembly acquires water in the oil. As the unit sits the water goes to the bottom of the banjo and 3 or 4 teeth sit in water for however long it takes to cause pits in the teeth.


Before I install an R&P gear set I bead blast it so that it looks almost new. This way I can inspect it easier and also look for a matching number on the ring gear. If you don't have a bead blaster spend some time wire wheeling both ring and pinion.

Look for cracks at all the gear teeth, this is a cause for rejection.

You indicate you have two rear axle assemblies. I would advise against mix and matching an R&P gear set. Use a set that ran as a mate in a rear axle assembly.

Most all original R&P's had matching numbers. What I think went on at the Ford factory is that a worker pulled a pinion gear from one stock bin and a ring gear from another. The pinion gear has a number stamped in the end. The worker would take an etching tool and in free-hand write the number on the back flat surface of the ring gear. Since I have seen this so many times, I believe this was the practice. After a few thousand miles the R&P became a matched set.

In the 2014 revision of my dissertation there is a photo of a numbered gear set.

I also count the teeth on both ring and pinion to know what ratio gear set I am working with. Nine on the pinion and 34 on the ring is the common 378:1 ratio.

There was an earlier post from a fellow in New Zealand who was attempting to install a Mitchell overdrive in an early car and discovered a 370:1 gear set. These early R&P's were completely different in construction and are not compatible with a Mitchell and requires a conversion to a 378:1 gear set. It would be a mistake to replace only the pinion gear of a 378:1 and leave the old 370:1 ring in place. The two are not compatible.

Once you get the R&P gear set cleaned up it will look a lot better and can be better evaluated. Good luck with your project. Take your time and follow the dissertation.

Tom Endy
 
Tom, the fellow in NZ is me. The car is a late 1930 phaeton, so the early diff in it was a surprise. All new parts are on their way now.
I have been rejecting diff gears for minor pitting and slight roughening on the ring teeth, but wonder if I am too fussy in view of your experience and your advice on this post. I have in the back of my mind the thought that small pits on the pinion teeth are a sign that hardening has gone through, but I guess these gears are through-hardened and not cased like some Japanese stuff was/still is?, so pitting may be corrosion or abrasion and not incipient surface flaking. Any opinions or help on this aspect?
Do you apply the same wear and pitting criteria to gearbox gears? My Roadster gearbox whines a bit, but the gears look good. All bearings etc are new. My wife's Tudor is very silent in the gears, in contrast to my roadster. Otherwise I would have assumed mine was better than it is. I have not looked inside her transmission to compare the appearance of the teeth with mine yet
SAJ in NZ
 
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Thanks Tom, I don't mind spending the money to do it right but I'd rather not spend $500 for gears I don't need. I had planned on keeping them as a set. The first ring gear from the rear end that had water and sludge had no rust per say but after soaking in Chem Dip there are 4 teeth in a row with pits worse than the one tooth on the second gear that I pictured. I won't ever plan on using that one. Also it had a lower portion that was stained I assume from the water as its brown. Both gears have three different sets of number(s) stamped and them as you noted both have a hand scrawled set of numbers. Both also have a shiny spot the size of a dime with a dimple that looks like something the size of a BB was tapped against it on the back side. What is that for? I've rebuilt an engine and transmission but this is new adventure. Thanks for your input and the time you spent on your guide.
 
mkmskm said:
Thanks Tom, I don't mind spending the money to do it right but I'd rather not spend $500 for gears I don't need. I had planned on keeping them as a set. The first ring gear from the rear end that had water and sludge had no rust per say but after soaking in Chem Dip there are 4 teeth in a row with pits worse than the one tooth on the second gear that I pictured. I won't ever plan on using that one. Also it had a lower portion that was stained I assume from the water as its brown. Both gears have three different sets of number(s) stamped and them as you noted both have a hand scrawled set of numbers. Both also have a shiny spot the size of a dime with a dimple that looks like something the size of a BB was tapped against it on the back side. What is that for? I've rebuilt an engine and transmission but this is new adventure. Thanks for your input and the time you spent on your guide.
Click to expand...

I believe that is a Rockwell hardness test.

Tom Endt
 
Here is a good looking ring gear. I was so pleased to see this when I pulled my rear end apart. I ended up with two bad bearings, she is as quit as a mouse now.
 

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Tom Endy said:
Most all original R&P's had matching numbers. What I think went on at the Ford factory is that a worker pulled a pinion gear from one stock bin and a ring gear from another. The pinion gear has a number stamped in the end. The worker would take an etching tool and in free-hand write the number on the back flat surface of the ring gear. Since I have seen this so many times, I believe this was the practice. After a few thousand miles the R&P became a matched set.

Tom Endy
Click to expand...

All original ring an pinion sets had matching numbers. Every set was lapped in, making them a matched set prior to installation. This is why Ford only sold them as sets through service.

I suspect the reason the number was etched into the ring gear as opposed to stamped was due to the risk of distorting the ring gear from the pressure of stamping.
 
Marco Tahtaras said:
All original ring an pinion sets had matching numbers. Every set was lapped in, making them a matched set prior to installation. This is why Ford only sold them as sets through service.

I suspect the reason the number was etched into the ring gear as opposed to stamped was due to the risk of distorting the ring gear from the pressure of stamping.
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Marco, I am not disputing what you are saying but have you looked on the prints to verify this is true? Why I question this is I have a picture on my computer at work that shows a Ford dealer with Model-T Ring & Pinions inventoried in two separate bins (a bunch of ring gears in one bin and another count of pinions in yet another bin.) where it is obvious those were not a matched set.

Now here is the second reason why I am asking this. To lap these two gears to each other, you would need to put them in a fixture that would allow both gears to run in lapping compound in the proper relationship to each other. THEN, that same gear-to-gear relationship would need to replicated to exacting standards or otherwise the lapping process would be in vain. Therefore I am curious if there is any verbiage on the prints that tells the process the engineer's spec-ed on this. If you do not have a copy of the prints, I may see about pulling them the next time I am at Bensen and sharing them with you & Tom so we know for sure how they were originally done.
 
BRENT in 10-uh-C said:
Marco, I am not disputing what you are saying but have you looked on the prints to verify this is true? Why I question this is I have a picture on my computer at work that shows a Ford dealer with Model-T Ring & Pinions inventoried in two separate bins (a bunch of ring gears in one bin and another count of pinions in yet another bin.) where it is obvious those were not a matched set.

Now here is the second reason why I am asking this. To lap these two gears to each other, you would need to put them in a fixture that would allow both gears to run in lapping compound in the proper relationship to each other. THEN, that same gear-to-gear relationship would need to replicated to exacting standards or otherwise the lapping process would be in vain. Therefore I am curious if there is any verbiage on the prints that tells the process the engineer's spec-ed on this. If you do not have a copy of the prints, I may see about pulling them the next time I am at Bensen and sharing them with you & Tom so we know for sure how they were originally done.
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You can see signs of the lapping on NOS gears. Also note that ALL the parts books say "(matched)" without exception.

As far as maintaining the required tolerances, that would be a relatively simple task for Ford. Maintaining tight, repeatable tolerances was what made the car far better than what most current owners could ever imagine.
 
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