Rebuilding of a 12-bolt differential (1949)

[Sebastian S.]

Hi folks!

In this thread, I want to share the steps and my thoughts while rebuilding my 1949, 12-bolt differential. Please be aware that I am NOT a professional mechanic – I just own two different service manuals and I did hours of investigational work on the internet. One can see my learning curve by reading through my steps. I am not perfect and this is just a "serious hobby" for me!

You probably have better mechanical education – and thus have a better idea or way to do things differently – or you are just interested in my work and made your own profound experiences...in both cases, please feel free to give your opinion. I am willing to learn and I am open minded.


Why did I have to rebuild it anyway?
When I bought the car about a year ago, I only noticed little drops of oil coming out of the gearbox rear end. So I thought, that drivetrain noise during cruising was mainly because the lack of oil in the gearbox and "normal" wear of the differential. I could feel the wearing state of the differential between push and pull driving states as part of the "overall play of the drivetrain". The noise itself startet as a nasty rattling noise on higher speeds – the way it sounds when a bushing is worn out and a shaft starts to rattle in the bushing because of excessive play.
So first thing I did was to rebuild the gearbox with quality bearings (SKF) and a selfmade bushing for the outlet shaft.

Note:
I exchanged the shaft-outlet sealing ring by a standard (modern) shaft seal with dust lip, to be oil tight for the next 30 years or so. I also drained the old oil from the differential, which turned out to be 150-200 ml stinky liquid of dark honey-sirup consistency.


First test drive
My work on the gearbox reduced some of the noise. Shifting is now a little more "tight" or "direct" and the only noise coming from the gearbox is from the teeth inside. After a high-speed test drive on the motorway (75 mph for about 10 minutes) the car developed a very loud (!) nasty purring and vibrating sound. I headed back to my garage but even on lower speeds, the drivetrain was generating a resonance rattling. Really loud and scary.

Back in my garage, I drained the oil from the differential and again found only about 200 ml oil in there – so it seemed to be obvious that the new noise wasn't coming from the gearbox.

That was the initial point on which I started my investigation about how to rebuild a differential.


Spare parts:
I found a NOS ring and pinion set (ratio 41:9 = 4.55) for small bucks (120 $ + 70$ shipping) and I ordered new bearings at Bearing Revolution.




Putting things together again...
My first attempt was to put everything together without the help of any special and precise tools except my 20 tons hydraulic press and some nail varnish from my wife.




Note:
I figured out that the bearings were pressed on firmly and I had to apply approx. 6 tons in order to get the old bearings cups off the housing. I used 5 tons and a bit of ceramic lube spray to press on the new bearings. Some grease would have worked as well.









Be precise, take your time...read the service manual FIRST
In my first attempt, I reused the shim from the former ring and pinion set only to find out, that I had to disassemble everything again in order to get it right!

Note:

• Check the face of the pinion and the housing for the correct number of shims you have to apply. Refer to the table in the service manual!
  For example: If there is no number on your pinion and no number to be found on the housing, you don't have to install any shim at all.
  
  
• Nail varnish works pretty well for marking here. Only let it dry thoroughly one coke or beer long in order not to spoil the pinion! The main advantage of nail varnish is, that you can apply a thick layer which is as thick as the play between ring an pinion. That way, I only had to turn the ring 360° once to achieve markings on both sides of the ring. For me, that worked well.
  
  
• Use medium-strong thread glue and do not overtighten the 12 bolts by using the specified torque!! You must know that any lubricant (such as oil residues, grease or thread glue) will significantly lower the friction between bolt and thread. The specified torque from the service manual only applies to dry, clean thread and bolts. I wasn't thinking about that fact when I destroyed the original bolt by using the adviced torque and thread glue. To my luck, I didn't kill the thread of the ring.

The following picture shows my first setup with BAD markings. Actually, the contact zone should be streched in an oval shape right in the middle of the rings tooth flange – see service manual for more information. For my understanding of the service manual, the markings should show nearly the same picture on both sides of the rings teeth. Anyway, I didn't refer to the manual well enough, installed the shim from the original pinion setup and ended up with the following situation:




I will add some more information step by step. If you are missing something, just let me know. I didn't make pictures of every state though, because at that time, I wasn't thinking of a picture story. Sorry for that!

 

[Sebastian S.]

How to get the differential oil tight?

I love to improve things, if possible. Therefor, I started looking for a modern shaft sealing which fits for the differential and has a sealing lip + a dust lip. It wasn't easy to find out but BMW uses a good opportunity here with their E90 differential side oil seals.

Note:
The ring is not a perfect fit, so I had to machine the differential housing just a bit on my conventional mill.


All original:





Milled (widened) seat for the new sealing ring:

Note:
I widened the original seat only that much, that the new ring fits the seat with the help of a small assembly hammer. The original seat depth turned out to be perfect for the yoke / v-lip combination of the new sealing. Of course I measured it thoroughly before starting my milling action!

Something to laugh about...machining the seat has actually been my first "high precision" work on my mill. I own a Paulimot F207-V for almost 2 years now, but I never machined something more exciting than a lamp holder for a bicycle, drilling some holes in mild steel and a license plate holder out of aluminum. I was really scared to work on the housing. So it took me 2 hours to center it on the mill and some 2 hours more to gradually widen the seat.



You may see some slight rattling marks, which I couldn't avoid due to the medium-good precision of my mill and the urge to finally go to toilet after a whopping 5 hours work here. But don't be fooled - those marks are visibly only...I couldn't feel any roughness with my fingers.




The installed ring is equipped with three lips (sealing, dust and v-lip):

Note:
The sealing stands "out" that much, that the v-lip seals against the yoke. I did a test by applying some motorcycle chain grease on the yokes bottom plate and installed the yoke temporarily. When I removed the yoke again, I found a perfect circle from the v-lip showing up in the chain grease – the proof for perfect fitment! That's a perfect condition and as intended for a modern differential sealing like that and much better than the old leather sealing.






This setup is absolutely oil tight and the "new spare part" is widely available for a few bucks.

 

[melsg5]

thanks for the post. The car came with the service manuals or you had to find them? Your access to the press and the mill is not something every backyard mechanic has.

 

[Sebastian S.]

What ratio is best? A question of personal use and driving style...

In my first attempt to repair the defective differential, I bought a pinion&ring pair which was advertised as a 3.9 ratio with 39:10 teeth. Well...when the box arrived, I counted the teeth and found a much "slower" (is that the correct term here?) ratio of 4.55 or 41:9 teeth.

Note:
Original ratio was 4.1 or 41:10 teeth. The standard ratio for synchromesh manual transmissions at that time.


Some basic theory here:

• A gear set with a ratio of 3.9 means, that the pinion needs 3.9 revolutions to turn the rear wheels a single time.
• A gear set with a ratio of 4.55 means, that the pinion needs 4.55 revolutions to turn the rear wheels a single time.
• ...

My new gear set was so extremely slow, that I couldn't even find it in the service manual!



So which ratio is "the best"?

The slower the ratio, the more pinion revolutions per wheel revolution you need. In other words, a slower ratio makes your car slower "per engine revolution" as the engine is coupled via a transmission. It gives your car the ability to "climb mountains" easily at the cost of overall speed!

A "faster" (lower!) ratio makes your car faster "per engine revolution" instead. Such a ratio is more suitable for plain regions or highway speed, as you reach higher speeds at significant lower RPM.



So, I was really unhappy with the 4.55 ratio, because here in Switzerland we have a speed limit of 50 mph for country roads and 75 mph for highways. I really love my car and I would like to drive it on highways and plain country roads with low rpm rather than race it on a mountain pass.


Hence, I kept looking on Ebay for a 3.9 ratio or lower...and finally found a 3.63, which turned out to be perfect for my needs (keep in mind, that my car has only 107 hp!). Okay...by that time, I had already installed the 4.55 gear set completely.



I changed the gear set again and by that time, my new measurement tool had arrived and I was able to adjust the play between pinion and ring to a perfect 1/10th of a mm. It's just a cheap China set for about 15$, but sufficient for that purpose.

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[Sebastian S.]

thanks for the post. The car came with the service manuals or you had to find them? Your access to the press and the mill is not something every backyard mechanic has.

Hi melg5

I bought two different manuals on Ebay – they actually have different information inside, although they are written for the same car types.




About my tools...well...aren't shiny tools the manly replacement to handbags for ladies?

To be honest, I had to buy the workshop press to be able to service the differential and it is a cheap one! The mill is something different though, it took me two years of "not buying expensive new tools" until I had the budget for the machine.

 

[melsg5]

I'm just amazed you can still find so many different ratios for a 1949 differential. Dealing with more modern cars compared to yours, say the 1960's, a 3.55:1 ratio is considered a perfect compromise between acceleration and cruising. Your 4.55:1 set would be considered serious drag racing territory.

 

[Sebastian S.]

Rear axle bearings - an odyssey and my personal recommendation


My learnings just right at the beginning:

Buying bearings of unknown quality is not a good idea at all. Period.


On my odyssey to find a pair of good rear axle bearings:

1. I started with a "medium priced" set of "quality bearings" from Ebay, including the retaining rings for 59$ plus 30$ shipping, I received a pair of extremely cheap China bearings! Don't get me wrong: We all know some quality products made in China. But those where filled with sticky grease, smelly...and worst of all, the sealing rings wandered in the outer races when turning the inner race! I even made an attempt to glue the sealing onto the outer race with silicone sealing compound – before I threw them away.
   
   
   
   
   
2. My next buy was from an US seller on Ebay who stated, that he only uses "high quality" China bearings with high quality, American heavy-duty grease. Those where a little cheaper (around 49$ plus shipping), but came without retaining rings, as specified. This time, the bearings where overall well machined, but the sealing lips of both bearings have been partly twisted and squeezed on one side. I assume, that happened during the refill process of the heavy-duty grease?
   During use, fresh air can get into contact with the grease and oxidate it when the sealing is not good! Plus there is a risk of certain loss of grease and that moisture and dust will be able to get into the bearing! Waste.
   
   
3. I ended up with agricultural bearings from INA, which is a European quality manufacturer. Those only cost 22$ per bearing, made in Europe and filled with quality grease. They came in air-tight plastic bags with silicate pillows to keep them dry. No bad smell, perfect outer appearance and even smooth and easy to turn at a cool 4 °C winters day.
   
   

In short: I really had some bad experience with those China bearings.

 

[Sebastian S.]

I'm just amazed you can still find so many different ratios for a 1949 differential. Dealing with more modern cars compared to yours, say the 1960's, a 3.55:1 ratio is considered a perfect compromise between acceleration and cruising. Your 4.55:1 set would be considered serious drag racing territory.

Hi

Well...maybe I have been just very lucky! It took me several month to find the 4.55 ratio set on Ebay. Being tired of manual search, I ended up with an automatic search in the meantime which I forgot to disable on Ebay after my first purchase. That's why the robot-search "accidently" found the 3.63 ratio for me a few month later.

To be honest, there aren't many sets around...

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[Sebastian S.]

Now, what about that initially noisy drivetrain?

Well, I think the best noise reduction happened with the rebuilt of the differential. But both, differential and transmission had pitted and worn bearings and oil loss.

I finished the repair work on Friday and yesterday, we drove about 120 miles with the car:

• The play between push and pull riding condition is minimal now, the noise level is very low. In fact most modern cars tend to be way louder than our Pontiac itself – my wife and I, we are not used to glide along that quietly and elegantly.
  
  
• I can only hear the first gear on higher revs when accelerating and the second gear, when I use it to "brake down" the car. The low humming engine (which is actually more of a low-level background noise) seems to drown any noise coming from the differential.

Awesome, we absolutely love it!

My work turned out to be definitely necessary to get rid of the combined noise of differential and transmission. In addition, I don't have to refill oil between the yearly oil exchange, which I perform on all my motorcycles and cars. Better for our environment anyway.


Next on my list is to renew the cabling. I will also make some minor improvements here, because I keep the 6V system but want to have less voltage drop for the headlights and so on...

 

[melsg5]

what viscosity differential fluid did you use? For instance 75W90 is made by Valvoline but they also have a 75W140 which can be useful for minor gear noise.

 

[Sebastian S.]

Thanks for your tip!

As for the transmission, all gear teeth are straight cut and so some minor gear noise is "normal", for my understanding. In my youth, I restored a tractor together with my dad and the transmission made very similar sounds.

After transmission rebuild, I used Liqui Moly 80w90 GL-4 and added some molybdenum additive in order to reduce friction and gear noise, but it turned out to be too slippy for the synchro rings and shifting was way too hard because of that! That's why I tend to use standard mineral oil, no synthetics.

Currently, I use low additive gear oil, specially made for classic cars (REKTOL GEAR 300 | SAE 80W-90 | API GL-3/4). It is advertised to be non corrosive to nonferrous metals like GL-3 but with pressure stability of GL-4. It seems to do the job and has the correct viscosity.


For the differential, I use very cheap standard mineral oil MANNOL Hypoid Getriebeoel 80W-90 API GL 4/GL 5 LS, for break in. I will change it after 200 miles, refill and change again after another 200 miles before I do the final filling with Castrol Axle EPX 80w90 + Liqui Moly molybdenum additive. That should be fine for the next few years then. What do you think?

 

[melsg5]

I agree with staying away from synthetics for older vehicles. Mannol sounds like a good choice and they have a 75W140 type oil

MANNOL Maxpower 4x4

mannol.de

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