Tech article model 404.1

Replacing the front axle output and the hub input seals - without pulling your hubs apart!

My truck suffered from the common ailment of leaking seals. Most notable was the seeping of gear oil on the front hub backing plates. The decision to replace these bad seals was not an easy one because of the special equipment required for hub dismantling. With the front axle assembly removed from the the vehicle, closer inspection revealed a possible alternative. Replace from the Cardan joint end.

The Mercedes shop manual for the 404 was no help at all. That whole part of the axle is completely ignored - what's with that?? The parts book has a decent exploded view ot the cardan joint and hub that I needed to make some rather large assumptions. RKI's, and EKI's were consulted (reasonably and extremely knowledgeable individuals), but no one could confirm my belief - that the hub input seal could be accessed this way.

I was far enough dismantled to go ahead and give it a try.

Here's how it's done - as I can remember it - sorry, no pics, I forgot the camera on my way to my friends shop who owns the press I used.

Tools needed:
Metric 3/8 and 1/2 inch drive socket sets to 24 mm
Metric combo wrench set to 24 mm
Dead blow and regular hammers
Assortment of chisels
Impact screwdriver set with standard head
Minimum 12 ton shop press
Pry bars, screwdrivers, and assortment of other stuff commonly used.
Scrap metal and stuff lying around waiting to be useful some day.
Method of cleaning parts.
Shop rags or towels, which ever you prefer.
Several pairs of nitrile and one pair of leather gloves - unless you are not required to show up at work the next day with clean fingernails and scabless hands :-)

The axle assembly does not have to be removed from the vehicle to complete this, so it is much less of a job than I made it, but it was out anyway.

1. Order your seals and wait for them to arrive. The parts you may want to change - per side - are:
- one front axle output seal
- one hub input seal.
- one hub input seal retainer gasket.
- four small cotter pins for the cardan joint bolts - don't reuse your old ones.
- You may want to also include two or four (depending on how much of the cardan joint you want to service) of the cardan joint pin plate gaskets (for lack of the correct name)
- optional - two cardan joint pin grease seals (again, I'm making this name up, but will describe where they go in due time.)

2- Start sweating.

2b- I always like to pressure wash things before dismantling, less to clean later, and nicer to work with quasi clean stuff.

3- Place the front axle of the truck on sturdy jack stands and remove the front wheel.

4- Drain the oil of the hub(s) you are servicing, and of the differential (this may be optional, but you might as well change it anyway.)

5 - Removing the brake drum will make it easier to handle the hub and axle once out, but not altogether necessary. My brakes were out, wheel cylinder everything, and it was still a monster to handle.

6. Disconnect the brake flex line at the wheel cylinder.

7. Engage the front locker and make sure your kids don't unlock it - the axle disassembly section of the manual says something about not required if you have the upgrade, but how are you supposed to know this? So lock it anyway.

8- Loosen and remove the hub retaining bolts to the outer steering knuckle. Pay attention here, one is slightly shorter than the rest, note where it goes.

9- Pull off the hub with the cardan joint and axle attached - this may need persuasion - use the dead blow and tap gently - if that does not work, tap less gently ;-). While removing it from the axle housing, try and keep the proper alignment so as not to damage any machined surfaces on the axle or in the housing. Don't grab the cardan joint - it is very floppy and will take one of your fingers off. Place the hub/axle shaft assembly on a clean working surface.

10- Stuff a shop towel into the axle housing so that your three year old does not find a new place to store his marbles.

11- Inspection time - you can now see the axle output seal located just inside of a retaining ring/washer. That retainer is retained by four punch marks. Also inspect the brass bushing/sleeve that the axle rides on - I was told that these are frequently cracked - mine were fine. To replace these, one would have to disconnect the steering, and remove the steering knuckles - I think - consult some RKI's or EKI's about this if you have to go in any deeper.

12- To change the axle output seal, you have to remove the retainer by removing the punch marks on the axle housing holding in the retainer - you'll see once you're there. The seal can then be removed with a seal puller , pry bar, or screwdriver - use your imagination.

13- Replace the seal, preferably with a seal driver, but a large socket or such can be used to drive the seal in evenly.

14- Replace the retainer and punch new dents into the axle housing - my retainers got somewhat mangled on removal, but a hammer and flat surface restored it in a jiffy.

One down, one to go.

15- Back on the bench, inspect the cardan joint. You want to press out the pin holding the joint on the hub input shaft. Once off, the seal and gasket can be changed without taking the input shaft out of the hub - which can't be done from this side anyway.

16- Remove the four nuts and bolts holding the cardan joint together on the hub side of the joint - they have cotter pins as locking devices. A dead blow or two separated the joint, and the U-joint ends were easily slid off - caution, 31 needle bearings and a seal per side. Also, there are two small rubber grommets between the halves you just separated - don't lose them - they are to seal a grease port between the two parts.

16- Remove the eight screws (4 each side) holding the "cardan pin plates" (my description). This requires an impact driver - don't attempt without one or you could risk stripping the heads of these screws. Under this plate there is a small gasket that I was able to reuse, but probably inexpensive to replace. Note that this gasket has a slight irregularity in it to clear a grease port.

17- Revealed now are 31 more needle bearings per side. You're going to have to pull these out. I found that gravity, along with a little tapping of the pin exposed them enough to get get a hold of them. Inspect. If yours are bad, sorry, but that may mean a lot more work for you.

18- Now you are ready for the shop press. 20 ton if you can, but I was able to squeak by with 12 tons. I first tried with a Brass drift, then a large c-clamp, not a chance. Then upgraded to a 5 inch bench vise, and broke it. It's in TIGHT! You are also going to have to use your neurons a little. The pin needs to be completely pressed out of the hub input shaft. Therefore, it is crucial to have dead center alignment of the press shaft with the pin all the while supporting the U-joint from the other side. I used a piece of pipe cut with a horizontal metal cutting band saw to assure trueness of this support piece. Probably a large deep impact socket with spacers would work. I also used an impact socket on the press side of things just large enough for the pin, and later changed for a deep well of the same size to continue the pressing. An extra pair of hands, or an octopus friend is helpful here as the hub is still heavy despite lack of the axle, and it is clumsy to handle.

19- SLOWLY increase pressure on the pin, all the while monitoring alignment of the pressing. I about maxxed the press on one hub and was about to give up, until I decided to at least try the other side. It let go a "little" easier. I remember this vividly - I was pulling down on the jack handle with considerable force while standing behind the backing plate attached to the hub using it as a scatter shield when a loud "CRACK" was heard. A warm, not-so-fuzzy feeling overcame me as I my thoughts went to wondering what this sound just cost me. I expected to see a splintered input shaft. What I saw was displacement of the pin toward the ultimate goal.
After that the pin moved considerably easier, but not without the worrisome cracking noise signifying progress.

20- With the pin removed and the joint completely off the input shaft, remove the seal retainer by unbolting the six 8 mm bolts. Caution, as one of these is a banjo fitting for the breather tube, and therefore the bolt is hollow.

21- Remove the retainer and seal. Replace. Don't forget to change the paper gasket as well.

22- Pressing the pin back in is not straight forward. There are grease seals (remember "cardan joint pin grease seals") on either side of the input shaft where the pin enters, and these must be properly aligned. You can't "feel" your way through with a 12 ton press, so easy does it.

23- Reassemble. You may also want to service the other side of the cardan joint. And remember:
- the grease port grommet between the U-joints,
- the cotter pins for the cardan joint halves,
- the smaller hub retaining bolt,
and to remove the shop towel from the axle housing before you return the axle home - and look for marbles!

24- Bleed the brakes, and test drive!

As of this writing, the seals are not leaking, but it hasn't been driven either. The truck awaits completion of the body restoration and various other items. But, prior to replacement of these seals, there were puddles of gear oil at each front wheel.

If you are considering doing this job, and have further questions, please feel free to e-mail me: