Unimog Maintenance Guide: Diff & Hubs Oil Change (Checklist & Specs)

WARNING: My Unimog is a model 435.117 with the older 3-piece axles model 747.113. All technical details pertain to tthose axles and those axles only! Do the research into your own axles and hubs and it’s particular needs. Also, there is a very possible chance I am wrong about anything and everything. I am not a professional. This information is for entertainment purposes only.

Portal Hubs

Draining

1. Get something to catch the oil. You can’t just set an oil pan on the ground because then the oil will run onto your rim and tire. You need something you can angle up into the wheel. I use a shaker bottle sitting on two 4″x6″s.
2. Remove drain plug (M14x1.5 / 19mm socket) – located at the bottom of the hub. Let oil drain.
3. Remove fill plug (M14x1.5 / 19mm socket) – located above and to right or left depending on which wheel.

Filling

1. Replace drain plug (M14x1.5 / 19mm socket)
   Washer: New brass crush washer (14mmx18mm)
   Torque: I cannot find a reliable source for this
2. Fill with Gear oil until it overflows – Mobil Delvac 1 gear oil 75W-90 (0.25 Litres/per hub (0.26 quart))
3. Replace fill plug (M14x1.5 / 19mm socket)
   Washer: New brass crush washer (14mmx18mm)
   Torque: I cannot find a reliable source for this
4. Clean up your mess

Fill & Drain port location

Front hub

Rear hub

Diagnosing Your Hubs: Reading the Old Oil

What you find when draining the hub oil could tell you some things about hub and axle health. I always use an empty container for each hub so I know exactly what is going on with each corner.

• Magnetic plug: The plug with the black top is magnetic. This is the drain plug at the bottom. Look for metal debris. Obviously metal debris is bad and you should investigate further if you find anything.
• Metal debris: Clear shaker bottles are helpful. I think I was told that the tiny little debris that you see below is normal and not to be worried about.
• Oil level: If it is lower than it should be or higher than it should be then I believe this means you have a bad seal somewhere between the diff and the hubs and oil is migrating from one to the other – unless, of course, you can see it leaking somewhere. I haven’t ever addressed this situation so you will have to look elsewhere for remedies.
• Color: I change all four hubs at the same time. Why is the rear hub oil darker than the front? I don’t know – is there a problem? It could just be the time of day affecting the lighting. Look at the difference in tape color and table color.

Parts list – Portal Hubs

Component	Cost	QTY	Source
Mobil Delvac 1 gear oil 75W-90	$50.00/gal	1 Litre (1.04 Quart)	Amazon
Copper crush gasket 14mmx18mm	$0.40	8	Belmetric

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Differentials

The differentials are probably the easiest fluid to change, especially the rear. They use the same oil as the hubs.

Warning: The front differential tool access issue

Before you start, ensure you have a ‘Stubby’ 14mm Allen key. The front panhard rod blocks access to the drain plug, and a standard key will not fit. See this post for details.

Out with the old

1. Remove drain plug (M24x1.5 / 14mm Allen) – located at the bottom of the case. Let oil drain.
2. Remove fill plug (M24x1.5 / 14mm Allen) – located above and to the back side of the case.

In with the new

1. Replace drain plug (M24x1.5 / 14mm Allen)
   Washer: None
   Torque: I cannot find a reliable source for this
2. Fill with Gear oil until it overflows – Mobil Delvac 1 gear oil 75W-90 (2.5 Litres/per hub (2.6 quart))
3. Replace fill plug (M24x1.5 / 14mm Allen)
   Washer: None
   Torque: I cannot find a reliable source for this
4. Clean up your mess

Parts list – Differentials

Component	Cost	QTY	Source
Mobil Delvac 1 gear oil 75W-90	$50.00/gal	5 Litre (5.28 Quart)	Amazon