Unimog Maintenance Guide: Main Transmission & PTO Oil Change (Checklist & Torque Specs)

WARNING: My Unimog is a model 435.117 with a UG 3/40 transmission model: 717.901 and Very fast transmission PTO – 0.745. All technical details pertain only to that transmission and PTO! Do the research into your own transmission and PTO 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.

Main Transmission

Changing the transmission fluid in your Unimog U1300L is one of the most critical maintenance tasks you can do. The transmission and PTO share specific oil requirements that, if ignored, can lead to very expensive repairs. One of my biggest fears as a vintage Unimog owner in the US, is having to do gear work. It is way beyond my ability, complicated, and the parts are big, heavy, and hard to find. Here is the step-by-step process, including the hard-to-find torque specs.

Draining

1. Remove drain plug (M24x1.5 / 14mm Allen) – located at the bottom passenger side of the case. Let oil drain.
2. Remove 2nd drain plug (M24x1.5 / 14mm Allen) – located at the bottom front side of the case. Let oil drain.
3. Remove fill plug (M24x1.5 / 14mm Allen) – located on the back side of the case.

Filling

1. Replace both drain plugs (M24x1.5 / 14mm Allen)
   Washer: None
   Torque: I cannot find a reliable source for this
2. Fill with Gear oil until it overflows – Red Line synthetic GL4 MTL 75W80 (11.5 Litres (12.15 quarts))
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 – Main Transmission

Component	QTY	Source	Photo
Red Line synthetic GL4 MTL 75W80	11.5 Litres (12.15 quarts)	Amazon

---

PTO (Power Take-Off)

There are a bunch of different PTOs that came on Unimogs. Mine is Code: N10 – Very fast transmission PTO – 0.745. Mine is attached to the bell housing (I know, not the correct name) and not the main transmission so it is filled separately.

Do you need to change it?

I did have a thought as I was dumping over $100 worth of fancy oil into this thing that I don’t even use: Since I don’t use the PTO, do I even need to put oil in it? Or, at the very least, do I need to put fresh oil in it? I am scared to death of damaging anything gear related on this truck so I filled it, but the question still stands.

Draining

1. Remove drain plug (M18x1.5 / 17mm Socket) – located at the bottom passenger side of the case. Let oil drain – be ready, it shoots out pretty aggressively and it hits the steering bars.
2. Remove fill plug (M18x1.5 / 17mm Socket)- located on the back side of the case.

Filling

1. Replace drain plug (M18x1.5 / 17mm Socket)
   Washer: New brass crush washer (18mmx24mmx1.5mm)
   Torque: I cannot find a reliable source for this
2. Fill with Gear oil until it overflows – Red Line synthetic GL4 MTL 75W80 (8 Litres (8.5 quarts))
3. Replace fill plug (M18x1.5 / 17mm Socket)
   Washer: New brass crush washer (18mmx24mmx1.5mm)
   Torque: I cannot find a reliable source for this
4. Clean up your mess (I made a huge one on this)

The Missing Torque Spec: M18 Drain Plug.

I searched the manuals for a drain and fill plug torque spec and could not find it. I only publish torque specs when I can find a Mercedes source. That said, I do also run things by Gemini AI. I did not take it’s advice, I just tightened them by feel, but you are welcome to take this info or leave it. Or… maybe you can give me a solid source and I can update this post.

Gemini had the following to say:

Since the manual is silent, we revert to the DIN 7604 standard for light-series drain plugs, which Mercedes adheres to for these auxiliary gearboxes.

Recommended Torque: 37 ft-lbs (50 Nm)

Here is the breakdown of why this is the safe, correct number for your specific M18x1.5 plug:

1. The Bolt Size (M18 x 1.5):
   • This is a “Fine Thread” plug.
   • Standard torque for a hollow M18 plug with a copper washer is 45–55 Nm.
2. The Sealing Mechanism:
   • You are crushing a Copper Washer (A18x24).
   • It takes roughly 30 ft-lbs just to deform the copper enough to create a seal.
   • 50 Nm (37 ft-lbs) ensures the washer is fully seated and the plug won’t vibrate loose, but stays well below the 70+ Nm point where you might strip the threads in the PTO housing.

Parts list – Main Transmission