Or maybe wrap the splines in electrical tape, then lightly oil it. Seal should slide over that like butter.

1+ on electrical tape/paper on the splines.

Re seal tool. Over the years, I have used Plumbing PVC Coupler with success.
All you need is to bring the seal to Home Depot etc. and match it with the appropriate coupler, which should be a tiny bit smaller than the O.D. of the seal.

In my situation, most of the time, it is either 1.5" or 2" coupler, which is about $2-$3 or so.

So, should the seal be driven only until it's flush with the mounting flange for the guide tube (not recessed at all)? I think that's how the original one was, but I didn't pay much attention since tis said drive it home, which usually means until seated on whatever is behind the seal.

Good question.

Even 1-2mm can cause a leak.
Best is to observe the factory setup.
Take photos, feel the factory seal with your finger so you know EXACTLY where it should be.

I have made this mistake before in a Volvo axle seal, installed the seal 2mm deeper than factory: it leaked! I had to redo it.

Anyway, most bore hole has the following setup:
- Air ---> beveled edge (about 2mm) ---> straight edge.
The question is: the factory seal is how far from the beveled edge?
Most of the time, it is about 1-2mm and that is it.
This is why at factory they have a tool so all the seals sit at the same depth for consistency.

Below is a GENERIC anatomy of a seal setup...


---

Thanks for that info. I would think that unless either the shaft or the bore is tapered, the seal should seal as long as it's in there square.

My problem is I already replaced the seal and drove it in recessed, but I can't remember fully where the original seal was placed. I think it was flush with the guide tube flange, but I did so many seals from the RMS, to differential side seals, pinion seals, etc., I'm not 100% sure.

I'm betting, though, that my mistake that's causing the leak is that I didn't wrap the spines before putting the seal on. The shaft diameter where the splines are is a bit smaller than the i.d. of the seal, but I'm sure they still made contact even though I was aware of the danger and tried to be careful not to have them touch.

The good news here is I think I can test the seal before reinstalling the transmission by tilting it forward a bunch so the fluid level rises to the seal.

Re seal installation...

- Sometimes during installation, part of the seal INNER lip folds on itself, causing leak. This happens when the seal just goes past the splines, once it is the shaft, rotate it as you push it in (oil the inner lip first). The rotation will prevent folding over of the inner lip.

- Once the seal hit the bore, you cannot rotate any longer, then just drive it in using appropriate tool (factory tool or coupler as mentioned). The depth of installation is important. Get some photos from the web for the MT input shaft seal to get an idea where the seal should be.

- At least you are getting good at removing the trans...

To keep this going in case someone comes across this thread in the future, I'm still waiting on the new seal, but I pulled the old one and can clearly see what I did wrong.

BTW - this applies to any of the ZF 5 speed/S5 xxx transmissions used in E46/E39/E53 etc.

The seal should definitely be driven only until it's flush with the flange that the clutch throwout bearing guide tube mounts to. Do not drive it all the way in! If you drive it all the way in (like I did), the outer seal case will land on the input shaft bearing, and it will be square. However, at this depth, there is a rather thick thrust washer and circlip that will put pressure on (and eventually chew up) the seal lip.

I've attached a picture of what's behind the seal, but it doesn't really show the thickness of the thrust washer and circlip. Just trust me that that circlip will be rubbing on the seal lip, and it will leak.

I was in a rush when I did this part, and, as always, when I'm hasty is when I get burned.

New seal is in, and after about 50 miles - STILL BONE DRY! This confirms that the new seal should only be driven flush with the guide tube mating surface. I didn't use a special tool. I just tapped it flush by using a 1/2" metal dowel and slowly working my way around the outside of the seal - same technique as I used for the RMS.

Glad to have the underside clean and dry. If you've done your OFHG and VCG, but still have a mist of oil all over the place, unfortunately, it's probably the oil pan gasket. If you see oil at the right front of the motor, it's almost certain it's the oil pan gasket.