This is the oil seal between the crankshaft and flywheel (transmission end). If you remove the flywheel and your car has very high miles on it, it's a good idea to replace the rear main oil seal. This project is rated 3/5 difficulty only because it requires transmission removal, once the transmission is out it's a very easy job.
If you have a mk5 or newer Volkswagen TDI, do not use this article because there's a ring sensor on the seal which functions as the engine speed sensor. See 1000q: mk5+ RMS replacement. For the mk3 VW TDI, search the FAQ.
The rear main oil seal (RMS) on the TDI is a teflon/PTFE seal. The teflon type (pictured below) is a papery-plastic like seal with no metal spring and should be applied dry (with no oil or grease). The advantage over a rubber-spring type is that it has a wider lip that sweeps across the running surface of the crankshaft and leaves no groove. Teflon seals are also more durable and slippery.
Your teflon RMS should come with a plastic guide sleeve installation tool. It only fits one way and guides the RMS onto the crankshaft by stretching the seal a little bit for installation. If the rear main seal isn't leaking then I would leave it alone unless you have very high miles.
Note the raised rubber gasket on the flange. The service manual says to apply a bead
of gasket maker to the flange, your flange may already have a rubber seal on
it. In this case I would only apply gasket maker around the bottom of the
flange or the corner. Do not use a paper gasket. The
service manual suggest sealant D176 404 a2. An acid free hylomar gasket
maker is an exact substitute for the VW sealant.
Related links: 1000q: general TDI clutch FAQ
Remove the transmission. See 1000q: mk4 transmission removal for more details. Counterhold the flywheel using a counterhold tool and remove the pressure plate bolts (12 point) and the 6x flywheel bolts. The flywheel bolt holes will only align at one orientation. This ensures that TDC is at the same point. If you are installing a new flywheel, make a mark on the engine to note where TDC is. This lets you double check the position of the TDC mark on the new flywheel against the old flywheel.
Remove the shield. It should just pull off. Note the alignment dowels on the side of the transmission.
Remove the RMS flange. One of the bolts is circled below in
yellow. Don't forget the 2x T40 torx bolts on the bottom of the flange
that hold it to the
oil pan Do not pry close to the sealing surfaces or else you could scratch
Below is the end of the crankshaft exposed. Scrub away any corrosion with green scotch brite and wipe clean. Green scotch brite will not scratch the metal surfaces. Note that on this car, there was no gasket maker around the upper part of the flange, just the rubber seal built into the flange. There was only gasket maker at the corner and the oil pan seam.
The factory service manual's method to replace the RMS flange is to remove the oil pan, install the rear main seal, and then install the oil pan with new oil pan sealant. This ensures that the flange is sealed correctly at the corner and that the bead of gasketmaker along the bottom of the flange forms a tight seal. It also ensures that the seal is pressed on straight with no damage to the seal. This is the best way to install the seal.
If you wish to remove the oil pan, see 1000q: mk4 engine oil pan removal.
Because this car was sitting on the oil pan, it got a dab of gasket maker at the corners and a thin, even smear along the oil pan-flange surface (the bottom part of the new flange) to seal it. As long as you press the seal on straight is should be OK but there's always the risk of a leak. Since it's your car the installation method is up to you.
More is not better, a thin 2-3mm bead of gasket maker is sufficient. Any gasket maker should be inboard of any bolt holes to seal against oil.
One person in the forum reported that they had an oil leak coming from one of the flywheel bolts. I have never heard of this before but their shop said some gasket maker around the bolt fixed it. Be careful if you do this because any lubricant in the threads or around the head of a bolt may change its torque spec.
Install the flange and teflon seal dry (no oil or grease). The service manual diagram may have a note to lube the RMS but do not lube it unless using a rubber-spring seal! If you have a Teflon seal, instal it dry! If using a teflon seal, let it sit for at least a few hours before filling with oil and at least 4 hours before starting the engine to let the seal seat.
Also note that the shorter bolts are horizontal, the longer bolts are vertical for the oil pan. *Thanks for the tip, YMZ! He found that a prior mechanic had installed a shorter bolt for the oil pan.
Press the flange onto the crankshaft and the install guide sleeve will get pressed out as you guide the flange onto the crankshaft. This prevents folding or damage to the seal.
Torque for the bolts on the rear main seal flange is 11 ft-lbs.
Wait a few hours before refilling the engine with oil. This lets the teflon seal "seat".
Do you have any questions on replacing the rear main oil seal on your VW TDI? Please ask in the myturbodiesel.com forums