Kid comes in the shop wringing his hands, all upset. He's just paid a lot of money for someone to rebuild the engine in his bug and it's leaking oil from the front seal. The guy who did the engine sez it's not his fault, it must be coming from the rear tranny seal, wants to sell the kid a rebuilt tranny. And besides, all VW's drip oil; no big deal. To further the kid's education you show him how the seal has been ruined; whoever installed it used a hammer. At least half the drippy oil seals I see are caused by improper installation of the seal. A few are due to improper engine assembly. 1 bearing is simply worn out, allowing the crankshaft to move back & forth. When it does, it acts as a nice oil pump, defeating the purpose of the lip-type seal. 1 main bearing oil seal is neoprene or silicone rubber bonded to a metal ring with a circular coiled spring inside to maintain a leak-free sliding fit around the center boss of the flywheel.
The oil seal fits into a recess cast into the crankcase; it's a tight fit. Properly installed, the oil seal ends up slightly below the level of the casting. The factory service manual shows the seal being pressed into place using a screw-type fixture, but an experienced mechanic can install one using repeated light taps from a plastic mallet. A leaky seal due to improper engine assembly is a bit more subtle. 1 is generously supplied with pressured oil form the main oil gallery. In normal operation the oil lubricates the journal and escapes from both sides of the bearing. On the flywheel-side of the bearing the oil collects between the bearing and oil seal, flowing back to the sump via a drilling in the left half of the crankcase. Want to guess what happens if that drilling gets blocked? 1 main bearing oil return is blocked, oil pressure will build up behind the seal and the thing will eventually leak. That's the obvious answer.
But a blocked oil return port also results in accelerated wear since the thrust face of the bearing and the associated shim stack is not being provided with a circulating supply of oil. Most often, the oil return passage is blocked by an over zealous application of sealant when the crankcase halves are joined. 1 main bearing. And that's just on the inside of the crankcase. Outside, the sealant is oozing all over, including down into the recess for the oil seal. Being in the 'corner' of the recess, the oil return passage gets more than its share when it shouldn't have gotten any at all. I've also seen engines with sealant deliberately painted into the oil seal seat, apparently hoping to stave off leaks. In those cases the oil seal itself was always damaged by hammer blows. Think about that for a minute. The guy builds an engine, hammers in the oil seal and sure enough, the puppy leaks like a sieve.
So the next time he globs on a lot of sealant, hammers in another oil seal and this time it leaks even worse, convincing him it's impossible to keep a VW engine from leaking. And besides, everyone sez VW's leak. In effect, the builder has just created a direct path from the oil pump to the ground under the engine. So why do people install oil seals with a hammer? First, because they see a real mechanic do it successfully and never understand that it takes considerable skill to do it right. Correctly installed, the seal will be slightly below that level. But the most common reason for all those drippy engines is the fact everyone assumes that hammering requires no skill. Oil seals are designed to be pressed or pulled into their seats. It's possible for an experienced mechanic to install them with a plastic mallet, or even a hammer in the case of some axle seals, but it's also possible for a skilled surgeon to do an appendectomy with a pocket-knife. The emphasis here is on the skill, not the tools.
