Sunday, December 3, 2006

Leaky Oil Seal

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. But the kid is sure it's leaking more than it did before it was overhauled and he's never had his tranny leak and the leaky stuff doesn't have that thick sulphury smell like tranny lube and his mom is all upset about the gunk on the driveway.

That last is the real reason he's here :-)

So you drop the engine, pull the old seal, check the end-play ('way off), clean the oil drain drilling, put in a new shim stack and a new oil seal. To further the kid's education you show him how the seal has been ruined; whoever installed it used a hammer.

Hammer = Drips

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. The remainder are from excessive end-float; the thrust face of the #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.

The #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. An unskilled mechanic will try to do it with a hammer and while it might look okay, about half the time the hammer blows cut the silicone rubber where it's molded over the invisible metal ring inside of the oil seal. Oil quickly discovers the cuts and you've got a leaker on your hands.

Too Much Sealant = Drips

A leaky seal due to improper engine assembly is a bit more subtle.

Like all of the main bearings, #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? (Be careful, it's a trick question.)

When the #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. Here's how it happens: The left half of the crankcase is in the fixture, parting line up, the guy swabs on about four times more sealant than needed and when he drops the right half of the crankcase into place it squeezes the sealant out, which flows downhill into the oil return passage from the #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. (Hint: After closing the crankcase for the last time, check the oil return drilling with a Q-tip to ensure it is clear.)

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. Conventional Wisdom wins again.)

Indeed, when you combine an improperly installed oil seal with an improperly assembled engine (ie, the blocked drain hole) the engine doesn't just leak, it gushes. 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. Secondly, they do it because most of the manuals say it's okay to hammer it in... and about half the manuals show the seal installed incorrectly, flush to the outside of the crankcase. 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.

Push or Pull = No Drips

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.

Oil seals aren't expensive and they don't look very sophisticated but there's more to them than meets the eye. If you toss an old one on the barby and wait for a while you'll be able to examine what's under the rubber. You'll see that most of them start out as a segmented ring of thin sharp steel. Cover that with rubber, tap on it with a hammer and it cuts the rubber as neatly as a knife.

So press them in. Or pull them. You can make a dandy puller-presser for your front brake drums (the seals you'll replace most often) using nothing more than a length of all-thread, some washers and three nuts. And you can buy a screw-type pressor for the #1 main bearing seal, although it's easy enough to make one, assuming you have a lathe. (See the drawing at the top of this article.)

Or you may drive them in with a 'seal-seater,' if the seal is small. By distributing the force of the hammer blows uniformly, a seal driver lets you pop the things into place with one or two well placed blows of a hammer. If you've a lathe, making oil seal drivers is a spare-time sorta thing; all are simple turnings, and aluminum or even hardwood works as well as steel.

Big seals are different. Because of their tendency to cock in the bore, large-diameter seals are best installed with a press or fixture. Rear axle seals are especially troublesome due to their deeply recessed position in the seal cover. Because of their proximity to the brakes and the fact that any leak could leave you without brakes, the wiser course is to always press-in rear axle seals.


With the exception of the Muir manual, books on maintaining your Volkswagen assume a certain level of competence. Learning to tap a seal into place with a plastic mallet isn't difficult; it's one of the many minor skills acquired during the apprenticeship all mechanics must endure. It is also one of the minor skills many self-taught mechanics never bother to master. (Hint: Start with an old seal. And an old engine case. When you can tap the thing in a dozen times in a row without damaging either the case or the seal you're probably ready to try it with a new seal on a good case. Along the way, you will have learned how to remove the thing as well.) But installing a seal with a hammer falls into the category of 'Field Repairs;' things a skilled mechanic must do when the proper tools are not available. Pressing the seal into place is not only safer, it's usually faster. And a pressed-into-place seal is cheap insurance against oil leaks.

Copyright © 1995 Robert S. Hoover

1 comment:

Anonymous said...


Good advice in replacing a seal. I have the same troubles in replacing a rubber seal between the oil indicator pipe and the oil pan. I have tried a few times tapping the pipe and the seal into the pan with a wood hammer, but it seems I need to be more gentle than that because the seal was cut in two.

This is the schema with the pan:

The rubber seal is at no 13. I guess I will try next in pressing the pipe into the pan by using lever. But if you can direct me to a better solution, I would be much grateful.