A fellow attempting to repair a flywheel seal leak on a recently rebuilt engine had the seal pop out of the case, and he wondered why. A helpful participant posted this:

.

.

My response:

.

This is unfortunately internet myth. There is only one spec seal regardless of transaxle type, 12mm depth. There are 10mm deep seals on the market which can be used, they can be driven in flush or the extra 2mm until they bottom on the step within the seal recess. Makes no difference except sometimes by bottoming a 10mm deep seal, the lip may ride on an unworn part of the f/w or driveplate seal hub. But it’s better to use a 12mm depth seal for its shell strength (read on to learn why), and resurface, sleeve, or replace a worn seal hub.

.

The 12mm deep seal is preferred, and there are notable quality differences between the commonly available seals, which I described in a post long ago. Here’s the relevant passage, edited to correct outdated info:

.

“The ones that come with the gasket sets are pretty inferior. Three obvious measurable aspects differ from the OEM seal: the depth of the shell front to back, the thickness of the tension spring around the seal lip inside, and the weight of the seal, reflecting the thickness of its shell material.

.

I have 4 different ones, and took measurements: a Reinz cheapo that come in the gasket set; an Elring cheapo from their set; a Reinz orange silicone one that used to sell for $10 by itself at VanCafe*; and the Sabo all-black Viton seal, which is the OEM VW seal.**

.

The Reinz cheapo seal is dark purple in color. The shell depth is 9.8mm, spring width only 1.9mm. Weight 48g.

.

Elring is black, grey, and red. Depth 10.1mm, spring 2mm. But this one weighs only 40g! Very thin shell material.

.

Reinz orange silicone is 11.8mm, spring 2.25mm, weight 51g.

.

Sabo black is 12.1, spring 2.5mm. You don’t need a scale to feel how much heavier this one is: 61g!

.

So the more expensive ones have a deeper and meatier shell, and stronger spring to keep pressure on the seal lip. The difference in weight between the cheap Elring and cheap Reinz purple is by itself remarkable. The Sabo is much heavier still than the Reinz orange even though dimensionally they’re very close. There are slight differences in the lip design, but all have the minute directional fins around the seal lip that lead oil back inward, a common feature on lip seals and the reason you must be sure the seal you use is built for the direction your shaft or hub will spin. And of course they are made of different polymer compounds, Viton (fluoroelastomer) and silicone being the best overall polymers for low wear, high conformity and high heat tolerance.

.

I think the Reinz orange is a good value, and they worked well for me. But the Sabo is truly bomber.”

.

*NLA at VC/RMW. This was promoted as Viton back then but I later learned it’s silicone, which is also a great material for this application, and that has been corrected in the text.

.

**This is in fact the OEM seal. Now sold at GW for $7.95 https://www.gowesty.com/product/mechanical/3944/flywheel-flexplate-seal-rear-main-?v=

.
and VC/RMW has what appears to be the same for $6.50

https://www.vancafe.com/029105245B-p/029105245b.htm

.
These were hard to find when that was written, and GoWesty claimed to have bought all the stock and was charging $30 apiece! But, they became commonly available years ago from many places, which even GoWesty had to admit to by charging a reasonable price.

.

.

A 12mm deep seal driven in to the step inside the bore will be less than 1mm below flush. If the seal is set at this depth or at any depth below flush, it is impossible for either f/w or driveplate to contact the outer seal face. That can only happen if the seal starts coming out from the bore, but if it does emerge only a couple mm its face will meet the spinning f/w or driveplate which will spin the seal and/or tear it up. In either instance, the problem is the seal not staying in its bore.

.

Reasons for this? Well, seal shell strength provides the basis for the seal to continually exert the outward tension needed to stay in the bore, so the heavier, stronger shelled ones have a decided advantage in staying put; you can really tell by how much easier it is to drive one of the cheapo ones into place. Often people drive a seal in unevenly so that parts of the outer rubber covering are torn, invisibly until removal, which results in a reduction in outward tension. And blockage of the seal cavity drain and breather channels from excessive sealant will obviously cause hydraulic pressure to build in the cavity which will drive out the seal with ease. Even if the drain hole is blocked but the breather is open, at high rpms the amount of oil escaping the #1 crank bearing may be able to overfill the seal cavity at a rate faster than the oil can escape out the top breather channel alone. With its large total surface area, it would take very little internal pressure to drive any of the seals right out of the bore. Both those channels need to be clear!

.

Of course the way to ensure those small channels aren’t blocked is to use a minimum of sealant when joining the crankcase halves, but if you’re dealing with someone else’s work, I’ve found that a fairly thin, stiff wire can be poked into the breather and drain channels on an assembled block to check that the channels are clear. I keep a spool of gardening twist ties (tomato ties) in the shop for many uses, it’s plastic covered and stiff enough to push thru the channel while flexible enough to make the curve, and the wideness of the plastic coating strip may help clear a bit of excess sealant. Concerns about bits of sealant bead loose in the sump are overstated, things like that will in most cases lay harmlessly in the sump, and if they’re sucked up by the oil pickup the large and fairly fine pickup screen will catch them. Any bits small enough to pass the screen (extremely unlikely) will pass harmlessly thru the oil pump and be picked up in the filter. Compared to anything that small, all the engine oil passages are enormous and the idea of such things blocking them are the fever dreams of amateurs. Don’t worry about imaginary stuff, get the job done.