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Crankshaft Oil Seal

We are currently having problems with an 2.2 911T that has just been rebuilt and leaks oil from the rear crankshaft oil seal.

The engine has been out 3 times and the owner has been told that it is likely he will have to replace the crank - Ouch.

We have now inherited this problem and have learnt the following:

The end of the crank has been re-surfaced to about 64.6mm diameter from the nominal 65mm and we assume that this is to remove a wear mark on the 'journal'.

This means that the seal doesn't locate very well and continually weeps oil.

Porsche used to make a seal that was 64.5 x 85 x 10 but they are NLA and I have looked hard to find a NOS part.

It is possible to buy an NBR seal with dimensions of 64 x 85 x 10 but the maximum speed of this seal will be around 3000rpm and with the journal at 64.6 it will probably overheat very quickly. Even using Viton which is good for 8000rpm the fit would probably still cause problems.

We could take the crank out and reduce the size to 64mm but I can't find a Viton version of this seal and again I have looked.

I can have a seal manufactured at 64.5mm but tooling is around $1500 and I have to order a minimum of 250 seals. Lead time is 2 months.

If I could find a 64mm dia Viton Seal it would still mean dismantling the bottom end.

We have looked at Speedi-Sleeves but am not 100% convinced and we have also considered hard chroming the journal.

We finally decided to have a custom PTFE seal made by a local specialist.

It has been made to a 64.6 size and should be good for at least 10 000rpm.

It cost $50 and was delivered in 3 days.

We fitted to the engine which is at least initially bone-dry.

Fingers crossed



Last edited by chris_seven; 08-26-2016 at 04:50 AM..
Old 08-26-2016, 04:45 AM
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wow!
Old 08-26-2016, 04:48 AM
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I was wondering about PTFE vs. heat so I pulled up the wiki. It's some pretty amazing stuff and was discovered completely by accident. I'm surprised there aren't more PTFE parts in cars in general.

Another fun fact...geckos and, in fact, pretty much all insects can't climb PTFE because of it's resistance to van der Waals forces. So, your rear seal is small lizard AND bug proof.

Well done, sir!
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Old 08-26-2016, 06:31 AM
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Quote:
Originally Posted by tharbert View Post
So, your rear seal is small lizard AND bug proof.
Well that's a relief
Old 08-26-2016, 07:39 AM
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That is a reasonable price for a custom made teflon seal. Yes, they get installed dry, absolutely no oil, grease, etc. or they will leak like a sieve.
Old 08-27-2016, 06:28 AM
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Although an improved seal is always a welcome development, my concern with your dilemma would center around the undersized crank dimension.
If I read your description correctly, the crank is .4mm (.016 ") undersized.
Without checking the book, I believe the spec for the dimension is -.005" .
This dimension is important because it locates the flywheel.
The better repair for this situation is to repair the crank with a sleeve.
I would be hard pressed to use your seal to repair this oil leak.


BTW: Back in my VW type 1 days when we had a flywheel seal that continued to leak we would reduce the seal dimension by removing the tension spring and trimmings off 4 or 5mm and replacing it. This tightened the seal and quite often worked until the next rebuild.
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Last edited by Henry Schmidt; 08-28-2016 at 12:48 PM..
Old 08-27-2016, 07:21 AM
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The car came to us running and didn't show any signs of being out of balance due to flywheel run out.

The flywheel fit is quite snug and clocks up as well within an acceptable run out - which we did check. I would agree that this is surprising.

The problem with the seal is that with the diameter already being reduced the active part of the seal will not run at is correct angle and this is why it will leak. It will not run on the seal edge but on the front face of the seal and is likely to overheat.

Taking out the spring will not affect the geometry so I am not sure how this help.

We would have tried an R22 or R23 but again we can't find them in Viton and NBR will nit take the speed.

If we could have stripped the engine I would use a speedi-sleeve and if the PTFE Seal doesn't work this will be the next attempt.

Last edited by chris_seven; 08-28-2016 at 01:41 AM..
Old 08-28-2016, 01:06 AM
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Interesting dilemma.

I'm not surprised about the lack of PTFE in automotive use. PTFE, while very slippery and chemically inert, is also relatively soft and has low yield strength. You can take a block of PTFE and squish it readily to permanently deform the shape and it won't readily spring back. It's soft enough that you can pick up contaminants in your wear/contact surfaces unless things are very clean.

I have attempted use of PTFE seals in "the industrial world" for similar situations on rotating machinery. (RPM's were lower but diameters much larger so surface speeds were similar to what I think you'd find in a 911 engine.)

It was very important to keep those seals clean during use. I found some long-term sealing problems where minor exterior grit (like you might get with "clutch dust") would find its way to the sealing surface and then abrade the surface away. I remember another situation in which the outer diameter of the seal took a cold-flow set and and leaked from the OD under pressure. (Funny thing about sealants is that they won't stick to PTFE either unless you etch PTFE surface.) Final solution in both cases was to install a sleeve and go back to elastomer seals.
Old 08-28-2016, 08:07 AM
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Originally Posted by chris_seven View Post
Taking out the spring will not affect the geometry so I am not sure how this help.
Hi Chris,

Henry made a good suggestion, but its simply shortening the spring a little bit, not wholesale removal.

Certainly, its a stopgap measure, but an effective one for an engine that doesn't require rebuilding at this time.
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Old 08-28-2016, 10:02 AM
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I certainly wouldn't use this machinist again who took it upon himself to turn it down that much.

Spring trick usually creates slightly more wear on crank surface.
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Old 08-28-2016, 01:53 PM
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Quote:
Originally Posted by chris_seven View Post
The car came to us running and didn't show any signs of being out of balance due to flywheel run out.

The flywheel fit is quite snug and clocks up as well within an acceptable run out - which we did check. I would agree that this is surprising.

The problem with the seal is that with the diameter already being reduced the active part of the seal will not run at is correct angle and this is why it will leak. It will not run on the seal edge but on the front face of the seal and is likely to overheat.

Taking out the spring will not affect the geometry so I am not sure how this help.

We would have tried an R22 or R23 but again we can't find them in Viton and NBR will nit take the speed.

If we could have stripped the engine I would use a speedi-sleeve and if the PTFE Seal doesn't work this will be the next attempt.
Pushing the edge of a tolerance to extend the life of an engine seems reasonable but 3 or 4 times the tolerance limits challenges what I would consider acceptable.
A low RPM 2.2T already suffers from a non-counter weighted crank so an inconsistent flywheel fit is enough to force the issue for me. Imagine the misery created for the customer if the flywheel shakes loose?
Seems like an easy sell to a customer that wants it right.

Quote:
Originally Posted by manbridge 74 View Post
Spring trick usually creates slightly more wear on crank surface.
I have never seen a minimal increase in seal tension create a noticeable increase in surface wear but experiences may vary.
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Old 08-28-2016, 02:53 PM
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Hent,

The flywheel clocks up as running true so unless it shakes loose due to a torsional vibration then it should run without too much trouble.

I don't see why it is any more likely to shake loose than in a standard engine.

It runs quite smoothly .


There seems significant conflicting opinions about T cranks.

Many people seem to recommend them for race engines and others seem to think they are a problem.

On a standard engine I can't see why they should be an issue as even if the flywheel was eccentric it would introduce a lateral force and not create the torsional vibration that would make it loosen.

If you create a basic Holzer Table for a T crank they seem to be quite stable to around 7800rpm.

This engine is already a misery and the PTFE seal is a last ditch attempt to solve the problem before removing the crank.

I think that the shop who re-surfaced the crank made a huge mistake but as we didn't rebuild the engine to start with I don't know who carried out the work.

I do agree that shortening the tension spring is a better idea and will try that if we encounter this problem again.

I spent some time discussing a solution with a seal specialist and we didn't come up with this idea, which does make sense.
Old 08-29-2016, 12:23 AM
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Originally Posted by fanaudical View Post
I'm not surprised about the lack of PTFE in automotive use.
PAG started using PTFE rear-main seals in the 911 GT models back in 2005.
Old 08-29-2016, 08:42 AM
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There are companies in USA that'll make custom seals. Last one I had done was $200. Few years ago though...
Old 08-29-2016, 11:14 PM
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Interesting note, JasonAndreas. Are those PTFE seals on the 911 GT all PTFE, or are those PTFE-lipped seals? (Meaning elastomer seal with a PTFE wear ring on the contact surface?)
Old 08-30-2016, 08:56 PM
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