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Reconditioning Flywheels, and a Slight Circumcision

May 13, 2011 | Troubleshooting & Repair
By Paul Christensen

When we rebuild our trusty little engines correctly, we send parts out to be inspected and reconditioned. Often the reconditioning process means removing metal from a surface to expose a new smooth surface for better sealing or contact area. Items that need attention include rocker arms, cylinder head surfaces, bearing bores, cam followers, flywheels and so on down the list. With reconditioning in mind, I would like to bring some attention to the flywheel and clutch surfaces.

Depth gage from clutch surface to friction surface, and the Porsche Spec’s book.

When the flywheel is reconditioned, the proper method is to remove just enough material from the clutch friction surface so that you have a nice smooth area for the clutch disc to contact. A like amount of material must be removed from the surface that the pressure plate mounts to so that you have the correct depth which is 24,0 mm (.945”) on the 180 mm clutch and 25,0 mm (.984) on the 200 mm clutch. This is the standard repair procedure and the most common job performed by many shops.
However, when the machining is done, the distance from the clutch friction surface to the surface where the gland bolt contacts the flywheel (the web) has been reduced. This is a measurement that is often overlooked and can have some consequences (see red arrows below.)

Flywheel dimensions to be maintained: F-depth to recess of web. G-web thickness.
H-width of oil seal surface.


Knowledgeable vendors, like the ones that advertise in our magazines, will complete the job according to the factory measurements. If the tolerances are marginal, they will advise you of the status and the best option is usually replacing the flywheel. On the other hand, many of your local shops will recondition your flywheel clutch and friction surfaces as per normal procedures like any other job. With the Porsche specifications not readily available to them, this may not be the best option for you.
The Porsche Workshop Manual has some of the tolerances and the small “Porsche Spec’s information, please…” book has a few more. Neither one seems to be a complete guide for tolerances, so the following measurements are a combination of the two resources, which may help direct you, if you use a local shop, to take off as little as possible and be within tolerance.

By the numbers
The distance from the clutch surface to the web surface for a 180 mm flywheel should be 12,4 mm (.488”) to 12,0 mm (.473”) nominal measurement, with a finishing down measurement of 11,0 mm (.433”).
The distance from the clutch surface to the web surface for a 200 mm flywheel should be 11,4 mm (.449”) to 11,0 mm (.433”) nominal measurement, with a finishing down measurement of 10,0 mm (.394”).
The web thickness of the flywheel should be 6,30 mm (.248”) to 6,85 mm (.270”) new and a minimum of 4,8 mm (.189”)

Caliper measuring the web with a backing on the opposite side of the web.

To be on the safe side and insure that the job has been done correctly, I install the flywheel, the gland bolt with the correct washer and put a little clay on the clutch disc or gland bolt*. Next, use a clutch disc alignment tool and fasten the pressure plate down as you would if you were finishing assembly. Now, disassemble the unit and measure the thickness of the clay.

Clay on the gland bolt.

* Absolute trivia to avoid nitpickers: The Workshop manual refers to this fastener as a Gland Nut, the Porsche parts book refers to it as a Hollow Bolt. I always thought a nut was a fastener that had internal threads that fit over a bolt or stud, and a bolt had threads on the outside diameter that fits into an internal threaded item. So from now on, I’m calling this item a Gland Bolt.
Another variable could be the gland bolt and washer. There have been several different manufacturers producing these parts over the years and a few of the measurements can vary regarding the thickness of the head of the bolt and style and thickness of a particular washer. The head of the bolt should be about 11,5 mm (.452”) thick.
If the clay indicates a clearance less than .060” I get nervous. To insure that the clearance will be enough, I machine an appropriate amount off the face of the gland bolt. There is usually enough meat on the gland bolt for this operation so you can remove .060 before you get too close to the “O” ring or felt washer area.
If the disc were to contact the gland bolt, it would tend to chatter and attempt to unscrew the gland bolt. I doubt it would do so right away because of the 360 ft/lbs of torque, but I strongly dislike any metal-to-metal contact in an engine.

Measuring gland bolt with micrometer, and the gland bolt in a lathe for machining.

Now to the backside of the flywheel. Remember in the September/October issue, I reviewed the dowel pin area of the flywheel in the Zig-Zag: Special measuring tools article. This small area needs to measure a taper of 1 degree 30 minutes or 1.5 degrees. If it does not, your shop needs to be capable of machining this with the web tolerances in mind.

Back of the flywheel with the small flat straight edge. Taper of the outer side of the dowel pin web must be maintained.


Seal the deal
With all this being checked and done, you are ready to install the flywheel and get on the road. But wait, there’s more. In the Summer 2009 issue of the 356 Club magazine article, I focused on the seals that are available. Thank heaven we can still get gaskets and seals for the 356 and 912 engines. However, with the passage of time, flywheel seals have changed and even improved, mostly for the better. Some are directional and others have a new lip added to the outside area. On our cars, both the blue seals in the gasket kits and the Viton seals have this neat new extra lip. In other applications and locations in the car this is not an issue, however, with the 356 and 912 flywheels this may be problematic.
When I disassembled a couple of engines in the past, I found that the outer lip on the new seals showed signs of high heat. To find out what the problem could be, I set a seal on the flywheel and aligned the inner seal lip flush with the beginning of the flywheel seal surface.

Flywheel, seal, and radius gage.

When the seal is pushed down on the flywheel about 4,72 mm (.185”) where it usually rides in our engines, the outer lip of the new generation of seals contacts the radius of the flywheel. When you check the back of the flywheel you can see the seal surface and radius of the flywheel surface as it converges with the back of the flywheel. In checking with the spec book, the seal surface is 13,25 mm (.521”) deep with a 3,5 mm (.140”) radius. The seal measures 10 mm (.393”) thick and the seal sets in the rear 9,05 mm (.356”) deep into seal seat area of the case.
If you add, subtract and confuse everyone with the above measurements, it appears that the new style of seals with the additional lip can then be forced up on the radius. With more than normal pressure put on the seal lip, limited lubrication, and excessive friction, heat is generated which burns and reduces the life of the seal.

An illustration of the lip’s interference with the flywheel’s radiused edge. 
Below: the circumcision of new seal on the left, original seal at right.


This is just my humble opinion and solution. Check it out on your components before assembly. As such, I hate to alter or “improve” a new part, but I take a new razor blade and carefully circumcise the extra lip off of the seal. The seal now has the same cross section as the older generation of the replacement and/or original flywheel seals. This eliminates the possibility of the outer lip contacting the flywheel, which could result in a shorter life.


Once again, I thought I was really clever and was first to discover and provide a solution for this problem. I was really pleased to hear from Vic Skirmants. He complimented me on the article and confirmed seal problem and remedy. He had experienced the same seal problem and has been altering the seals for quite some time. It’s nice when we hear from those who have such extensive knowledge of cars and all of their workings.
Remember to have all your rotating and reciprocal parts balanced prior to assembly.

Somewhat balanced, Paul C.

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