http://powertorquesystems.com/Corvette/ ... eC4MN6.pdf
SAVING THE C-4 MN6
For 1989 car production, Chevrolet introduced option code MN6 for Corvettes. This 6 speed manual
transmission/clutch assembly was primarily engineered and built by German companies, with the gearbox
coming from Zahnradfabrik Friedrichshafen (known as ZF). Their model number for the Corvette transmission
was S6-40. If you have tried to purchase a critical C-4 ZF S6-40 clutch related component recently, then you
have undoubtedly noticed something, well, troubling. It appears that if in some sort of twisted magic
performance, all of the Original Equipment Manufacturer (OEM) parts have disappeared. In an effort to keep
our 1996 LT4 powered race car up and running, I certainly have noticed this disturbing phenomenon. After my
initial disbelief, frustration, and yes anger, I decided to dig in and do some research. Please join me in my
journey to discover what has happened to lead us to this point in time. A word of warning: During this ride
along, put your race helmet on and strap yourself in….there are some major potholes in the road course ahead.
And yes, it is a good idea to have some extra strength Tylenol close by.
I think it is helpful to take a look at the history of the major OEM suppliers of our clutch systems. Let’s start
with the heart of the beast, the now infamous dual mass flywheel (“DMF†for short). The DMF was utilized in
conjunction with the ZF S6-40 6 speed manual transmission because it is very good at one thing; dampening the
vibrations and harmonics coming from the small block Chevy motor. The DMF was made by LuK, a German
company. C-4 Corvette DMF’s were produced until sometime in 2007. Towards the end of production, LuK
notified GM that the tooling used to make the DMF was worn out and needed to be upgraded or replaced. GM
declined to spend the money on new tooling. As orders from the General had been steadily declining for this
component, LuK also opted not to make an investment in tooling. At the bitter end, the production line was
shut down and DMF’s were painstakingly assembled by hand. That may explain why the price of them shot up
from around $600 each to $1100 a copy.
The pressure plates and solid friction discs for our cars originally came from Valeo. Around 2000, the pressure
plate production was moved from Virginia (USA) to Brazil. Sometime during 2007 – 2008, the company that
was casting the aluminum housings for the pressure plates went bankrupt. Not only was that bad for us
Corvette owners, but it gets worse. No one could find the molds used to make the aluminum housings. No
molds, no pressure plates. Easy to understand, difficult to swallow (might be a good time for one of those ES
Tylenols). Because Valeo could no longer manufacture pressure plates and therefore clutch kits for the C-4
Corvettes, they allowed their inventory of friction discs to dwindle as well.
Another small but important clutch component is the throw out (release) bearing. The originals were supplied
by INA (the “I†is pronounced as an “Eâ€), another German manufacturer. For simplicity I am going to state that
INA merged with LuK, and then they merged with FAG of Germany. More mergers/acquisitions occurred and
this conglomerate is now known as the Schaeffler Group Automotive. Trying to determine the INA throw out
bearing production history has turned out to be a major archeological unearthing. I’m still digging……
The story line for the next component is my personal favorite. The clutch master and slave cylinders were
designed by Lucas-Girling (German company) and originally manufactured by Lucas at their Cincinnati, OH
facility. At some point, Lucas merged with Kelsey-Hayes, which was subsequently bought by TRW. The last
clutch master and slave cylinder production was completed mid 2008, at which time the contractual agreement
between GM & TRW expired. In this situation I could not definitively determine whether the original tooling
used to make these cylinders was owned by GM or TRW. Really, it doesn’t matter as I do know that this
tooling was sold at auction and that the pieces that did not sell went to the scrap yard.
The dual mass flywheels are history. Schaeffler Group has done several physical inventories at their U.S.
warehouses and no DMF’s have been found. There are zero units within the GM distribution system. Rebuild
kits do not exist. Resurfacing is difficult but if done correctly, is possible. That gives you a fresh friction
surface, but it certainly doesn’t rejuvenate the 100 plus parts found inside these units. The only good news here
is that you no longer have to buy “F†body flywheels and machine .090†off of them. Plenty of dimensionally
correct single mass flywheels are available from companies such as Spec, Fidanza, and Carolina Clutch.
The Valeo LT4 pressure plates with their high clamp load (10,000N) are no longer available through any means
of distribution. Valeo made them for everyone, including GM, aftermarket clutch manufacturers and the large
retail automotive chain stores. All units you see advertised today, such as the ones shown in the Corvette mail
order specialty catalogs, are made in China. Valeo has tested the Chinese models, and the results are
disappointing. Stress cracks showed in the steel diaphragm fingers as early as 50,000 cycles, with some
complete failures at less than 100,000 cycles. Original Valeo plates test out at 1 million cycles or better. Valeo
is exploring other options at this point in time….I will keep you posted. During my research I was made aware
of a pallet of Valeo race version pressure plates, originally manufactured for Chevrolet Motorsports. These are
highly machined pressure plates which are rated to 11,000 rpm! The plates are now at my shop.
The solid friction discs from Valeo are gone as well. I bought the last two discs from them in early February,
2011. Valeo has told me that they do not anticipate any more production on these discs unless a worthy
pressure plate replacement can be found. I have been working with RAM Clutches to make an aftermarket (or
better!) equivalent, and I believe they have succeeded in doing so. These are custom made discs and they weigh
in .5 pounds lighter than the Valeo. They are in stock at my shop. If you convert to a single mass flywheel,
then you will need a “sprung hub†friction disc and there are several excellent choices available from different
aftermarket performance manufacturers.
I have been unable to locate any 1989-1996 throw out bearings from INA. In fact, I received no response from
them after weeks of inquiring about the status of these bearings. Fortunately, Valeo is manufacturing the
bearings along with a heavy duty wire retainer. These are available from a number of sources, just not your
local Chevy dealer.
There are no more original extruded steel clutch master or clutch slave cylinders available through normal
distribution outlets. I purchased the last OEM slave cylinder from Corvette Central in April of 2010. All units
out there (and I do mean all), are the cheap, cast cylinders coming from China. It doesn’t matter if it’s in an AC
Delco box, a Raybestos box or a Sach’s box, they are all the same. Some components used in these cylinders
are extremely poor quality. Our cars deserve better. At least I think so. In an effort to save the integrity of the
hydraulic portion of our clutch systems, I was able to find and purchase the entire inventory of individual
cylinder parts from TRW Automotive. These parts were destined for steel scrap salvage and I intend to breathe
life into them. I also have a limited number of NOS (new old stock) completed slave assemblies, and I can
totally rebuild clutch master cylinders through a core exchange program (please don’t throw away your
originals!).
I would like to thank the good people at Valeo and TRW Automotive for their help in locating these forgotten
Corvette parts. They were extremely generous with their time and knowledge. If you have a C-4 and you are
interested in any of the parts mentioned in this article, please feel free to contact me at powertorque@msn.com
via e-mail, or phone me at 641-342-7446 (Central Time, M-F 8:00 am to 5:00 pm). You can also visit our new
website. The address is http://www.powertorquesystems.com. Have a great race season!
Submitted by CCI member Jim Jandik
SAVING THE C-4 MN6
For 1989 car production, Chevrolet introduced option code MN6 for Corvettes. This 6 speed manual
transmission/clutch assembly was primarily engineered and built by German companies, with the gearbox
coming from Zahnradfabrik Friedrichshafen (known as ZF). Their model number for the Corvette transmission
was S6-40. If you have tried to purchase a critical C-4 ZF S6-40 clutch related component recently, then you
have undoubtedly noticed something, well, troubling. It appears that if in some sort of twisted magic
performance, all of the Original Equipment Manufacturer (OEM) parts have disappeared. In an effort to keep
our 1996 LT4 powered race car up and running, I certainly have noticed this disturbing phenomenon. After my
initial disbelief, frustration, and yes anger, I decided to dig in and do some research. Please join me in my
journey to discover what has happened to lead us to this point in time. A word of warning: During this ride
along, put your race helmet on and strap yourself in….there are some major potholes in the road course ahead.
And yes, it is a good idea to have some extra strength Tylenol close by.
I think it is helpful to take a look at the history of the major OEM suppliers of our clutch systems. Let’s start
with the heart of the beast, the now infamous dual mass flywheel (“DMF†for short). The DMF was utilized in
conjunction with the ZF S6-40 6 speed manual transmission because it is very good at one thing; dampening the
vibrations and harmonics coming from the small block Chevy motor. The DMF was made by LuK, a German
company. C-4 Corvette DMF’s were produced until sometime in 2007. Towards the end of production, LuK
notified GM that the tooling used to make the DMF was worn out and needed to be upgraded or replaced. GM
declined to spend the money on new tooling. As orders from the General had been steadily declining for this
component, LuK also opted not to make an investment in tooling. At the bitter end, the production line was
shut down and DMF’s were painstakingly assembled by hand. That may explain why the price of them shot up
from around $600 each to $1100 a copy.
The pressure plates and solid friction discs for our cars originally came from Valeo. Around 2000, the pressure
plate production was moved from Virginia (USA) to Brazil. Sometime during 2007 – 2008, the company that
was casting the aluminum housings for the pressure plates went bankrupt. Not only was that bad for us
Corvette owners, but it gets worse. No one could find the molds used to make the aluminum housings. No
molds, no pressure plates. Easy to understand, difficult to swallow (might be a good time for one of those ES
Tylenols). Because Valeo could no longer manufacture pressure plates and therefore clutch kits for the C-4
Corvettes, they allowed their inventory of friction discs to dwindle as well.
Another small but important clutch component is the throw out (release) bearing. The originals were supplied
by INA (the “I†is pronounced as an “Eâ€), another German manufacturer. For simplicity I am going to state that
INA merged with LuK, and then they merged with FAG of Germany. More mergers/acquisitions occurred and
this conglomerate is now known as the Schaeffler Group Automotive. Trying to determine the INA throw out
bearing production history has turned out to be a major archeological unearthing. I’m still digging……
The story line for the next component is my personal favorite. The clutch master and slave cylinders were
designed by Lucas-Girling (German company) and originally manufactured by Lucas at their Cincinnati, OH
facility. At some point, Lucas merged with Kelsey-Hayes, which was subsequently bought by TRW. The last
clutch master and slave cylinder production was completed mid 2008, at which time the contractual agreement
between GM & TRW expired. In this situation I could not definitively determine whether the original tooling
used to make these cylinders was owned by GM or TRW. Really, it doesn’t matter as I do know that this
tooling was sold at auction and that the pieces that did not sell went to the scrap yard.
The dual mass flywheels are history. Schaeffler Group has done several physical inventories at their U.S.
warehouses and no DMF’s have been found. There are zero units within the GM distribution system. Rebuild
kits do not exist. Resurfacing is difficult but if done correctly, is possible. That gives you a fresh friction
surface, but it certainly doesn’t rejuvenate the 100 plus parts found inside these units. The only good news here
is that you no longer have to buy “F†body flywheels and machine .090†off of them. Plenty of dimensionally
correct single mass flywheels are available from companies such as Spec, Fidanza, and Carolina Clutch.
The Valeo LT4 pressure plates with their high clamp load (10,000N) are no longer available through any means
of distribution. Valeo made them for everyone, including GM, aftermarket clutch manufacturers and the large
retail automotive chain stores. All units you see advertised today, such as the ones shown in the Corvette mail
order specialty catalogs, are made in China. Valeo has tested the Chinese models, and the results are
disappointing. Stress cracks showed in the steel diaphragm fingers as early as 50,000 cycles, with some
complete failures at less than 100,000 cycles. Original Valeo plates test out at 1 million cycles or better. Valeo
is exploring other options at this point in time….I will keep you posted. During my research I was made aware
of a pallet of Valeo race version pressure plates, originally manufactured for Chevrolet Motorsports. These are
highly machined pressure plates which are rated to 11,000 rpm! The plates are now at my shop.
The solid friction discs from Valeo are gone as well. I bought the last two discs from them in early February,
2011. Valeo has told me that they do not anticipate any more production on these discs unless a worthy
pressure plate replacement can be found. I have been working with RAM Clutches to make an aftermarket (or
better!) equivalent, and I believe they have succeeded in doing so. These are custom made discs and they weigh
in .5 pounds lighter than the Valeo. They are in stock at my shop. If you convert to a single mass flywheel,
then you will need a “sprung hub†friction disc and there are several excellent choices available from different
aftermarket performance manufacturers.
I have been unable to locate any 1989-1996 throw out bearings from INA. In fact, I received no response from
them after weeks of inquiring about the status of these bearings. Fortunately, Valeo is manufacturing the
bearings along with a heavy duty wire retainer. These are available from a number of sources, just not your
local Chevy dealer.
There are no more original extruded steel clutch master or clutch slave cylinders available through normal
distribution outlets. I purchased the last OEM slave cylinder from Corvette Central in April of 2010. All units
out there (and I do mean all), are the cheap, cast cylinders coming from China. It doesn’t matter if it’s in an AC
Delco box, a Raybestos box or a Sach’s box, they are all the same. Some components used in these cylinders
are extremely poor quality. Our cars deserve better. At least I think so. In an effort to save the integrity of the
hydraulic portion of our clutch systems, I was able to find and purchase the entire inventory of individual
cylinder parts from TRW Automotive. These parts were destined for steel scrap salvage and I intend to breathe
life into them. I also have a limited number of NOS (new old stock) completed slave assemblies, and I can
totally rebuild clutch master cylinders through a core exchange program (please don’t throw away your
originals!).
I would like to thank the good people at Valeo and TRW Automotive for their help in locating these forgotten
Corvette parts. They were extremely generous with their time and knowledge. If you have a C-4 and you are
interested in any of the parts mentioned in this article, please feel free to contact me at powertorque@msn.com
via e-mail, or phone me at 641-342-7446 (Central Time, M-F 8:00 am to 5:00 pm). You can also visit our new
website. The address is http://www.powertorquesystems.com. Have a great race season!
Submitted by CCI member Jim Jandik
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