interesting bits of c4 vette clutch info

grumpyvette

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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


c4manualtrans.jpg

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1989-1996 CORVETTE 6-SPEED MANUAL TRANSMISSION
CLUTCH REPLACEMENT TORQUE VALUES & TORQUE TIPS
The following torque values are taken from the 1996 GM “Y” Platform Service Manual. Torque
tips are provided by Jim Jandik and Bill Boudreau, the ZF Doc.
FLYWHEEL BOLTS: 74 FT-LBS
CLUTCH PRESSURE PLATE TO FLYWHEEL BOLTS: 30 FT-LBS
BALL PIVOT STUD: 33 FT-LBS. 12MM INTERNAL HEX
LOCKING SCREW FOR PIVOT STUD: 11 FT-LBS. 6MM INTERNAL HEX
FLYWHEEL HOUSING TO ENGINE BLOCK BOLTS: 37 FT-LBS
TRANSMISSION TO FLYWHEEL HOUSING BOLTS: 37 FT-LBS
DRIVELINE SUPPORT TO TRANSMISSION BOLTS: 37 FT-LBS. 18MM HEX
DRIVELINE SUPPORT TO DIFFERENTIAL CARRIER: 59 FT-LBS. 21MM HEX
GEARSHIFT SUPPORT BRACKET: 18 FT-LBS
PROPELLOR SHAFT RETAINING BOLTS: 18 FT-LBS
SLAVE CYLINDER NUTS: 18 FT-LBS

IMPORTANT TORQUE TIPS:
Any time Loctite is specified or recommended, you should cleaner/degreaser both the male
threads on the fastener as well as the female receiver threads.
Flywheel bolts should have torque applied in a three step process, each time using a 6 bolt
pattern: clockwise, 1-4-2-5-3-6. Torque to 15 ft-lbs, then 37 ft-lbs. At 37 ft-lbs, remove the
bolts one at a time, apply red (262) Loctite to the threads, re-install them and final torque to 74
ft-lbs. I do one additional clockwise pass at 74 ft-lbs, using a sequential pattern 1-2-3-4-5-6.
This ensures all bolts have an even torque.
You can accurately torque the pivot stud into the flywheel housing. On the back side of the stud
there is a 12mm hex input; go in from the outside of the housing and set your torque wrench in
ccw rotation at 33 ft-lbs. Because you do this from the back side it will draw the stud into the
housing. I would use blue (242) Loctite on the locking screw threads. Torque the locking screw
clockwise to 11 ft-lbs and do not exceed this value.

Pressure plate bolts should not be tightened more than one revolution at any given time. Use the
6 bolt pattern for alternate bolt torque: 1-4-2-5-3-6. Do this incrementally. I recommend 5 ftlbs
at a time. This will load the diaphragm fingers evenly. Take your time here, as this process
is critical for smooth engagement/disengagement of your new clutch assembly.
For the transmission to flywheel housing bolts, use the torque pattern 1-5-3-2-4 (clockwise).
Mark bolt 1 as the top right side bolt, which is close to the 1 o’clock position.
ADDITIONAL HELPFUL TIPS
Be sure to use a good cleaner/degreaser on your flywheel and pressure plate friction surfaces.
There is a storage coating on them and this will help to remove the residue. Additionally, you
may use a Scotch-Brite pad or dual action sander with 220 grit sandpaper to lightly roughen
these friction surfaces. This will help prevent glazing on the lathe finish surfaces.
Never put grease on the pilot bearing/bushing or the tip of the input shaft. You may place grease
on the splined portion of the input shaft. Following these suggestions will eliminate glazing.
Make sure the air intake duct stays attached to the transmission and does not get kinked.
 
BTW your not necessarily stuck with the transmission that came in your car, or corvette, if you find a really clean c4 corvette or muscle car, thats got a different transmission than you really want, if you get the car at a great price it might be well worth the time and cost to convert the car to a manual trans , or a 4L80E yourself
c4manualtrans.jpg


lt1g5.jpg

the C-beam differs between the dana 36 and dana 44 rear differentials
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84_96_corvettedriveshaft.jpg

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ZF6 Information: 04/16/2008 Rev3

There are two ZF6 transmissions that were used in the C4.

onesealvstwo.jpg

A "Black Tag" ZF6 that was used from 1989 to sometime in late 1993.
It was produced in Germany and literally has a black service tag. The Black tag makes more gear noise due to the different
angle of the gear machining. The black tag also has a 1.375" diameter front collar, therefore requires a clutch disc
with an integrated 1.375" throw out bearing.

A "Blue Tag" ZF6 was used from sometime in 1994 until 1996. It was produced in the United States, and has
a Blue service tag. Re-angled gear machining lead to more quiet operation. The collar on the front has been reduced
to 1.311" so that a nylon insert could be pressed into the Throw out bearing. (To reduce wear, I'm told). Therefore,
your clutch disc will need to be specific to the transmission you have.
http://garage.grumpysperformance.com/index.php?threads/hydraulic-throw-out-bearings.1122/#post-2236

ZF6 Clutch Specifics:

In stock form, the ZF6 uses a "Pull style" clutch, similar to the LT1-T56 transmission. The only difference is,
a ZF6 does NOT use a sprung hub clutch disc, but rather uses a dual-mass flywheel with integrated springs between
the mounting surface and the friction surface. The OEM flywheel sells for nearly $700 US.

There are a few options for clutch replacement, other than stock. For example, you can change to a singlemass flywheel
and re-use a OEM type ZF6 clutch kit, OR use fbody clutch components, OR convert to a push style clutch.
flydie.jpg


there are 153 (about 12")
and 168 tooth (about 14")
fly wheel,
but in every case the starter must be mounted, on the block with the correct matching bell housing on the block to correctly contact the flywheel SIZE your using


Default SBC Flywheel & Flexplate part numbers

Small Block Chevy Flywheel Part Numbers
Part # DIA. Teeth Comments
3991406 Flywheel, LW Nodular Iron for HD 10.5" Clutch
3991469 14" 168 two-piece seal, internally balanced
3986394 14" 168 400 External balance, two-piece seal
10105832 14" 168 86-later, one-piece rear seal, ext. balance
14088671 12.75" 153 86-later, one piece rear seal, int. balance
14085720 12.75" 153 Lightweight (15 lbs.), nodular iron, two-piece seal, int bal Will work with a clutch diameter of 10.4" only. Crank flange bolt pattern is 3.58". Will work with a clutch diameter of 10.4" OR 11.0". Crank flange bolt pattern is 3.0"
14088646 12.75" 153 86-up, Lightweight (15 lbs.), nodular iron, one-piece rear seal, ext bal
14088648 14" 168 86-up, one-piece rear seal, ext. balance
14088650 12.75" 153 86-up, one-piece rear seal, ext balance Will work with a clutch diameter of 10.4" only. Crank flange bolt pattern is 3.0"
10174482 89-up with six-speed m/t, requires starter (10455709)
Common clutch sizes are: 10.4", 11.0", and 11.85". Some flywheels are drilled for two different clutch patterns.
12.75" are usually 153 tooth and the other larger 14" diam. are 168 tooth flywheel or flex plate designs, the larger 14"/168 tooth designs fit the much stronger 11" clutches.... obviously the 14" can use a larger clutch, but require a larger bell housing that is why some stock bell housings won,t fit the larger fly wheels and of course the starter used must match

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http://www.4speedconversions.com/chevy-bellhousings.html

1) If using a Singlemass T56 flywheel out of a 93-97 V8 Fbody (OEM# 10125379), you can use a Fbody sprung hub clutch disc (OEM #12551309),
but must use the propper ZF6 pressure plate for your transmission. Additionally, the flywheel must be resurfaced .090" for clearance
with the clutch fork to bell housing. This will only work on 1986+ motors with 1pc rear main seal!

2) If using a Singlemass 4+3 flywheel (OEM# 14088646), you can use an OEM type ZF6 clutch kit, but due to the lack of a dual mass flywheel
OR sprung clutch hub you will get some gear noise.

3) You can use a Singlemass 4+3 flywheel with an OEM type 4+3 PUSH clutch, but NOT with the ZF6 hydraulics!
You need to convert to a hydraulic throw out bearing (such as Howe 82876 for Black Tag ZF6 with a 1.375" TOB). LD85 has completed
this swap and has added the following information:

"Essentially, I use a 4+3 FW , PP and Disc, .100" spacer between the BH and Block, and a Hydraulic Howe Push Type slip on TOB,
a Wilwood 3/4" bore Master Cylinder, and a Clutch Pedal travel stop."

Expect to pay about $160 for the TOB, $160 for the master cyl, and another 40-50 in materials to make the spacer + pedal stop.

LD85 added the following:

"To setup the push type.

1. Bolt the BH only to the block with the clutch and pp in place.
2. Measure from the BH/tranny face to the PP fingers.
3. Measure from the tranny face to the TOB face on the Hyd-TOB.
4. Subtract the 2nd dimension from the 1st above.
5. Once you know this dimension then you add a spacer to the face of the BH so that the clearance from the TOB face to the PP Fingers is a gap of .150-.200" including the gap you obtained from step # 4 above.
NOTE: the Howe TOB comes with .040" thick spacers to add behind the TOB if necessary to reduce the gap.

In reality, using my 5.875" deep BH, I had a gap of @ .030" BEFORE I added a spacer to the BH. I added two spacers made from steel from Home Depot that measures .060 each, and ended up with a gap, including the ZF BH spacer that comes with the BH, of @ .200"

Actually the bigger the gap the better because the Howe .040" thick shims just slide over the input shaft and rest behind/under the Howe TOB.


If you use a thicker BH, then you do not need a BH spacer, you only use the Howe spacers.

Next time I will get a standard non-safety Lakewood Chevy BH which is 6.312 deep with no Block spacers, and weld some nuts for the ZF tranny mount bolts onto it , and use the Howe TOB and you are done, except you will need to move the C-beam holes by @ 5/8" because of the thicker BH.

If I would have done this last time, my gap would have been too large and I would have used the Howe 82870 tob which is .136" longer then the #82876, which would make my gap a total of @ .300 with no spacers, but the good news is that the travel for the #82870 is longer so really no spacers would be required or maybe 1 or 2 at worst.

To release a Push Type Clutch disc, the PP only need travel .375" according to McLeod, RAM and Centerforce.

Or you can do the same as above and add a sapcer of .500 to the fron the the ZF stock 5.875 thick BH."

http://www.4speedconversions.com/chevy-bellhousings.html

ccl1.png

ccl2.png

ccl3.png

ccl4.png

ccl5.png

keep in mind any serious performance application is best served with SFI certified billet flywheel,
clutch assembly and a blow proof bell housing


** AGAIN, A DUAL MASS or A T56 flywheel will only work on a 1986 AND UP MOTOR WITH A 1PC REAR MAIN SEAL!



Automatic, OR 4+3 to ZF6 swap:

A few points to add:

1) You can re-use your 4+3 pedals, and thats IT.
2) You need an OFFSET LT1 starter because of the bell housing
3) Stay away from ZR1 bell housings. You will run into major geometry problems!
4) Some folks say you need a ZR1 pivot stud when using a 4+3 singlemass flywheel. The reason
is because of the thickness difference with the 4+3 flywheel. Let me explain:


The 4+3 flywheel is 1 1/16" thick
The ZF6 flywheel is 1 7/16" thick

Therefor the ZF6 flywheel is 3/8" thicker than the ZF6 flywheel. This puts the angle on the clutch fork
wrong in a way.

In stock form, the ZF6 fork is about level, and exactly 1.428" from the slave cyl mount flange
to the center of the pocket where the slave cyl rod contacts.

The slave cylinder will provide .435" of total travel on a clutch stroke.

The pressure plate requires .290" - .315" of travel to release the clutch.

The absolute MAX travel of the slave cyl is about 2.600"


The ZF6 L98/LT1 pivot stud is 3/4" tall
The ZF6 LT5 (ZR1) pivot stud is 1 1/6" tall, therefor about 5/16" longer

When using a 4+3 flywheel as stated above, the flywheel is 3/8" farther away from the transmission,
so the clutch fork is now at a serious angle. It's distance from the slave cyl is now reduced to
1.230". (aboout 7/32" closer).

When using a 4+3 flywheel in the above configuration, but using a ZR1 pivot stud the distance to the fork
from the slave cylinder is increased to 2.00", or about 9/16" farther away then spec. ** NEED RE VERIFICATION **


Some users have reported clutch slippage problems with using a 4+3 flywheel (or any 1 1/6" thick flywheel) with
a L98/LT1 stud. The theory is some master cyls won't compress much beyond the stock setting of 1.428", and is
preloading the clutch.

However, when using a ZR1 pivot stud you can risk over-extending the clutch slave cylinder (not fully releasing clutch).
In theory, with a 2.00" installed length, and a max travel per stroke of .435" puts us at 2.435". This figure tells us
we have roughly 5/32" to spare. If this causes worry, adding 9/16" ** NEED RE VERIFICATION **
to the slave cyl rod would put the geometry back to stock specifications.

Another option, which I have not tested other than by mocking things up is using a washer under the stock
pivot stud. This will raise the stud by the thickness of the washer, which could be a solution if a ZR1/LT5 stud
is not readily available. Anything between 1/8 - 1/4" should both work and leave enough threads for safe stud mounting.
** THIS HAS NOT BEEN ROAD TESTED **


Centralcoaster and myself have done the swap. This is the minimum parts list from him with
my edits:

Pedals:
For 4+3 owners, your pedals can be used for the ZF6. Keep in mind
that in mid 1986 GM changed from the "large" stud to a "small" stud
that connects to the eyelet in the master cylinder rod.

Newer style replaement masters could have a plastic insert, OR
simply a smaller hole drilled. You MAY have to remove the plastic
insert, or drill out the hole to the larger diameter.

For Automatic owners, you need a set of pedals. It's a safe bet that
pedals for 84-89 cars will be the same, and 90-96 cars be the same due to
dash differences.

Transmission:

89-93 black-tag ZF6 and 89-93 clutch set
or
94-96 blue-tag trans and 94-96 clutch (Remember the 1.375" vs 1.311" difference!)

Flywheel:

Stock flywheel from manual trans vette of your year (Cheapest route)
Keep in mind you may need a ZR1/LT5 pivot stud, or washer
or
ZF6 Dual mass flywheel asssuming you have an 86+ motor with a 1pc rear
main seal.
or
T56 Flywheel with propper machining, and a Fbody clutch disc (OEM#12551309)
assuming you have an 86+ motor with 1pc rear main seal.

Starter:
91-96 or LT1 starter. Has offset gear to clear BH. I tried a race
starter and it crashed into the BH.

Driveshaft:

Driveshafts depend on your rear end, so if you have a Dana36 or Dana44
you need the respective driveshaft. Your 700R4 or 4+3 yoke however will NOT
fit the ZF6.

Everything else:

91-96 ZF6 bellhousing (Stay away from ZR1 bell housings)
89-96 ZF6 clutch fork
89-96 ZF6 clutch master cylinder
89-90 ZF6 clutch slave cylinder
89-90 ZF6 clutch slave hose
89-96 ZF6 Yoke
TH400 Rear seal, Timken #9449 if using a used ZF6
backup switch connector, a.k.a. horn connector. (AcDelco PT1357 or Napa ECHEC11)
(5) metric M12 x 45mm transmission bolts
(6) standard 3/8 x 1-5/8" pressure plate bolts ($2.50 each from GM)
3 bottles of GM 1052931 or Pennzoil Synchromesh
Sheetmetal and rivets for patching the trans tunnel, and some extra carpet insulation.

Speedo sensor from my 85 4+3 fit the ZF6. I bought a red 37 tooth driven gear from GM for it to match my 3.07 rear end ratio.
Getting the 37 tooth driven gear to mesh requires installing the speedo sensor upside down and tweaking the hold down clamp a bit.
The 3.07 rear makes 6th gear unusable under 85mph. I recommend upgrading to a 3.45 or 3.55 ratio.


Some Generic ZF6 myths:

1) You do NOT have to have your flywheel "match balanced". That is bogus information. You need to have the correct flywheel for your motor.
1985 and earlier used a 2pc rear main seal flywheel. Any 153 tooth 2pc rear flywheel for a 350 is balanced correctly if it's an OEM replacement.
Any 1986 and up 153 tooth 1pc rear flywheel for OEM replacement is balanced correctly, including a T56 flywheel. *

Aftermarket flywheels for "race use" may need balancing. Make sure the flywheel is for OEM replacement! Some flywheels will specify
things such as "internal balance 383". This is not what you want. You want an OEM direct replacement 153 tooth flywheel for your year
motor.

2) You cannot "re-use" a 4+3 clutch setup with the ZF6 hydraulics. No way.

3) You cannot use 4+3 hydraulics with the ZF6. Not happening.


sources:

"push style" clutch - LD85
"4+3 to ZF6 swap" - CentralCoaster
"Flywheels + balancing" - Miller machine shop, GM parts manual, Sofakingdom
"Some specs and measurements" - byebyel98
"Additional specs" - ZFDoc (Bill)
 
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