ZF6 Information:                                       04/16/2008 Rev3

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

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. 

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.

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."

** 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.

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

	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. 


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


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

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


	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

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. 


"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)