Before anyone takes my head off, YES I have read and re-read every swap thread I could find. This thread is more for me to layout my plan/understanding of the process and hopefully get some feedback on weather I am missing the mark or I have a good plan.

First a little background...Bought my 85 a few years ago, Gold Metallic manual with Bronze leather standard seats. The paint was in good/decent shape, it started, moved and stopped and I have wanted a corvette since I can remember...so I may have overlooked a few key issues when I agreed to buy it. The car was mostly original, but there were signs of neglect and a backyard mechanic or two had been at her over the years. Most of the issues I've been able to fix or have fixed (frame rot). This past winter I pulled the 4+3 and with the help of a buddy with a lot of knowledge and a lift, we tore into the trans and rebuilt it. Someone had been in it before and even after replacing bearings, seals, clutches..etc there was not a lot I could do with the damaged pump and planetary gear housing. On my second time out, the trans lost all forward gears. Luckily I was at a bar close to home and being late at night I was able to drive it home in reverse without hitting anyone or anything.

Without going into why I chose the ZF6, I got what I feel was a decent price on a low mileage black tag out of a 91. It's the entire set up from shift *** to drive shaft, including the tunnel section and hydraulics. All in good working order with a 30 day return policy.

My plan is as follows and this is where I need some feedback from those of you familiar with the swap.....Please.

-Using my brand new (21 mile) single mass flywheel with the ZF clutch and pressure plate (I understand this can cause a little noise but I'm ok with it)
-ZF Hydraulics
-purchased a clutch fork pivot stud for a ZR-1
-My understanding is you only need the mini starter if the bell housing/trans is from a 92 and up, which means I can re-use my starter as I have a 91 ZF?
-The rest is cutting, measuring and fitting? oversimplification but other than hydraulics it will bolt right up to my L98?

Some of the swap write ups are a few years old now....has anything changed in terms of clutch options to absorb the gear noise? The 2 piece rear main really limits the options without spending myself out of the bedroom and onto the couch.

Based on everything I've read, with the total assembly that I've purchased.....Other than cutting the tunnel the only part I needed to make this work was the ZR-1 clutch fork stud?
Any positive feedback or advice on the swap is welcomed....if it's negative about this trans choice, there is no need to beat this dead horse.....Thanks

 

subbed. i may be doing this to my 86 soon.

 

-There are two types of flywheels, for a one piece main, or a two piece main crank. Make sure you have the right one.
-I'm not sure if the ZR1 pivot ball stud will not work with the L98, LT1/4 ZF6 transmission setup. There is a difference in size between the two.
-ZF Doc sell a counter shaft shim kit (p/n 1052CSSK) that may reduce the rattle noise. Give him a call and ask first if it will work on a 91.

 

Have you considered the difference in the rear gear ratios ? The 85 is a 3.07. The ZF equipped cars were 3.33 or 3.45 s.

ZF

Gear ratios - 1st-2.68, 2nd-1.80, 3rd-1.29, 4th-1.0, 5th-0.75, 6th-0.50, Reverse-2.50

Doug Nash

2.88 1.91 1.33 1.00 and the OD was .67 or .62

So the car's acceleration won't be the same after the swap unless you also add in rear gears to the plan.

 

….. As long as your dual mass flywheel is for a 2pc rear main you should be good to go … I'm not sure what the ZR1 fork pivot will do for you that the original won't but so be it … the 3.07 rear gear will make 6th gear pretty useless unless you cruise @ 80mph + but that's the worst of it … I doubt you would ever notice the difference in gear ratios between the 4+3 and ZF6 , they are minimal … the ZF6 rattling complaints are mostly from those "Corvette Guys" … you know ... the ones that buy a 'vette only to complain about the stiff ride , the squeaks , and especially the rattles … Good luck with the swap …..

 

Same here. Once my auto goes out again I'm going to have a hard time not doing a swap as well.

 

My understanding from what I have read is that you don't really feel a difference gears 1-4 with 5 operating like 4 OD. 6th is where people are stating that unless you are doing 75-80 it's useless without the rear end upgrade.

 

Thanks for confirming the only real noticeable difference is with 6th gear. As for the zr-1 pivot stud, I've read in a few different threads that the geometry would be off using the pivot stud from the zf or the 4+3 set up while using my 85's single mass flywheel. My understanding is that the 4+3 flywheel is 1 1/16" thick while the ZF dual mass is 1 7/16" thick. The difference causes clutch engagement problems because the clutch fork is at an incorrect angle. There are a few posts where I've read this and the most detailed write ups I've found (by Anesthes, Id85 and Z51racer). From their explanations: The ZF for L98 pivot stud is 3/4" long where the ZR-1 pivot stud is 1 1/16" long. This should allow it to make up around 5/16 of the gap in flywheel thickness.

If you or anyone else can confirm this please let me know. If I can get away with using what I already have that would be great.

 

..... How long is the 4+3 pivot stud ? … I have a '90 ZF6 dual mass and it measures 1 15/16" thick … FYI … I have a complete auto to ZF6 conversion for sale in the Parts for Sale section … from a '90 with 70K miles …..

 

I will measure it when I get it out tomorrow. Great question though,, I figured it had to be shorter than or the same length as the ZF because it was never mentioned in any of the write ups. I have a complete ZF from a 91 including pedals, flywheel....etc just can't use the flywheel in my 85.

 

You will feel the difference,it's definitely a step back. I have a T56 in my '86 (same ratios as the ZF) and i 1/8 mile drag the car kind of regularly. I have exactly one sub-2sec 60' timeslip. I have to beat it like rented mule to launch it. I have a set of 3.73 gears under the bench to go in this winter.
Do the swap. Don't let that change your mind, just be aware going in.

 

Pics to come soon, but tonight I removed the tunnel covers, carpet and the center console. Everything went well with the exception of snapping off one of the passenger side seat rail bolts at the floor. The prior owner really worked the plastic console over when adding the stereo. I have a plastic welder used to fix the door cards so it should be salvageable.

My ZF delivers tomorrow so I'll be pushing to get this swap done within the next week. I'll document everything the best I can but if anyone has something they want more pictures of it more details please let me know.

 

Make sure that FW is machined properly for a Y body and not F body or some other application. For example if it is for a F body, it will be too thick, you will have everything buttoned up and the starter will not be able to turn the engine. Ask me how I know that.................

Also, with gears, the main concern other than engine RPM is the stress on the clutch with EOM gearing for the auto. The 3.07 is a high ratio for this 1st gear in the ZF which can cause premature clutch wear just under normal stopping and going conditions.....ask me how I know that as well.........

Anyway, sounds like you are doing your homework and you have a stick car as a base so it could be an easier proposition than converting an auto to manual.

 

BTW; I have a custom,steel single mass FW for your 2 piece rear main seal 85 if interested. I know you have a FW, but if you are looking for an alternative, let me know!

 

That's why I left the 700r4/4l60

 

Haven't updated this in a while but making decent progress for everything else going on in life. I changed my plan and will be using a Howe hydraulic TOB with the 4+3 flywheel, clutch disk, PP. I am ready to bolt up the bellhousing to take the measurement to the PP fingers but can not get the bell housing to seat correctly on the block. I was under the car for an hour last night and couldn't figure it out. I sat the ZF bellhousing next to the 4+3 bellhousing and noticed there are fins in the ZF bellhousing. Could this be the reason it won't seat? It doesn't make sense to me because the dual mass flywheel in the 91 was much thicker than the single mass in my 85. I should have a ton of clearance between the pressure plate and the back of the ZF bellhousing. Has anyone who has completed this swap had issues like I described?

Also, for anyone else looking to swap in a ZF to an early C4, I read in a few different posts that you could reuse your starter in ZF bellhousings 91 and older. Unless I received a different bellhousing or if there was a change mid year 91, my 85 starter will not work. The bellhousing won't fit over the nose of the starter. Just an FYI that you will need to add a mini-starter in your budget.

I'll post pics soon, other than nagging issues like the bellhousing and breaking a seat rail stud off at the floor it hasn't been too bad. I've found where a lot of the squeaks were coming from...broken tabs in the center console, the rear under the glove box was broken out at the screws and the heads wore thru the bottom of the glove box. The driver power seat switch bracket was taped in, the plastic screw mounts were broken off...etc. On days I was waiting on parts I sat in the basement with a cold beer and a cheap plastic welder. I've found that lego's work the best and bond well to the C4 plastics. The broken areas are all in the lower console and my "welding" will be hidden under the top console or under the glove box. While I've been under the car, I noticed that I have an oil leak on both sides of the engine coming from either the head gaskets, the valve covers or both. If I can get the ZF in, looks like I'll be pulling the heads next. They needed new valve seals anyway and probably a valve job so I'm looking at the leak as a good thing.

If anyone has info on the bellhousing I would appreciate your feedback, the little issues take the wind out of momentum.

Thanks, Buckeye

 

Good luck. Nice upgrade..

 

I dunno, but having a 96 and having done a clutch job, which involved replacing anything replaceable, I'd be very careful; I think you'll also need the LT / ZF bellhousing; I don't think the early style bellhousing is gonna work; not that it won't work, but I'd double check to be sure.

As you know, the ZF uses a (goofy and expensive) PULL style clutch; every other chevy back to 1955 uses a push style clutch. Why the change? Who knows?

 

Surprise, surprise. Checked the parts numbers and the L98's and LT's both used the same bellhousing, p/n 10126448; sorry for any confusion.

 

Thanks for checking. I looked into it before starting the swap, but there is always the one off that previous swap posts may have missed.

I figured my issue out, the bracket for the hydraulic hose on the BH was hanging up on the trans tunnel. I pulled it off and the BH bolted right up....as it should have. Will torque it down tonight and taking the required measurements...hope I won't need to make a BH spacer and can get on with the install.