Buckeye88

Looking for help validating my thought process to fix the following problem. In 2020 I ZF swapped my 85 corvette. I used the 4+3 flywheel, pp and clutch with the original master cylinder but went with a Howe 82870 hydraulic throw out bearing. I purchased a Wilwood GS compact 3/4 master cylinder but didn’t install it because the 4+3 worked and at the time I wasn’t up to fabricating an adapter to go from the OE offset mounting point to the straight up and down mounting ears on the Wilwood MC.
now my issue: I lost pressure in my clutch pedal last weekend. I found the Master Cylinder was leaking and decided to finally fabricate the bracket and install the wilwood MC. I got it installed and bled yesterday, went to start it and the starter isn’t turning. I crawled back under the dash and everything is hooked up, all wiring is still intact. The only thing I noticed was the clutch pedal stops about an inch off the floor because the travel is limited by the Master Cylinder. I used a female Hiem joint to connect the MC Rod to the pedal but it’s adjusted all the way to the backend of the threads. The clutch pedal is also an inch higher than the brake pedal. At this point the only option is see is to make the spacer/adapter between the MC and the firewall thicker, this will move the MC back and allow more travel for the clutch pedal. Before I tear it all back apart, does this make sense? My thought is that the clutch pedal isn’t moving far enough towards the floor to actuate the clutch safety switch. This was working fine prior to tear down.

here are a few pics of the spacer and the mounted MC.

MC mounted

MC spacer/adapter mounted to firewall

MC mounted to firewall

Spacer/adapter made from 2 pieces of 3/8 aluminum bar

Spacer/adapter showing offset for OE firewall mounting points

Backside of Spacer/adapter. Attached the 2 pieces of aluminum by drilling/tapping 6/32 Allen head screws. The 8mm allen head bolts hold the new MC

Buckeye88

Based on the lack of responses nobody else has gone this route and had to find a solution. After lots of trial and error, I finally figured it out. Somebody much smarter would have understood the problem before they got as far along as I did, so this is for those of you that know in the back of your mind that it won't work but push on anyway.
First, when I emailed Wilwood about the spacing from the firewall and an adaptor bracket the response I got was "We don't make anything for your car, you should have used a MC with remote reservoir." At first I felt dumb, and then I went out and measured from the center point of the hole in the firewall to the mounting holes for the OE MC and the mounting hole on the inward (Lower left) side is closer than the one to the upper right. You would need to fab something to mount any Wilwood MC as the mounting holes in the Wilwood MC's I looked at were all even distance from center. Just including this info for anyone who might call /email them and get the same person I did.

After my initial install didn't work as stated above, I went back and measured everything with a caliper. The OE/reproduction Master cylinder is 4.5 inches long, the Wilwood GS compact is 3.9 inches long. I started down the path that I had to make up a 1/2 inch somewhere but then I looked at the throw of each MC. The word COMPACT in the name of the Wilwood MC kept pinging in the back of my mind. That word usually means does the same job in a smaller form. I had to move the MC an additional 10 MM (not 1/2 xinch) away from the clutch pedal linkage in order to engage the clutch safety switch while keeping the clutch pedal even with the brake pedal.

In the end it all worked, I had to order a hydraulic line and fitting to get it all hooked up. I think it looks good and I've driven it enough to know everything works as it should.


Additional spacer

New MC and hydraulic line installed and bled properly. Now I can see how dirty everything else really is.

ChumpVette

Never saw your thread initially. This is what I fabbed for use.

Buckeye88

Thanks ChumpVette, your design is much cleaner than mine. Do you remember the thickness and what MC did you use?

ChumpVette

It was a tilton master cylinder. It matched up pretty good with stock. I want to say the spacer is like .75" thick.

1993C4LT1

Thanks for the info, OP! I have a tilton clutch master from after dark speed. And they pretty much did what you did to make it work. Nice work, btw.

dizwiz24

Thank you for the info

ACMX92

I have the parts to install a Tilton M/C on my 85 Zf6 swapped car, but gave in and put a stock style master on after having difficulty figuring out a good way to attach it to the stock pedal. It was supposed to be temporary, but I am actually quite happy with travel and pedal feel. I am running a Spec supertwin clutch.

How do you guys like the aftermarket M/Cs compared to stock?

Buckeye88

I like the aftermarket with the ZF swap, the pedal felt too light. I won't like it as much the first time I'm in a traffic jam but when able to row thru the gears a Corvette clutch pedal should be hip dislocating stiff.

yakmastermax

ACM in your ZF6 swapped 1985, what clutch pedal assembly are you using with what clutch hydraulic master?

My 1985 is a factory 4+3 car so has factory 4+3 pedals and clutch hydraulic components.

I was hoping the donor 1991 ZF6 clutch hydraulic master would bolt to my 1985 firewall and that the actuating piston rod of the 1991 ZF6 master would hook onto the pedals already in the 1985 from the 4+3

Does that sound right?

ACMX92

It has to be the clutch assembly I'm using because the clutch effort in my car is plenty stiff with a relatively short throw. I had a read a ton about clutch effort being pretty weak with the stock master cylinder but I was pleased with it once I got it all together. Compared to the 85 Celica GT-S I have and the 70 Charger with a TKX 5 speed I helped build, my Corvette is easily 5 times the pedal effort.

Another factor is likely the hydraulic throw out bearing I am using. I keep forgetting that stock C4s use a slave cylinder and clutch fork.

I am using a stock 84-89 pedal and a 4+3 master cylinder. The rod end on it had a plastic insert but once I removed that it fit just fine on the stock pedal. The zf6 swap parts I bought had a bunch of extra spacers in it, plus an adapter for a tilton master cylinder. I was able to dial in my pedal height with that and then used an afterdarkspeed pedal stop to keep from over extending the hydraulic tob.

1993C4LT1

I am still using the stock clutch master. Only reason I haven't installed the tilton clutch master, is because I have no clue how to set proper pedal travel. I am using a RAM hydraulic TOB. How do you set pedal travel properly?

ACMX92

In the C4 it is pretty easy since you can pretty much reach the rear wheels while sitting down and pressing the clutch. With the car on jacks I slowly depressed the clutch until I was able to easily move the rear wheels, and then added about another 1" or so of travel to the pedal stop to ensure full disengagement of the clutch. Of course, you want to limit how far you push the clutch pedal past the disengagement point as you can overextend the Ram TOB, but not enough clutch movement will result in difficult shifts and increased synchro wear.