Clutch master cylinder woes and confusion... help needed

[bookite03]

So I'm in the middle of my ZF-5 swap into my 1987 C6 truck. Installed the correct pedal assembly and removed the factory cover for the master cylinder hole in the firewall. I have a couple used master cylinders I accquired for this project but am coming up with issues. I also have one of the small factory clutch master cylinder support brackets from the same 87-91 truck I pulled the pedals from... Its not fitting either. The bracket is running into the "doghouse"/firewall...the rolled edge is hitting and the top hole will.not line up.

So I did a little research, found two separate PN's for clutch master cylinders when you compare 1987 to 1988...which one should I use? Is the slave and hydraulic line between the two different also?
Second, any guess why the plate doesn't fit? Is there a difference in the cabs of these trucks over the years? I know 87' was a transition year for the body style and 5-speed so maybe I'm missing something here. Any advice is appreciated.

BTW here's the plate I'm.talking about;

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[riotwarrior]

HAMMER is your friend...Ball Pien hammer actually and adjust the firewall accordingly.

As said the BIG plate TSB even says bending of plate may be required...well Jason's plate did not need be bent but my tunnel firewall did need a little massage..

Do the MC's you currently have physically fit into the firewall as required? If so adjust the tunnel and enlarge the plate hole to make it work with the MC

That would be my best 2CW of advice.

 

[bookite03]

Sounds like its hammer time...works for me.

The master cylinders fit, the ones I have were used and showed some signs of rust/corrosion also stiff. Maybe it was just me. I was more concerned with the difference in part numbers and if either one matters in my application.

 

[LCAM-01XA]

That plate you have is for the '88-up trucks, I got one in my '90 and it was a direct bolt-on affair. I also have a plate I pulled from an '86 IIRC, it's different and the could not be used in my cab, bet it will fit yours tho... Like you said '87 is a transition year, your cab still uses the '80-'86 transfer case shifter arrangement and also IIRC the clutch safety switch on the pedal assembly, wouldn't be one bit surprised if the master cylinder is also of the '80-'86 setup.

Answer this, how do its mounting studs sit on the firewall - are they perfectly vertical inline with the pushrod, or are they offset so the upper stud is closer to the brake booster and the lower one is closer to the engine? The offset-stud arrangement in the '88 and newer trucks is what that plate you have is made for, when installed in the correct cab its long sits vertical and runs along the doghouse and clears it all. But if you are attempting to use it on a vertical-stud setup then the whole plate will get tilted and its rounded corner will hit the doghouse and the holes don't line up right, which is what you're experiencing - personally if I ran into that situation, and a correct factory plate was unavailable, instead of swinging the hammer like a madman I'd source a piece of flat steel and cut and drill it to make my own reinforcement plate that fits properly.

I can post a picture of the '80-'86 plate that works with vertical-stud masters, just gotta dig it out tomorrow...

 

[laserjock]

87's still had 4 speeds right? I think that could be your part number variation. Might be your fitment issue too if you pulled your stuff from an 88 with a 5 speed.

 

[bookite03]

LCAM -
Wow, can't thank you enough for the great info, very helpful! I ended up modifying the 88'-91' support plate to work on my 87' cab. Just some quick grinder work and it fit well. Should be good to go until I do the whole support plate. I also picked up a new 87' correct clutch master cylinder and it hooked right up to my existing line and slave cyl. Looks like I'm back on track, hoping to go for a test drive tomorrow as long as my driveshaft gets done today. Again, THANK YOU

 

[freebird01]

FYI the larger support plate will work on all F-series 83-91

The small plate like you have there will help prevent cracking but if your firewall is already cracked you may want to look at the larger plate.

 

[bookite03]

The firewall is solid, but I would like to do the big plate in the ffuture to prevent any long term damage.

 

[freebird01]

i make them and sell them....but they have been known to turn up in junkyards as well.

 

[bookite03]

i make them and sell them....but they have been known to turn up in junkyards as well.

Copy that, I'll be getting in contact with you on it when I'm ready.

 

[riotwarrior]

Now that I've gone out and looked over my collection of stuff I noticed two distinct different small plates one that has not got the round nub on the corner, it's just two bent angles and a plate, I'm suspecting that's for the 87 down firewall and the other one I have is for the 88 - 91 firewall.

I'll see if I can get some pictures of the two different ones.

 

[freebird01]

the one with the 2 bent angles is for 83-86....his is the small one for the 87-91...

i used to make the 83-86 plate but it offers marginal coverage and they were a pain to make and i didnt sell many of them....