Firewall Flex Issue

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Bob Jasinski

Bob Jasinski

FABO Gold Member
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I've had my '65 FS running and driving now for about 2 weeks. I've got issues with firewall flex like others on FABO have written about, that I didn't pay much attention to until recently. My car has been converted to manual front discs using the Raybestos dual pot master with the 15/16" bore, and the clutch is a Borg and Beck type, and both require a significant amount of pedal effort. I get firewall flex so much that when I clutch or brake it will visually move the other pedal and you can see the master cylinder move. This is a big problem and is affecting the clutch operation and making the brakes feel like pressing on a spring. I clearly need to strengthen the firewall and trying to find the best way to add reinforcement. I know one approach is making a strut from the shock mount to one bolt on the master cylinder, but before I start fabbing something up, does anyone know of a vendor that makes that bracket, or can you share pictures of the bracket you made and works for the application?
 
Bob Jasinski said:
I've had my '65 FS running and driving now for about 2 weeks. I've got issues with firewall flex like others on FABO have written about, that I didn't pay much attention to until recently. My car has been converted to manual front discs using the Raybestos dual pot master with the 15/16" bore, and the clutch is a Borg and Beck type, and both require a significant amount of pedal effort. I get firewall flex so much that when I clutch or brake it will visually move the other pedal and you can see the master cylinder move. This is a big problem and is affecting the clutch operation and making the brakes feel like pressing on a spring. I clearly need to strengthen the firewall and trying to find the best way to add reinforcement. I know one approach is making a strut from the shock mount to one bolt on the master cylinder, but before I start fabbing something up, does anyone know of a vendor that makes that bracket, or can you share pictures of the bracket you made and works for the application?
Click to expand...
Have you considered a hydraulic setup?
 
Cuda416 said:
Have you considered a hydraulic setup?
Click to expand...
No, not going to happen, and that wouldn't fix the flex when the brakes are applied. A simple strut from the shock tower to a bolt on the master should work and be easy to do, just wondering what others have done.
 
Bob Jasinski said:
No, not going to happen, and that wouldn't fix the flex when the brakes are applied. A simple strut from the shock tower to a bolt on the master should work and be easy to do, just wondering what others have done.
Click to expand...

I missed the brake part, carry on..
 
Bob Jasinski said:
No, not going to happen, and that wouldn't fix the flex when the brakes are applied. A simple strut from the shock tower to a bolt on the master should work and be easy to do, just wondering what others have done.
Click to expand...
that is the right idea, just don't have any pictures to show. I've seen one done with square tubing. always wondered if using the late model A-body firewall to fender brace could be modified to use?
 
relatively easy way to reinforce the firewall would be to get a section of firewall from a donor car and panel bond it to the inside of the original.

this much would make a big diference

1760557488119.png


I made a 3/4" thick aluminum plate when I hung brakes on the firewall of a 56 Ford Pickup It mounted outside in the engine bay and the power Brake booster mounted to it. It had cutouts for the ribs on the firewall. it worked great

you could also make L channels to screw to the firewall on the inside of the car at strategic points that mount the brake pedal mount / MC mount and the structure of the clutch linkage
 
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Bob Jasinski said:
I've had my '65 FS running and driving now for about 2 weeks. I've got issues with firewall flex like others on FABO have written about, that I didn't pay much attention to until recently. My car has been converted to manual front discs using the Raybestos dual pot master with the 15/16" bore, and the clutch is a Borg and Beck type, and both require a significant amount of pedal effort. I get firewall flex so much that when I clutch or brake it will visually move the other pedal and you can see the master cylinder move. This is a big problem and is affecting the clutch operation and making the brakes feel like pressing on a spring. I clearly need to strengthen the firewall and trying to find the best way to add reinforcement. I know one approach is making a strut from the shock mount to one bolt on the master cylinder, but before I start fabbing something up, does anyone know of a vendor that makes that bracket, or can you share pictures of the bracket you made and works for the application?
Click to expand...
Are you able to add a plate to where the firewall is flexing where your components are attached. Mopar used plates under the master cylinder on several cars I've seen.

I believe it was in the Mopar Chassis manual that I saw them running a 1 1/2" tube horizontal under the dash attached to the 2 "A" shaped braces at the top of the door pillars portion of the cabin. They also connected it in the middle of the dash on the firewall.

My convertible was rust free and never hit, but the cowl shake it had over washboard roads was bad. Adding the US Cartool under fender braces, the under radiator brace and the sub-frame connectors transformed the car, however there was still a bit of cowl shake left.

I made a Monte Carlo bar that bolts in and I only had to drill 3 holes near the windshield wiper motor to attach it. That eliminated the remaining cowl shake.

Novi 1500SL.jpg
 
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Bob Jasinski said:
I've had my '65 FS running and driving now for about 2 weeks. I've got issues with firewall flex like others on FABO have written about, that I didn't pay much attention to until recently. My car has been converted to manual front discs using the Raybestos dual pot master with the 15/16" bore, and the clutch is a Borg and Beck type, and both require a significant amount of pedal effort. I get firewall flex so much that when I clutch or brake it will visually move the other pedal and you can see the master cylinder move. This is a big problem and is affecting the clutch operation and making the brakes feel like pressing on a spring. I clearly need to strengthen the firewall and trying to find the best way to add reinforcement. I know one approach is making a strut from the shock mount to one bolt on the master cylinder, but before I start fabbing something up, does anyone know of a vendor that makes that bracket, or can you share pictures of the bracket you made and works for the application?
Click to expand...

I had the same firewall flex issue with a strong clutch in my 65 Barracuda. Probably not what you are looking for but I added a chain from an eye bolt mounted to the k frame up to one of the MC mounting studs using an L bracket. It cleared my fenderwell headers and did the job. So no permanent mod and I could take it on and off. FWIW.
 
Ford has same issue with their pick ups, Bromcos, etc., for a short while. Clutch master would push away thus the neutral safety switch could not be closed/activated. Their patch was a piece of plate added on. I won't say 11 gauge but fairly heavy gauge with its edges rolled outward for more rigidity. My memory sux but I'm thinking 3 added bolt holes to drill. Some fun,,, NOT.
Edit; brief search refreshed my memory. May be something out there that would easily modify to serve your need.

Screenshot_20251015_200726_Google.jpg
 
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Just remember that when you fix an area of flex in one spot, often times, you transfer it to another, so you may need to reinforce more than one area.
 
If you are flexing the firewall you are flexing the clutch cover as well.

The issue is you have way too much plate load (spring pressure) and that’s causing the flex. If you plate the firewall the cover will still flex. And that’s bad. Even if you have a heavy duty cover.

And when you get the firewall stiff enough to not flex, the load will find the next weakest spot and flex it. It could be the cover is now taking all the flex and it will fracture the cover at the mounting points and at the yoke bolts.

Or the linkage will flex. It probably already is. So you stiffen that up and the flex keeps MK in to some other part of the system.

You are just moving the issue from one location to another.

The correct fix is to pull the clutch and get rid of all that excess spring pressure. If you have an organic disc you only need 2400 pounds of spring pressure to get the clutch to lock up.

If you have an organic ceramic/organic disc you need less pressure than that.

There are 12 springs in that cover. If you need to, you can remove three, four or even six of those springs. Whatever you do, you need to reduce the spring pressure or you will keep flexing parts.
 
Thanks all for the feedback. The picture below is what I ended up doing to strengthen the firewall, running a formed piece of 1/2" (O.D. =.70") steel conduit from the side of the shock tower to the bottom MC bolt. I tried a support from the top of the shock tower but that failed due to the top shock rubber mount absorbing some of the movement. I'm satisfied with this and it doesn't look bad, only had to drill one hole.

Yes, the clutch pressure is high, even though it is a standard clutch kit from Rock Auto. I really didn't anticipate these issues, and thought I'd be fine with the stock kit I bought in 2020. The dual pot master was also moving quite a bit, and that surprised me. The factory had drum brakes on all 4 wheels with a single pot master, so the manual discs take more pedal pressure (not self energizing like drum brakes). I probably will do something to reduce the clutch pressure at some point, but I really don't feel like pulling the transmission right now, I need to have the car running to get it aligned and the headliner installed.

Newbomb Turk mentioned that 3 or 4 springs could be removed, I wasn't aware of that. That may be an answer or changing the system out for a diaphragm clutch. I'd like to know how those springs come out. Here is the installed clutch package I bought in 2020 and pictures of the added support.






DSC_8648.JPG
DSC_8649.JPG
DSC_8645.JPG


DSC_8647.JPG
 
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Bob Jasinski said:
Thanks all for the feedback. The picture below is what I ended up doing to strengthen the firewall, running a formed piece of 1/2" steel conduit from the side of the shock tower to the bottom MC bolt. I tried a support from the top of the shock tower but that failed due to the top shock rubber mount absorbing some of the movement. I'm satisfied with this and it doesn't look bad, only had to drill one hole.

Yes, the clutch pressure is high, even though it is a standard clutch kit from Rock Auto. I really didn't anticipate these issues, and thought I'd be fine with the stock kit I bought in 2020. The dual pot master was also moving quite a bit, and that surprised me. The factory had drum brakes on all 4 wheels with a single pot master, so the manual discs take more pedal pressure (not self energizing like drum brakes). I probably will do something to reduce the clutch pressure at some point, but I really don't feel like pulling the transmission right now, I need to have the car running to get it aligned and the headliner installed.

Newbomb Turk mentioned that 3 or 4 springs could be removed, I wasn't aware of that. That may be an answer or changing the system out for a diaphragm clutch. I'd like to know how those spring come out. Here is the installed clutch package in bought in 2020 and pictures of the added support.






View attachment 1716467768View attachment 1716467769View attachment 1716467765

View attachment 1716467767
Click to expand...

That doesn't look bad....my b-body has the reinforcement plate but the MC goes up like 1/4" or so at the front.. i might try a brace like that, i was thinking kinda the same thing but inside at the pedal box itself.. a winter thing to figure out.
 
To reduce the plate load on my CenterForce PP, I just put thin washers between the flywheel and the cover.
Worked perfect.
The bonus is that it slips just enough at low rpm that I can blip the throttle to put some energy into the factory heavy flywheel, then more or less dump the clutch, which drags the engine back to idle, at 3.5 to 4mph, and my little 367 just purrs away.
My CF has flyweights so by 2800rpm/20mph she is clamped up tight... unless the tires are spinning lol, then less than 20, lol. I control that with my gas-pedal foot. Once the tires are broke loose, I can bring the spinning tires right back down to idle.

Great mod. Plus you don't have to remove anything except the bolts. I just loosened them all, slipped the washers in, two bolts atta time, then tightened her down.
 
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Bob Jasinski said:
No, not going to happen, and that wouldn't fix the flex when the brakes are applied. A simple strut from the shock tower to a bolt on the master should work and be easy to do, just wondering what others have done.
Click to expand...
What shock tower? Shock is rubber mounted.
Brace to fender mount bolts & another to frame rail, if needed. Might need both.
A plate behind M/Cyl mount covering more area, thru bolted, might also help. Still will need bracing. You're right inside may be best.
Someone will post pics.
 
Woody500 said:
What shock tower? Shock is rubber mounted.
Brace to fender mount bolts & another to frame rail, if needed. Might need both.
A plate behind M/Cyl mount covering more area, thru bolted, might also help. Still will need bracing. You're right inside may be best.
Someone will post pics.
Click to expand...
See post #14. Its braced to the side of the shock tower and the bottom of the MC .
 
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AJ/FormS said:
To reduce the plate load on my CenterForce PP, I just put thin washers between the flywheel and the cover.
Worked perfect.
The bonus is that it slips just enough at low rpm that I can blip the throttle to put some energy into the factory heavy flywheel, then more or less dump the clutch, which drags the engine back to idle, at 3.5 to 4mph, and my little 367 just purrs away.
My CF has flyweights so by 2800rpm/20mph she is clamped up tight... unless the tires are spinning lol, then less than 20, lol. I control that with my gas-pedal foot. Once the tires are broke loose, I can bring the spinning tires right back down to idle.

Great mod. Plus you don't have to remove anything except the bolts. I just loosened them all, slipped the washers in, two bolts atta time, then tightened her down.
Click to expand...

AJ/FormS, I had to read your post a couple times to have it sink in. Brilliant idea. I have the 10" B&B style clutch installed with scalloped pressure plate. It has the special 5/16" -18 bolts from Brewers.

www.brewersperformance.com

PRESSURE PLATE BOLT PACKAGE, 10.0" SCALLOPED PRESSURE PLATES - Mopar A833 4-Speed Transmission & Component Specialists

PRESSURE PLATE BOLT PACKAGE, 10.0" SCALLOPED PRESSURE PLATES
www.brewersperformance.com www.brewersperformance.com

So, as I see it the washer OD could not exceed the diameter of the special bolt, or there would be clutch disc interference. They'd be installed under the pressure plate flange and the special bolt goes through it. I've got some 5/16" small diameter washers here about .050" thick. How thick were your washers?
 
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