Adding a clutch pedal

[eric8]

I have always read that on 67-72's, "autos and manuals used the exact same pedal box. GM just slid a clutch pedal on the end of the manuals." Well I now have located a manual transmission complete pedal box and pedal assembly, and they indeed appear to have a small difference.

If you'll reference the attached pic, I am pointing to part of the pedal box that is not part of the stamping on the original auto pedal box on my 72 C10. This stamping is where the very heavy clutch return spring attaches. Why are there two return springs?

I'm doing a T56 hydraulic clutch swap. Do I need both return springs? I'd like to keep the original pedal box in place and just add on the clutch pedal if possible.

[eric8]

This factory diagram only shows a light return spring at the top of the pedal arm. Any ideas? M

[imjeff]

Looks like a PO probably added a spring to get the clutch pedal to fully return. Sometimes, depending on the adjustment, they can hang down a little. The hydraulic set up should only require one spring.

[eric8]

Interesting idea Jeff but I'm almost confident this looks to be a factory setup. If you look in the diagram above, the item labeled "production brake pedal support" looks like my auto setup. If you look at the shape of the manual setup I have, you can see it is shaped much different and extends over toward the driver's side to the clutch pedal.

[imjeff]

I believe you're right after looking at the diagram and pics again. I will keep pondering...maybe my old mind will recall a useful fact! Lol! That spring looks a lot like the generic ones they used to sell in parts stores. There are a couple I've launched in about the same orbit as Sputnik.

[eric8]

It is much much larger than it looks in the pics. Super heavy spring. After playing around with it and pushing the pedal through its range of travel, the big spring appears to be the exact opposite of a return spring. Rather, it feels like it is assisting the pedal to go to the floor once it gets through the first 25% of its travel. Maybe it was used on C20s with super heavy duty clutch setups, so the pedal effort wouldn't seem so high? You'd still think they would show the spring in the parts breakdowns or assembly manuals if that were the case though.

[ray_mcavoy]

Originally Posted by eric8

After playing around with it and pushing the pedal through its range of travel, the big spring appears to be the exact opposite of a return spring. Rather, it feels like it is assisting the pedal to go to the floor once it gets through the first 25% of its travel.

That's often called an "over center" spring and does exactly as you described. It assists in pushing the pedal down once the pedal travel goes beyond the center pivot point of the spring.

Originally Posted by eric8

Maybe it was used on C20s with super heavy duty clutch setups, so the pedal effort wouldn't seem so high? You'd still think they would show the spring in the parts breakdowns or assembly manuals if that were the case though.

This diagram is from the assembly manual (sorry, scanner clipped off the LH side a bit but I think you can still read most of it). Looks like it was used in medium duty trucks:

[eric8]

You are the man Ray! I don't know how I missed that in the assembly manual I have (actually, I do know how.....as I'm sure I don't need to tell you how badly disorganized that thing is. It is seriously in need of some type of index or table of contents). Thank you for putting the mystery to rest!

[imjeff]

Thank you! I started out looking at the wrong spring, but now the mystery is solved!

[ray_mcavoy]

Originally Posted by eric8

You are the man Ray! I don't know how I missed that in the assembly manual I have (actually, I do know how.....as I'm sure I don't need to tell you how badly disorganized that thing is. It is seriously in need of some type of index or table of contents). Thank you for putting the mystery to rest!

You're welcome!

Yeah, I have a bunch of different assembly manuals for my various projects and I have to say the 67-72 truck one is one of the worst in terms of organization. Mine is full of pieces of paper I've used to bookmark certain pages but it still takes a while to find stuff in there. I've also noticed that it doesn't show the original GM part numbers like a lot of the other assembly manuals. Oh well, at least it's better than not having anything available ... as is the case for some of the cars I own.