1970-72 Chevy/GMC Frame differences

[Dieselwrencher]

I have a 72 C1500 long bed 2wd frame listed for sale. I listed it stating it has leaf springs because it was from a GMC but noted the frame is closer to a coil spring truck. I originally wanted to 4x4 swap the front of the frame but since the rear is shaped differently, I decided just to sell it. A member posted in the thread stating that this frame is basically a leaf spring frame, not a coil spring frame. I'm intrigued now and don't want to be misleading.

I realize the Chevrolets were standard with coil spring suspension and leaf springs were an option. And the GMC's were opposite. I also believe all the K frames had straight frame rails over the rear axle. They don't bump out in the middle of the wheel well/over the rear axle.

Here come the questions. I've had a few Sierra Grande C15s over the years but all were parted and scrapped. I never paid any attention to these frames. Same with C10s.

Were all half ton 2wd frames based off the coil spring suspension and have the bump outs over the rear axle? (C10 and C1500)

Is it correct that all the K, 4x4, frames were straight and had no bump outs?

In the pics below, you can see the center cross member has factory punched holes for the mounting brackets for the trailing arms. Yes, it is missing the cross member and reinforcing plates for the coils to mount to. I firmly believe these are a coil spring frame with leaf spring hangers bolted on the side.

Do any of you have pics of a leaf spring C10 that the rear rails are straight and don't bump out?

My experiences with these trucks are mainly C20s or K10s and K20s. I like learning new stuff on these trucks though. I've had a couple coil spring rear C20s but once again, those were pars trucks and were scrapped long ago.

[Dieselwrencher]

Here are a few more pics of the C1500 frame. The next 2 are of a rusty 71 K10 and the final pic is a 71/72 short bed C10 frame. I don't have the K10 frame anymore to get a better pic of the rear rails straight on for comparison to the coil spring suspension frames.

[Big Kev-O]

I converted my 68 GMC from leafs to coils. The C20 frame has the same curves section/ bump outs (circled in the picture) as the C10. The frames is about 3/32" taller between the top and bottom of the frame rails and is made from slightly thicker steel. Leaf spring frames do not have rivet holes in them for the coil style shock mount cross member / spring perch. Leaf truck don't have panhard bars which is another requirement on coil trucks.

The hardest part of converting a leaf truck to coil springs is locating and drilling the holes for the coil style cross member.

[Dieselwrencher]

Originally Posted by Big Kev-O

I converted my 68 GMC from leafs to coils. The C20 frame has the same curves section/ bump outs (circled in the picture) as the C10. The frames is about 3/32" taller between the top and bottom of the frame rails and is made from slightly thicker steel. Leaf spring frames do not have rivet holes in them for the coil style shock mount cross member / spring perch. Leaf truck don't have panhard bars which is another requirement on coil trucks.

The hardest part of converting a leaf truck to coil springs is locating and drilling the holes for the coil style cross member.

So really, these C10/C1500 frames were designed to be universal and GM punched the necessary holes per the suspension selection. That being said, I'm gathering the only frames that where the rear rails are straight are the cab chassis, Longhorns, and K frames?

[CG]

I have zero answers to your question but have another question for you. If the rear of the truck is leaf springs and you were thinking about converting the front to leafs to make it a four wheel drive, why does it matter in the rear whether the frame is a little different as long as it is already leafs? Cant you install lifted springs or blocks just like you would do on a come from the factory 4x4? To me the tough part would be converting the front of the frame to leafs (no skills lol). Maybe the mounting pads are located differently? Seems like an easy fix for someone with your skill set.

[Dieselwrencher]

I definitely could swap it but it just bothers me that it has those bumps when real 4x4 trucks don't.

[57taskforce]

Originally Posted by Dieselwrencher

So really, these C10/C1500 frames were designed to be universal and GM punched the necessary holes per the suspension selection. That being said, I'm gathering the only frames that where the rear rails are straight are the cab chassis, Longhorns, and K frames?

Bingo! Ryan, That’s correct as far as I know, C10/20 (non longhorn on the 20) are the only ones to have these bends in the rear.

[pjmoreland]

I saw these frame braces under the engine mounts on a 1972 C20 the other day. Are these braces on all C20 trucks?

[71meangreenc10]

Originally Posted by pjmoreland

I saw these frame braces under the engine mounts on a 1972 C20 the other day. Are these braces on all C20 trucks?

My 72 GMC 1 ton has them also.

Smitty

[Dieselwrencher]

Originally Posted by pjmoreland

I saw these frame braces under the engine mounts on a 1972 C20 the other day. Are these braces on all C20 trucks?

I've only seen them on C20 and 30s as well. Look at that crack in the firewall seam at the trans tunnel.

[special-K]

Originally Posted by 57taskforce

Bingo! Ryan, That’s correct as far as I know, C10/20 (non longhorn on the 20) are the only ones to have these bends in the rear.

Bingo x2! Can there be a tie in bingo??