Leaf spring differences for 68 C-bodies and maybe other years. Soft to Firm.

[HWYCRZR]

Yesterday I started down the path of gathering some details before I ordered new springs for my ‘68 Polara. I figured I would find three or four different choices that came from the factory. But I discovered that each brand and model of C-bodies had the own P/N. So now I am trying to decipher. What ones are the softest/weakest to which ones are the stiffest and strongest. For now I am going to stick with the Dodge Polara and Monaco which does share a couple of spring sets with the Chrysler Newport. I didn’t recall any Plymouths with the same P/N as Dodge. Am I assuming correctly that the base Dodge and Chrysler got the weakest springs and as the performance and options go up the spring stiffness increases?
Dodge examples:

2dr HT DL, DM, and DH 318 -383 eng uses pn 3004 658

Convertible, 4dr HT DL, DM, DH and 2 dr. ht. D-P (Monaco 500) 318-383 used pn 3004 597
Getting stiffer?

Dodge taxi 318-383 pn 2585 240

Police, 4dr 383 440 eng D pn 2585 399
And
Except Sta wagon. Extreme Duty 318, 383, 440 D pn 2585 399
So can assume 440 only have the “extreme duty” springs?

Then there is “Heavy Duty” are these stiffer than “Extreme Duty”? Which ones were used with the trailering package?

Except Sta Wagon 318, 383, 440 eng pn 2585 240. So the Heavy duty seem to be the same as what was used on the taxi’s

I am not going into station wagons as they have left and right differences and by how many seats they have.
And why is it that the Newport is the only other c-body that shares the base spring of the Polara and Monaco and no Plymouth matches.

Just wanting to understand the thought process of the engineering department.

 

[HWYCRZR]

I am contemplating new springs to replace my existing modified springs. When I have a full tank of gas and then fill up the trunk for a road trip it seems to squat a bit. With trunk empty, no passengers and full tank of gas, the rocker panels are about perfectly level. Front height is adjusted to spec. As soon as I start loading the trunk, it gets a little lower in the rear. I realize that these aren’t corner huggers, I feel that I have a little more sway than I used to.
I am thinking I would like to go up to the next stiffness level. I believe the next level would be the spring pack used on convertibles, Monaco 500, and 4d ht. Not sure I need to go up to the HD or Extreme duty.

 

[LocuMob]

I am contemplating new springs to replace my existing modified springs. When I have a full tank of gas and then fill up the trunk for a road trip it seems to squat a bit. With trunk empty, no passengers and full tank of gas, the rocker panels are about perfectly level. Front height is adjusted to spec. As soon as I start loading the trunk, it gets a little lower in the rear. I realize that these aren’t corner huggers, I feel that I have a little more sway than I used to.
I am thinking I would like to go up to the next stiffness level. I believe the next level would be the spring pack used on convertibles, Monaco 500, and 4d ht. Not sure I need to go up to the HD or Extreme duty.

I'll be putting a set from a Monaco 500 in my Polara 500, you can tell the difference between the two springs.

 

[MoPar~Man]

How are the springs physically different between the various options? Did they throw in extra leafs? Did they make the leafs longer? Thicker? Wider?

 

[HWYCRZR]

How are the springs physically different between the various options? Did they throw in extra leafs? Did they make the leafs longer? Thicker? Wider?

That’s what I am hoping to draw out from the crowd.
I do know my Polara D-L w 383 engine would have had the 3004 658, but the pn on the bottom spring is 2835 354. Mine has 5-1/2 leafs. So at some time the part number was super seceded.

 

[Big_John]

I am contemplating new springs to replace my existing modified springs.

Modified springs?

If they were rebuilt, IMHO, you can't compare them to the original stock springs. They take a spring leaf and bend them to put the arch back in. The way that they allow for the metal fatigue of 50+ years is to add another leaf. It all looks good until you really understand it's at best a guess at matching the stock spring characteristics. A stock 5 leaf spring is now a 6 leaf, but the tired 5 leaves (or is it leafs?) are still there.

From what I've seen of factory leaf springs, the leaves look like the same thickness and the same width. I've never checked that thickness myself, so I could be wrong. I don't believe that there was any change to material or heat treating to change spring rates. I think change in rate was probably just done in adding half or full leaves.

 

[CBODY67]

If you look at the FSM specs, as to number of leaves for the std and HD applications on sedans, it's quite specific about what is what.

I believe the police car springs would be the stiffest, followed by others, until you get to the base 2bbl 318s.

I don't suspect that "Extreme Duty" would necessarily be stiffer than the police springs, but could have a higher-grade of metal to sustain more flexing before sagging.

Look at the FSM specs and you might see that the rh assy has an extra 1/2 leaf than the lh assy does. Torque reaction control? Especially on the 383+ cars.

Personally, I always considered Chrysler's rear leaf springs to be "works of art", done to a higher level of execution than similar GM items. Taking them to a spring shop can dilute or erase some of that initial goodness, to me. Making them more generic than specific. BTAIM

ONE thing to note is that the normal leaf springs have the wax-fiber interleaf pads at the end of the leaves. The factory HD items have zinc interleaf items. When the pads wore away, never did hear more "sliding sounds" from the leaves. IF that might matter.

If it were me, I'd just order the factory HD rear springs, at the factory ride height, and not worry about the other things. From Eaton, probably, although Espy can be a good source, too. THEN make sure appropriate HD rear shocks were on the car too.

Just some thoughts and observations,
CBODY67

 

[HWYCRZR]

Modified springs?

If they were rebuilt, IMHO, you can't compare them to the original stock springs. They take a spring leaf and bend them to put the arch back in. The way that they allow for the metal fatigue of 50+ years is to add another leaf. It all looks good until you really understand it's at best a guess at matching the stock spring characteristics. A stock 5 leaf spring is now a 6 leaf, but the tired 5 leaves (or is it leafs?) are still there.

From what I've seen of factory leaf springs, the leaves look like the same thickness and the same width. I've never checked that thickness myself, so I could be wrong. I don't believe that there was any change to material or heat treating to change spring rates. I think change in rate was probably just done in adding half or full leaves.

They are factory but I modified them in high school by pulling out the third leaf and putting in two leaves from a late 60’s early 70’s F250 4x4. I had a little too much rake and couldn’t adjust the caster so it was like steering a pig with two sticks. After a couple of months chasing it down the hiway my better sense kicked in. I removed one of the additional leaves and left the leaf in that was in similar length to my original leaf I pulled out. I used an hose clamp to “help” my original spring clamp that I couldn’t re fasten all the way. The replacement spring is a little thicker and a little longer than the original. So 4-/12 out of the 5-1/2 are original. Including the top two with the eyelets and the bottom 2-1/2. Thus the original p/n. It worked quite well for a number of years.
It’s the squared off tapered one.

 

[MoPar~Man]

For what it's worth, here's what's on my '67 Monaco (with 318):

Driver's side: 2539746

Didn't look at passenger side, I'm assuming it's the same number.

I'm not sure what counts as a leaf vs just a "plate", but I have 7 distinct leafs or plates. I measure the top leaf to be 35 inches from the rear face of the shock-bracket backwards to the center of the rear shackle mounting bolt. I then measured the distances from shackle bolt forward to where each next leaf starts:

Leaf 2: 5.5 inches
Leaf 3: 10 inches
Leaf 4: 13.75 inches
Leaf 5: 21.5 inches
Leaf 6: 29 inches
Leaf 7: 35 inches

I didn't measure the leaf gaps on the front side of the stack, but I can if requested.

Leaf 7 is at the bottom of the stack, but it doesn't extend in the rearward direction beyond the shock bracket. On the front side of the bracket it does extend, I didn't measure it (maybe 6 inches?) - that's where it has the P/N stamped on it.

I'm really hazy about what was done with the springs when the car was first R&R'd in 1987 but I'm fairly certain they were re-sprung, possible an extra leaf thrown in. My observation of 7 leafs should confirm whether or not there is an extra leaf in there.

I measure .26 or .27 for the leaf thickness, that could easily be wrong and they're all .25 inches. Width is 2.5 inches, or possibly 2 9/16. Distance from the ground up to the center of the rear shackle bolt is 15.25 inches.

After the 1987 R&R the car has never really carried a heavy load, maybe the rear does sit up - perhaps a little too much even after sitting on these springs for the past 37 years but I don't mind, I feel confident that it can take a couple of adults in the back seat without dragging the rear end.

Edit: Hmm, I could be wrong about the PN for my right-side springs being the same P/N as the left:

Rear leaf spring questions

I have purchased a old set of rear leaf springs out of a 67 Newport. I plan on rebuilding them and installing into my 67 300 convertible. The ones in the car now have added leaves installed in the early 70"s when my Dad was pulling a holiday trailer. The *** end of the car sits a little too...

www.forcbodiesonly.com

 

[HWYCRZR]

For what it's worth, here's what's on my '67 Monaco (with 318):

View attachment 691725

Driver's side: 2539746

Didn't look at passenger side, I'm assuming it's the same number.

I'm not sure what counts as a leaf vs just a "plate", but I have 7 distinct leafs or plates. I measure the top leaf to be 35 inches from the rear face of the shock-bracket backwards to the center of the rear shackle mounting bolt. I then measured the distances from shackle bolt forward to where each next leaf starts:

Leaf 2: 5.5 inches
Leaf 3: 10 inches
Leaf 4: 13.75 inches
Leaf 5: 21.5 inches
Leaf 6: 29 inches
Leaf 7: 35 inches

I didn't measure the leaf gaps on the front side of the stack, but I can if requested.

Leaf 7 is at the bottom of the stack, but it doesn't extend in the rearward direction beyond the shock bracket. On the front side of the bracket it does extend, I didn't measure it (maybe 6 inches?) - that's where it has the P/N stamped on it.

I'm really hazy about what was done with the springs when the car was first R&R'd in 1987 but I'm fairly certain they were re-sprung, possible an extra leaf thrown in. My observation of 7 leafs should confirm whether or not there is an extra leaf in there.

I measure .26 or .27 for the leaf thickness, that could easily be wrong and they're all .25 inches. Width is 2.5 inches, or possibly 2 9/16. Distance from the ground up to the center of the rear shackle bolt is 15.25 inches.

After the 1987 R&R the car has never really carried a heavy load, maybe the rear does sit up - perhaps a little too much even after sitting on these springs for the past 37 years but I don't mind, I feel confident that it can take a couple of adults in the back seat without dragging the rear end.

Edit: Hmm, I could be wrong about the PN for my right-side springs being the same P/N as the left:

http://[URL]https://www.forcbodiesonly.com/mopar-forum/threads/rear-leaf-spring-questions.84765/

[/URL]

Leaf 1 is actually what they call the 1/2 leaf. So you have the 6-1/2 leaves pack.
What make and model car do you have? Is it a convertible or a Monaco 500? Do you know if you have the trailer option?
As @CBODY67 said it is in the service manual, but does not address convertible or the difference for 4dr HT.