Rear leaf spring swap issue
 

I purchased a set of front multi-leaf springs that were listed as 2 1/2 inch drop from stock and rear multi-leaf springs that were listed as 3 inch drop from stock. The process of swapping out the front springs was fairly straightforward with minimal difficulty. As a result, I assumed the rear would be about the same.

After getting the wheel and shock off on the passenger side, I then made sure to take all the tension off the spring my having a jack under the diff. When all tension was removed, I took off both the threaded bolt on the front of the spring and disassembled the shackle on the rear and easily removed the mono spring. Once I got the front new threaded bolt lined up and installed, I used a small bottle jack to line up the rear of the new leaf spring and was able to get the shackle assembled.

It was at that point that I noticed that the bolt/nipple that is mounted at the center of the leaf spring and is intended to be centered into a shim, as well as used to line up the lower shock mount bracket was off center. While the leaf spring was lined up front to back on the axle properly, the center of the leaf spring was three inches too far forward of the axle. I tried lowering the axle to meet the spring and even tried raising the spring assembly up with the bottle jack to see if it centered.

The other thing I noticed was that the rear shackle was pitched forward rather than rearward like the drivers side. I'm not sure if that accounts for the three inches, but at that point I was frustrated and removed the leaf spring.

What am I doing wrong or what haven't I done that I need to, to get this leaf spring mounted?

Re: Rear leaf spring swap issue
 

Did you measure the length of the springs? Or compare them?

First thing, Grab your tape measure and measure the length of the springs to make sure that they are the same. Then measure from each end to the spring bolt to make sure you don't have one reversed. Simple things first.

Are you doing one side at a time and putting the tire on and putting the weight of the truck on the spring? Meaning that the left side has the weight of the truck and the right side is hanging? That would make a difference at the shackles.

Re: Rear leaf spring swap issue
 

response to questions above in BOLD so that they stand out....not yelling...lol

Re: Rear leaf spring swap issue
 

Did you measure between the front eye to the centering pin on both the multi leaf and the single leaf spring ? That measurement should be the same on both springs . It’s what determines where the axle center line is . Providing the front mount is still riveted to the stock location .

Re: Rear leaf spring swap issue
 

It appears that both leaf spring mount points are factory original given the Chevy rivets used to hold them in place.

The issue I have is that it does appear that the axle may not be in its original position, given that it is not the original axle. The current measurement from the center of the front mounting point to the center of the axle is 27 inches. It appears from my measurement of the new spring when it is not under any load is 24-25 inches depending on if I measure on a straight line or follow the curve of the spring.

The tail piece of the drive shaft that is in the transmission appears to have been pulled out about 2 inches, which could be part of the adjustment that the prior owner did to fit the non-original axle.

At the moment I have all weight off the mono springs, the frame is supported both in front and behind the two spring mount points on both sides. I have a bottle jack under the diff and have the wheels just slightly off the ground. I realized that the u-bolts that I have are not long enough to handle the increased width of the multi-leaf spring, so I've had to order longer ones and they won't be here until Wednesday.

My plan is to remove the mono-springs, mounts the multi-leaf, get them as close to in position as I can until the longer u-bolts show up and see if between compressing the springs to get them closer to the underside of the axle and attempting the front to back location of the axle itself if I can things lined up.

If all of this wasn't enough of a challenge, as the song "99 problems" goes, I realized today that the new rear tires/wheels that I mounted on the rear axle after installing a rear disk brake kit do not rotate. It seems that the backspace/offset of the wheels isn't enough to avoid pinching solidly against the caliper. Apparently, I will have brand new 15 inch chrome wheels and 1/2 moon hub caps, as well as brand new Firestone tires for sale, since it seems that I will need to move to 16 inch wheels....more to come.

Re: Rear leaf spring swap issue
 

If that 2 to 3” difference is on the new multi leaf spring you’ve got the wrong spring for your application.

Re: Rear leaf spring swap issue
 

When I purchased them for this truck, it was stated that they were for a 3" drop, not stock height. However, I expected a modest lift from the current mono-springs.

Re: Rear leaf spring swap issue
 

The two new ones are the same length each

This is what I was asking about. Making sure that they were indeed exactly matching springs.


Are you doing one side at a time and putting the tire on and putting the weight of the truck on the spring? Meaning that the left side has the weight of the truck and the right side is hanging? That would make a difference at the shackles.

That is correct, the drivers side is still on its tire, i have a jack stand under the frame on the passenger side and a bottle jack under that side of the axle. I can let the jack down or put it up to get the spring closer to the axle, but its still off from centering by several inches. Another friend suggested putting the tire back on, removing the jack stand under the frame and letting that side take more weight before securing the u-bolts.

That is going to happen on any leaf spring vehcicle that you jack one side up on one side only with the weight on the other side, the shackle is going to pull forward as you raise the vehicle. Some with the shackle that goes up and hangs the truck from the end of the spring are known to have the shackle flip when you raise the rig with the tires off the ground such as on a two post lift. I'm thinking that is somewhat common on TF trucks.


One thing that came to mind on your axle location. Normally we drill a hole 1-3/4 inches in front of the center line of the rear axle for the spring bolt to go through and center the axle in the wheel well.

If your axle seats or spring perches or ?? what ever the name of the week is for them only have the hole in the center that may be what is offsetting your axle or if you are using the hole in the center rather than the one that is drilled in front of that.
This screen shot of the classic parts add for shows the axle seats (what they call them) with the centered hole and the offset hole for 47/54 3100 AD trucks with stock measurement springs if you use the center hole your

Re: Rear leaf spring swap issue
 

i may have misread so bear with me but

a stock 53 3100 will be a torque tube rear

you dont have the stock rear (torque tube) any more. you do have stock replacement (drop) leaf springs.

I havent done much with torque tube axles but I know there is a special attachment from the axle to the leaf to allow the axle to move WRT the trans.

I seem to remember plates being sold/required to put an open drive axle on torque tube leafs. its possible you just need those plates.

Re: Rear leaf spring swap issue
 

what mr48chev said.

Factory axle has a pivot point just in front of the axle tube, the bolt through spring pack more or less lines up with that. The axle seat needs that extra hole to move it backwards on top of spring pack.

Here is a picture of factory '50s gm truck style axle mount point.
Attachment 2233547

might be fuzzy, had to enlarge it a lot

Re: Rear leaf spring swap issue
 

I'll admit that I'm both massively confused and massively frustrated. Today, I had the frame supported by jack stands in front and behind the leaf spring mount points. I had a bottle jack under the diff and the wheels/axle were just slightly off the ground.

Here are two photos of the setup. I already have the welded on bracket (with two holes for the spring to slip into)

Attachment 2233574

Attachment 2233575

I took the mono springs off and installed the multi-leaf springs, first at the shackle and then the front mount. Since I don't yet have the longer u-bolts necessary (they arrive tomorrow) I just wanted to see how close to fitting I could get the springs. Using a bottle jack I compressed the springs upward towards the axle. If I had the longer u-bolts I could have partially tightened the u-bolt nuts to get the lower shock bracket to get closer to the axle, but the nub that needs to mount into one of the axle holes were still not aligned, but far closer.

The next time I attempt this (after the longer u-bolts are here) I will likely remove the wheels and support the diff with my rolling hydraulic floor jack so that I can attempt moving/adjusting the position of the axle over the springs. Which of the two holes should I be aiming for?

Re: Rear leaf spring swap issue
 

you want to move the axle back and use the forward most hole. it looks like you could drill a 3rd hole ever further forward if needed.

is the rear shackle angle mentioned in first post working out? Spring eye needs to be behind upper shackle pivot. if it is in front the ride will be harsh, far enough in front the spring will be unable to lengthen as it flattens out under load.

The shim I see used with the monoleaf to adjust pinion angle may not be correct after the springs are changed.

Re: Rear leaf spring swap issue
 

Eyeball engineering says that the axle should be pretty well centered on the bump stop.

The locating hole in the spring pad on the axle is normally 1-3/4 inches in front of the center line of the axle. That is what I had to drill mine at when I swapped transmissions and rear axles in a one night after work (worked swing shift) thrash where I swapped rear axles and the transmisson and didn't drill the hole to center the axle and had to take the axle out, drill the holes and put it back the next night in 1973. Not having much yoke in the transmisison is also what caused me to have the yoke knock out the rear seal on I 35 by Marietta OK on my way to Tulsa a couple of days later to go to the 73 NSRA street rod nationals. Bob Davis who lived in Waco and Taught at TSTI then and I swapped tail shafts in the trans in a little wrecking yard in Marietta and were only a few hours late in the process.

Re: Rear leaf spring swap issue
 

I'm confused on what direction I want to move the axle. I could be wrong, but it appears that the axle needs to be moved forward enough to allow the springs to slot into the center hole on the axle mount. If I do need to drill another hole, would that be on the back side of the axle mount?

Here is a view of the current pinion angle and the amount of the drive shaft tail piece that is already sticking out of the transmission. Th angle at the diff is 5 degree and 6 degrees at the transmission.

Attachment 2233587

Attachment 2233586

Re: Rear leaf spring swap issue
 

it looks like you are measuring if your driveshaft is straight, not the relative angles on the transmission tailshaft and diff pinion shaft?

The yoke stick out does not look horrible.

My assumption: Your monoleaf spring was sold with the pin the right place to line up a modern axle in the fender. Your new OEM drop multileafs are sold with the pin in the right place for the original rear end to line up at the correct length for a torque tube on a factory trans.
Putting your modern rear end centered under the axle hole on the factory location pin will put it too far forward. Shifting the axle back to the other hole might work.

The measurements you reported earlier for front eye to pin seem to support this, the pin is further back on monoleaf

If you mount springs to frame front and back with the axle loose on both sides just sitting on the springs. With the yoke sticking out the same as it was before are the wheels where you want them in the fender? Where is the pin in the new leafs relative to axle tube and the two holes you have to choose from?

Re: Rear leaf spring swap issue
 

All good points. Just to be absolutely certain that I have the correct springs, I have taken measurements and photos and sent to the company to ensure I have the right springs. The longer u-bolts will be here tomorrow, but the spousal unit requested my assistance in getting ready for both Thanksgiving and her birthday (Saturday). I suspect I won't get to play again on the truck until Friday. More to come once I have the bolts.

Re: Rear leaf spring swap issue
 

i mean, this is everything I said just 6 posts later. :lol::lol:

I went looking for the setback plate i remember seeing but didnt come up with anything, so its probably like mr48 and lee said, just drill a new hole in your perch to center the wheels.

Re: Rear leaf spring swap issue
 

Jus to be clear, the drilled hole would be more towards the back or the front of the axle mount? A or B location?

Attachment 2233642

Re: Rear leaf spring swap issue
 

This is one of my go to to share driveshaft angle charts.

The output shaft on the transmisison and the pinion in the rear axle need to be parallell with each other. That is the main thing. That should make the angle between the output shaft and the driveshaft and the angle between the driveshaft and the pinion shaft the same. That is the important part.

https://hosting.photobucket.com/imag...720&fit=bounds

You need to know the difference between the output shaft and drive shaft and the drive shaft and the pinion shaft and that that those differences are the same. The angle of the driveshaft should be the same though the whole drive shaft.

Re: Rear leaf spring swap issue
 

i cant say, but only because i dont have a reference on which way is front on your axle from the pic. it might be there, but i am just not seeing it easily.

you said the pin was too far forward when the axle was centered so my guess would be to move the axle back and that is the direction you drill the hole.

Re: Rear leaf spring swap issue
 

Sorry....

Attachment 2233655

Re: Rear leaf spring swap issue
 

that's what they said about Stairway To Heaven too, but the Led Zeppelin version is still better :)


Based on how I have read this thread I vote for B

but I'd bolt the springs on,
Check that shackle angle that was worrying you.

Set the axle in there loosely,
check the yoke stick out,
check the fender/wheel look

and then look at where the pin is, if one of the holes is close enough, if a new hole can be drilled or something else has to happen

Re: Rear leaf spring swap issue
 

1-3/4 inches in front of the centerline of the axle.

This being the drawing from the PFD in the Vehicle information kit. This is the drivers side. Where you can see the centerline of the axle is behind the pad that bolts to the spring and that circle on that pad is the bushing for the bolt that the axle hinges on to allow the enclosed driveshaft to work without binding up the spring.

Re: Rear leaf spring swap issue
 

Perfect, thanks.

Re: Rear leaf spring swap issue
 

like the post 19 pics showing the driveline angle. I would also add that spring wrap will cause the axle end to change as the axle housing turns under load. that is the reason a lot of lifted 4x4 trucks wear out the rear u joint and also why the fix for them is to add a link between the axle and the frame, ahead of the axle. doing this keeps the axle from rotatingwhen the springs start to wind up under heavy acceleration (usually). some guys use the single link that bolts on at the center of the axle and connects up near the trans output, it came standard on some older camaros even. some call them torque arms or rods or links. just google it for a better explanation.