Leaf Springs and Axle location

[JohnFM3]

I am making a 1973 Plymouth duster road worthy again, see build thread here. I have ran into a issue where the axle does not appear to be perpendicular to the rear of the car, where the driver side of the axle is sitting 1/4 in behind the passenger side. I have heard opinions that poor manufacturing processes from the time these were built could explain that. Being a 1973, I find that hard to believe. 1/16 sure, 1/4 I dont believe.

But it brings the question, is it possible that I am ok? I am about ready to take the car for a 4 wheel alignment and want to rule out any issues before having that done.

I measured from the front edge of the rear tire to the wheel well at the bottom where the pinch weld ends.

Driver: 2in
Passenger: 1.75in

Is there the opportunity of mismatched leaf springs with a difference in the center pin measurement from the front bushing? I am finishing the car while being unemployed and my funds have about dried up, so new leaf springs are out of the question for the time being. I have considered the spring bushings could be bad on 1 side, but both of the shackles match relative to each other pretty evenly.

Thanks ~ John

 

[33IMP]

I'd guess that the axle is square. I'd bet the body is off by your 1/4 inch. Find the chassis points used to square the axle, and measure there.
Might require the car on a lift.

 

[Professor Fate]

Common but not ideal.
I wouldn't trust the bodywork to determine the "squareness" of your rear axle, though. There's a lot of tolerance stack how the body panels are attached that can change your numbers. Instead, triangulate your rear, ie, measure diagonally from a point on your axle to a point on the opposite side front frame- an assembly jig hole, K frame bolt or the like. Then take the same measurement for the opposite side- they should match. Any discrepancies are usually dealt with by using a shim plate behind one or the other front spring hanger brackets.

 

[JohnFM3]

I'd guess that the axle is square. I'd bet the body is off by your 1/4 inch. Find the chassis points used to square the axle, and measure there.
Might require the car on a lift.

It sounds like you would be talking about a Body on Chassis solution. The Duster is a uni-body car. There is no alignment option.

 

[JohnFM3]

Common but not ideal.
I wouldn't trust the bodywork to determine the "squareness" of your rear axle, though. There's a lot of tolerance stack how the body panels are attached that can change your numbers. Instead, triangulate your rear, ie, measure diagonally from a point on your axle to a point on the opposite side front frame; a assembly jig hole, K frame bolt or the like. Then take the same measurement for the opposite side- they should match. Any discrepancies are usually dealt with by using a shim plate behind one or the other front spring hanger brackets.

As commented to the prior person, this is a uni-body car. So other than comparing the rear axle to the K-Frame.

 

[RustyRatRod]

Just shim the short side at the front spring hanger. This ain't rocket science.

 

[33IMP]

It sounds like you would be talking about a Body on Chassis solution. The Duster is a uni-body car. There is no alignment option.

No actually I was talking about determining IF the rear is square in the car.... by something other than the body work.
And there actually is an easy way to correct it, if it's not. Shims between the body and the front spring hangers have been used for decades, for more tire clearance for tall tires.
Edit: RRR covered it while I was typing this. Point is, IF it's not square, it's easily fixable.

 

[Professor Fate]

As commented to the prior person, this is a uni-body car. So other than comparing the rear axle to the K-Frame.

I'm well aware of them being unibody cars. I've been working on them for fifty years. Unibody or body-on-frame doesn't matter, square is square. What I described is how it's done, like it or not. You can do it yourself or pay to have the alignment/frame shop do it, doesn't matter to me.

 

[RustyRatRod]

It sounds like you would be talking about a Body on Chassis solution. The Duster is a uni-body car. There is no alignment option.

No, he's telling you how to do it correctly.

 

[JohnFM3]

33IMP and ProfessorFate, Forgive me for asking clarification questions. I have no clue the experiences of who comments. Note that in your original response you did not specify that on Uni-Body cars there are issues like this where shims in the leaf spring bushing mounts may be needed. That statement (instead of assuming I know what your talking about) would have changed the entire direction of the conversation indicating to me that you understand the full issue of my problem and then I would have taken what you said and followed up with a chance of learning something new having a more constructive conversation than this pissing match with you upset with my follow up. When something sounds off I am going to question it and ask for more understanding.

I have interacted with RustyRatRod several times on my Barracuda build thread and now my new Duster thread over the last 10 yrs.
Hey man, how goes it? With the follow up comments, the possible issue make sense. I will measure from the K-Frame first as soon as I can get the car on flat ground. Then adjust if possible.

The big thing I was looking to hear was whether or not there was a possibility of leaf spring mismatch. I was always under the impression that Mopar A-Body springs are all the same geometry. At least for slant six versions. Unsure about V8 version. Considered the possibility of Center Pin locations being off 1/4 inch due to mismatch.

All of this was identified last night as it was getting dark. I was doing some research with intent of furthering my investigation today.

 

[JohnFM3]

BTW: Based on feedback here, it sounds like my neighbor may be correct in manufacturing errors. Hes a Ford guy building model A hot rods with frames. So I take what he says with a grain of salt worthy of addition investigation.

 

[Dana67Dart]

It is far more important that the rear axle is square to the front suspension for your car will be crabbing as it is driven.

The alignment shop is set up to determine that the front axle centerline is parallel to the rear axle centerline and that the track is inline with the body

 

[JohnFM3]

It is far more important that the rear axle is square to the front suspension for your car will be crabbing as it is driven.

The alignment shop is set up to determine that the front axle centerline is parallel to the rear axle centerline and that the track is inline with the body

Understood. Currently my wheel base measurement is 108 1/8 on the driver side and 107 1/2 on the passenger side giving me a 5/8 difference between the 2 sides. As I completely replaced the front end with HDK (as discussed in my build thread), I am going over and checking all this over. With assumption I was going to need to adjust the front upper control arm joints a bit. Now, I am finding the rear is not square as I expected.

Sadly, the car seemed to drive pretty straight upto 40mph with out having been to alignment shop.

Nothing more to add to this. I will update my build thread with my findings.

 

[33IMP]

If your wheelbase measurements are that far off, I agree with you, a trip to the alignment shop for a four wheel measurement is in order.
I don't know what the HDK entails, but the rear can be fixed IF it is out, with shims, as RRR and I suggested.

 

[JohnFM3]

If your wheelbase measurements are that far off, I agree with you, a trip to the alignment shop for a four wheel measurement is in order.
I don't know what the HDK entails, but the rear can be fixed IF it is out, with shims, as RRR and I suggested.

1973 Plymouth Duster rebuild thread

The HDK suspension removes the torsion bars and switching to QA1 coil overs. Its quite the conversion. Its well designed and Denny is known on the abody forum and is very free with his knowledge. Regarding my car, there is a possibility I have a upper control arm rearward adjustable joint turned IN too much (or Front joint OUT too much) pulling the front control arm back a bit resulting in the spindle center line pulled back. I am trying to sort out the rear before messing with the fronts. And I want all this set as close as possible so that the alignment shop can do their work. Most shops wont even touch my car due to the NON Factory front suspension and Ford 8.8 rear. I know, it makes no sense. But that is the BS I deal with in the Seattle area of WA. Leaving me with taking this to a high end shop for work. And being unemployed, I can not do that right now. And the car is so close to finished.

I do have 1 shop in the small town I live in with a old guy who is willing to assist. I am just trying to get it as close as possible to make it as easy as I can for him. Paying for him to do all that shim work and adjustment in the upper control arms before using alignment bolts will save me a lot of cash.

Before this thread, I didnt know the front leaf spring mount was removable. I assumed that the front leaf attached directly to the body and only the rear shackle mount unbolted which is why I had no clue about shimming it.

Thanks to everyone for the guidance. As soon as I can, I will get some measurements taken to the rear end and see what needs to happen.

 

[72bluNblu]

The factory tolerance on the chassis points was an +/- an 1/8”. So if one side is out one way, and the other is out the other way, that’s a 1/4” off. And that was the spec on the CHASSIS points. Bodywork is frequently off by up to a 1/2”. The factory tolerances on these cars were LOOSE.

In the FSM there are chassis measurements listed to identified chassis points. You need to check those, and you need to do that by leveling the car, dropping a plumb bob and snapping the lines in between those projections. This is what the diagram looks like, download an FSM if you don’t have one, the directions for how to use this diagram are in there too

And finally, if the front has not been aligned yet your wheelbase measurements are meaningless because you don’t know the front wheels are actually in the final location.

 

[JohnFM3]

The factory tolerance on the chassis points was an +/- an 1/8”. So if one side is out one way, and the other is out the other way, that’s a 1/4” off. And that was the spec on the CHASSIS points. Bodywork is frequently off by up to a 1/2”. The factory tolerances on these cars were LOOSE.

In the FSM there are chassis measurements listed to identified chassis points. You need to check those, and you need to do that by leveling the car, dropping a plumb bob and snapping the lines in between those projections. This is what the diagram looks like, download an FSM if you don’t have one, the directions for how to use this diagram are in there too

View attachment 1716502735

And finally, if the front has not been aligned yet your wheelbase measurements are meaningless because you don’t know the front wheels are actually in the final location.

OMG that diagram is great. Thanks. What is the FSM and where do I find it? Is there a paper version as well?

If there is any truth to the tolerances you shared, that's interesting. I would have expected tighter. I will take these other numbers you provided in the diagram and ignore what I have noted.

As far as the wheel base measurements being meaningless, I get that. When I saw the rear end so far off, I investigated that first so that I have the rear as a reference to pre-adjust the front too. My thought is, get the rear end locked in, then measure the wheel base between front and rear, then adjust the front as necessary to get it close. Finally take it to the shop for a 4 wheel alignment. They dont move/adjust the rear end, only use it as a reference to the front.

 

[crackedback]

I'd find the car centerline first, then start working on how things align from there

 

[72bluNblu]

OMG that diagram is great. Thanks. What is the FSM and where do I find it? Is there a paper version as well?

If there is any truth to the tolerances you shared, that's interesting. I would have expected tighter. I will take these other numbers you provided in the diagram and ignore what I have noted.

As far as the wheel base measurements being meaningless, I get that. When I saw the rear end so far off, I investigated that first so that I have the rear as a reference to pre-adjust the front too. My thought is, get the rear end locked in, then measure the wheel base between front and rear, then adjust the front as necessary to get it close. Finally take it to the shop for a 4 wheel alignment. They dont move/adjust the rear end, only use it as a reference to the front.

You can download the FSM’s here
Service Manuals – MyMopar

Paper copy’s are available on eBay, second hand etc.

The factory tolerances on American made unibody cars from the 70’s are terrible. If you’ve got a car square to a 1/4” you’re probably average at the worst, the same car isn’t even usually the same side to side. Again, a 1/4” on the rear axle is within spec.

You said you also have an 8.8 swap, so, there’s other things that can be at work here. Usually the perches available now have a larger hole than the pin on the springs, so, the axle can sit crooked even if everything is in the right spot just because of the oversized hole in the perch. And that’s assuming the perches are even in the right spot and the axle is true.

You don’t want to be locating the front off the rear or the other way around. You want both square to the chassis. And you do that with the measurements I posted from the FSM. You don’t want to screw up your front alignment trying to square it with the rear, the rear easier to move and the front alignment is more important.

 

[JohnFM3]

You can download the FSM’s here
Service Manuals – MyMopar

Paper copy’s are available on eBay, second hand etc.

The factory tolerances on American made unibody cars from the 70’s are terrible. If you’ve got a car square to a 1/4” you’re probably average at the worst, the same car isn’t even usually the same side to side. Again, a 1/4” on the rear axle is within spec.

You said you also have an 8.8 swap, so, there’s other things that can be at work here. Usually the perches available now have a larger hole than the pin on the springs, so, the axle can sit crooked even if everything is in the right spot just because of the oversized hole in the perch. And that’s assuming the perches are even in the right spot and the axle is true.

You don’t want to be locating the front off the rear or the other way around. You want both square to the chassis. And you do that with the measurements I posted from the FSM. You don’t want to screw up your front alignment trying to square it with the rear, the rear easier to move and the front alignment is more important.

Gotcha. As far as the 8.8 swap, as a temp solution I used grade 8 bolts which head fit the perch hole snug. I bought NEW mopar performance perches and feel confident with the help of my neighbor the measurements are good and we nailed the angles with the help of people on the forum. I am going to locate new pins which will work, I was hoping to get a job then buy new rear leaf springs and correct the center pin then. I did drill out the current slant six leaf springs for 3/8 studs, and I am hoping to find center pins with the correct head size thats 3/8 stud assuming the new springs has the same stud size.

Thanks.

 

[Dana67Dart]

If your wheelbase measurements are that far off, I agree with you, a trip to the alignment shop for a four wheel measurement is in order

Keep in mind if the front alignment is not correct the wheelbase measurements might show that.

Example

If the caster is correct on one side and way off on the other I can see the front wheel being 1/4" back or forward.

Compound that with the body not being square and the springs sagging more on one side than the other there's more fractions of inches added into the equation.

 

[JohnFM3]

Keep in mind if the front alignment is not correct the wheelbase measurements might show that.

Example

If the caster is correct on one side and way off on the other I can see the front wheel being 1/4" back or forward.

Compound that with the body not being square and the springs sagging more on one side than the other there's more fractions of inches added into the equation.

Under the assumption that the rear was good and locked in, I was under this same thought for identifying and adjusting the front caster before taking it to the shop. Then I measured the rear which is prompted starting this thread.

 

[Dana67Dart]

This is 67 Dart from the FSM

68 Valiant from the FSM

Couldn't find a Barracuda diagram