Rear end question

Mirada512

Active Member
Joined
Jan 22, 2022
Messages
40
Reaction score
4
Location
Marshfield, Wisconsin
So I have an 8.25 suregrip and I would prefer the 8.75 rear end. The measurements between the two for spring perches are as follows j body 44.46" and the 62-70 b body is 44". So my question is is the .46 going to make a difference when bolting up or will they have to be cut off and rewelded to 44.46. I'm new at this so bare with me, thanks for any help!

F37BC13E-E81F-4472-90CC-2B4F4A121FE6.jpeg
 

Camtron

Well-Known Member
Joined
Apr 25, 2019
Messages
1,814
Reaction score
1,376
Location
US
Don’t HAVE to be relocated, but will turn the springs inboard. With what you’re planning, it’s worth cutting and welding them in the right location for spring rate/geometry sake.
 

Mirada512

Active Member
Joined
Jan 22, 2022
Messages
40
Reaction score
4
Location
Marshfield, Wisconsin
Don’t HAVE to be relocated, but will turn the springs inboard. With what you’re planning, it’s worth cutting and welding them in the right location for spring rate/geometry sake.
Thank you, I don't want to have any issues with the rear. I'll have some one reweld the perches. I just had to ask to make sure.
 

Duke5A

Well-Known Member
Joined
Apr 6, 2018
Messages
1,638
Reaction score
855
Location
Michigan
Don’t HAVE to be relocated, but will turn the springs inboard. With what you’re planning, it’s worth cutting and welding them in the right location for spring rate/geometry sake.

I second this. I ran an 8.75" out of a '66 Coronet and just redrilled the purches. You could actually see the inward bow in the springs from behind the car. That 1/2" makes a huge difference. Eventually I had the perches put in the correct placement.

Overall width of the axle is just fine. 275 and even 285 tires will fit.
 

M_Body_Coupe

Well-Known Member
Joined
Aug 19, 2013
Messages
621
Reaction score
293
Location
Windsor, ON, Canada
I agree with the guys re: doing it right...however...I have not done this re-location, and I honestly can't say that has ever caused me a problem...maybe it's just because I don't know what to look for???

I will say the following: my setup has the correct pinion angle and is using shims to get it to where I wanted it to be...which perhaps meant that this 0.23" on each side really got eaten up in whatever +/- normally one sees.
 

SRTMirada

Well-Known Member
Joined
Dec 29, 2015
Messages
101
Reaction score
65
Location
Niagara
I opened up the perch holes and offset them at the same time. Makes for perfect drop in swap. The housing you want is from a 68 - 70 B-body. The chart you posted is inaccurate. There are better ones out there.
20201127_144228.jpg
 

Mirada512

Active Member
Joined
Jan 22, 2022
Messages
40
Reaction score
4
Location
Marshfield, Wisconsin
I opened up the perch holes and offset them at the same time. Makes for perfect drop in swap. The housing you want is from a 68 - 70 B-body. The chart you posted is inaccurate. There are better ones out there. View attachment 46206
Thank you so much! I was wondering about the measurements. Those are hard to find in the wild. What chart did you find?
 

BudW

Moderator
Staff member
Joined
Mar 4, 2012
Messages
5,121
Reaction score
1,485
Location
Oklahoma City
The measurement difference (between '65-70 B vs. FMJ) is actually 0.46 inch total or 0.23 inch per side. The spring and bushings can compensate for that 0.23 inch per side - but it would be better to either modify the perch hole OR modify the front and/or rear spring hangers and/or front spring bushing inserted offset.
There are several different solutions or combination thereof.

That said, FMJ rear leaf springs are not mounted parallel to car center-line.
This is from my '77 Factory Service Manual - but this will be the same for all FMJ's:
77 FSM pg 13-2b.png

The frame rails angle one way. The leaf springs angle the other way:
77 FSM pg 13-2b cl.png

The blue represents the leaf spring position. The red line is parallel to center-line of vehicle.
If you don't believe me or the above pictures, go measure the distance of rear tires to leaf spring - on front edge vs rear edge of tire and you can confirm. If you have car in the air, measure the distance of front of leaf spring (to leaf spring) to rear of leaf springs.
I had the measurements down somewhere - but if memory serves correctly, the rear of leaf springs are about 4 inch closer together than the front (but please don't quote me on that).

Personally I don't think 0.23 inch per side is going to hurt a thing to leave as is (between FMJ to '66-70 B-body rear differentials).

Also, this list is much more correct (but is not complete):
Screenshot 2022-02-01 215813.png

Note: the '62-64 B-bodies use bolt on hubs (and tapered axles) and therefore the housing ends are different from the '65 and newer 8-3/4" differential housings. Those bolt on hubs are a ROYAL PAIN to work with and anyone is best to leave one alone.
1963 Rear Axle a.jpg

63 FSM pg 3-3.png


The '65-67 B 8-3/4" is a different overall width (54.250" F-F) from the '68-69 B 8-3/4" (54.936" F-F) which is slightly different width from the '70 B 8-3/4" (54.906" F-F) - but all three have the same spring perch position and either version will fit into an FMJ without much, if any, modification.

I included my differential spreadsheet as a PDF (below) - which is still a work in progress.
BudW
 

Attachments

  • Chry Master Diff.pdf
    503.3 KB · Views: 190

droptop

Well-Known Member
Joined
Jan 12, 2012
Messages
289
Reaction score
148
I opened up the perch holes and offset them at the same time. Makes for perfect drop in swap. The housing you want is from a 68 - 70 B-body. The chart you posted is inaccurate. There are better ones out there. View attachment 46206


How much did you enlarge this hole? I am contemplating this same swap but want to use the f spring instead of the b body mount.
 

SRTMirada

Well-Known Member
Joined
Dec 29, 2015
Messages
101
Reaction score
65
Location
Niagara
I made a drill fixture to keep the drill aligned while I opened up the hole from 1/2" to 5/8" which is the size of the pin on an ISO- leaf. A die grinder could accomplish the same thing. That .46" doesn't sound like a lot until you're yanking on the leaf spring trying to get the perch to go on. Just my opinion, but I think a 68-70 b-body axle goes best in a j-body and a 65-67 b-body axle goes best in a f-body.

20220202_113031.jpg


20220202_113012.jpg
 

Duke5A

Well-Known Member
Joined
Apr 6, 2018
Messages
1,638
Reaction score
855
Location
Michigan
Are you keeping the ISO mount, or rubber sandwich? Use a 1 9/16' holesaw.
 

droptop

Well-Known Member
Joined
Jan 12, 2012
Messages
289
Reaction score
148
Are you keeping the ISO mount, or rubber sandwich? Use a 1 9/16' holesaw.

?????
Yes, keeping ISO mounts. Don’t you mean enlarge the hole diameter to 1 and 1/8”?
 

Duke5A

Well-Known Member
Joined
Apr 6, 2018
Messages
1,638
Reaction score
855
Location
Michigan
?????
Yes, keeping ISO mounts. Don’t you mean enlarge the hole diameter to 1 and 1/8”?
Nope, 1 9/16". You can measure though when you get it apart first before modifying anything.

The rubber isolator goes between the location pin on the leaf and the spring pad. That's why it needs to be so large. Also why I don't think you offset it and have it work.

I'm out of state right now. When I get back this weekend I'll find my photos. Did this exact swap 17 years ago.
 

SRTMirada

Well-Known Member
Joined
Dec 29, 2015
Messages
101
Reaction score
65
Location
Niagara
The discussion of axle swaps should certainly include the topic of whether or not to keep the ISO clamp. I don't think that a big block swap, especially a 500" big block would be happy with ISO clamps. If you only drive on the street and never put on sticky tires then I guess that you can get away with it. With all that torque you risk over angling your u-joint and dropping your drive shaft on the road. The stock leaf springs aren't going to be up to the task either. I'd recommend cop, heavy duty b-body or super stock springs. If you plan on going to the track then a Dana 60 is probably a better choice than a 8 3/4.
 

BudW

Moderator
Staff member
Joined
Mar 4, 2012
Messages
5,121
Reaction score
1,485
Location
Oklahoma City
The discussion of axle swaps should certainly include the topic of whether or not to keep the ISO clamp. I don't think that a big block swap, especially a 500" big block would be happy with ISO clamps.
I agree 100% - if using the rubber biscuits. I can't comment on urethane biscuits. Either way, the lower "C" or "U" bracket is not that strong and is prong for breakage. I think and friends that had made the ISO clamp changeover makes the car feel so much better when driving and braking - especially panic stops.

If anyone wants a better ride, then consider tossing the ISO clamp. Tossing the rubber and going urethane might work - but not tried them personally.

Not talked about much, but performing an ISO elimination also raises the rear of vehicle 1/2 inch (13 mm) without doing anything else (moves the spring perch about 1/2 inch closer to the leaf spring - which in turns raises the vehicle.

With all that torque you risk over angling your u-joint and dropping your drive shaft on the road.
Again, I agree. Keeping the pinion angle at check is another plus with the ISO elimination. A big plus on U-joint failure rate.

The stock leaf springs aren't going to be up to the task either. I'd recommend cop, heavy duty b-body or super stock springs.
I'm with you there. For general usage or mild performance, 5-leaf wagon or police springs are OK. For more power than that (big block or similar) you need better springs. I'm planning on making a custom set of springs for my cars. Using SS (or 440HP/Hemi) lower leaves and a stock style top leaf will help tons. The front half of FMJ leaf springs is longer than they should be - which is part of the problem. Shorting the front half of that top leaf and installing a longer front hanger should help a lot, as well.

If you plan on going to the track then a Dana 60 is probably a better choice than a 8 3/4.
On the money, yet again. I like the 8-3/4 inch differential for a large number of reasons - but even Chrysler has a TSB saying the 8-3/4 inch differential is not strong enough for the 383/440 HP cars and was paying to upgrade to the Dana 60 (9-3/4 inch) if they failed under factory warranty. I can produce that TSB if anyone wants to see it (its on my work computer).
My only problem with the Dana 60 is its weight and lack of available gear ratios for it (hard to find a gear ratio higher (lower number) than 3.54). Other than that, it is pretty much indestructible.
I'm going to use the 8-3/4 for my cars for I won't be going to the track and will use street tires. If I was going to use slicks or go racing, then the Dana 60 would be a must for me.

If the Chrysler 9-1/4 inch differential wasn't so much of a pain to adapt to an FMJ, then it would be an attractive option - for I think it will hold up to a ton of torture - but it is just too much work to make fit.

BudW
 

Mirada512

Active Member
Joined
Jan 22, 2022
Messages
40
Reaction score
4
Location
Marshfield, Wisconsin
The discussion of axle swaps should certainly include the topic of whether or not to keep the ISO clamp. I don't think that a big block swap, especially a 500" big block would be happy with ISO clamps. If you only drive on the street and never put on sticky tires then I guess that you can get away with it. With all that torque you risk over angling your u-joint and dropping your drive shaft on the road. The stock leaf springs aren't going to be up to the task either. I'd recommend cop, heavy duty b-body or super stock springs. If you plan on going to the track then a Dana 60 is probably a better choice than a 8 3/4.
I will be going with super stock springs since that is what we have ran them in the past on other cars. I do have a drive shaft loop just to keep damage down just incase I do snap a u joint. It'll be a street car and I won't be putting sticky tires on the car. The car will be a driver and not a racer.
 

BudW

Moderator
Staff member
Joined
Mar 4, 2012
Messages
5,121
Reaction score
1,485
Location
Oklahoma City
I am glad you brought up the rear leaf springs. Here is some data I have on rear Chrysler leaf springs - which are 2-1/2 inch wide. The full size pickups and vans use 3 inch wide leaf springs - so are not mentioned here.
Leaf Spring - by overall length.png

Above is sorted by overall length.
Below is sorted by front segment.
Leaf Spring - by front segment.png

The front spring hanger comes in several different lengths (from body mount to leaf spring hole).
AR Engineering FSH.png

The thing is I don't have those lengths for the different front spring hangers (yet), so not sure what we will need. The above list of hangers (for sale) is from Spring Hangers | AR Engineering.

You want the center bolt hole (differential) preferably in same location. If not, your propeller shaft length will change and not sure how it would look not centered in wheel well.
You also want rear bushing hole in same location - but this can be moved providing the rear spring hanger position is moved (forward or rearward) the same distance the rear bushing hole is located - but this might be a bit more difficult to do. To not do this will mess up the spring geometry.

I happen to have a used pair of '69 GTX 440 HP rear leaf springs in my garage - with the "Plymouth B" front spring hangers. Those spring hangers are about (measured attached to spring - so might be off) is 2-3/4 inch from front edge to center of bolt.

The front spring hangers I have (measured on car - so might be off) are 2-3/4 inch from front edge to center of bolt - so might be the same as above (...maybe).

The problem I see is none of the above leaf springs have the rear segment length, of 34 inch. I was wondering what you are going to do for this has been a stumbling block I have. To me, it appears I need to have a new top spring made to fit my car, but with a shorter front segment (using a different (longer) front spring hanger), then either use the lower leafs from my 53 year old '69 440HP springs, or get a new set of either SS or 440HP/Hemi springs and merge the two.
For those who don't know, the 440HP/Hemi leaf springs (B-body or E-body variants) have 5-leafs on one side and 4 full plus 2 half leafs on the other side. I "think" the SS springs are different.

I'd like to hear your thought - for maybe you have other insights I've missed.
BudW
 

Mirada512

Active Member
Joined
Jan 22, 2022
Messages
40
Reaction score
4
Location
Marshfield, Wisconsin
Wow what an awesome amount of information! This is truly needed info for me or anyone. There is a video on YouTube which if I can find it again I will post the link here. So in this video they put SS leaf springs in an 80 Mirada and the have special front mounts that they bought and it uses the factory mounts for the SS springs. I'll do some hunting and find it!
 

Mirada512

Active Member
Joined
Jan 22, 2022
Messages
40
Reaction score
4
Location
Marshfield, Wisconsin
I am glad you brought up the rear leaf springs. Here is some data I have on rear Chrysler leaf springs - which are 2-1/2 inch wide. The full size pickups and vans use 3 inch wide leaf springs - so are not mentioned here.
View attachment 46238
Above is sorted by overall length.
Below is sorted by front segment.
View attachment 46239
The front spring hanger comes in several different lengths (from body mount to leaf spring hole).
View attachment 46240
The thing is I don't have those lengths for the different front spring hangers (yet), so not sure what we will need. The above list of hangers (for sale) is from Spring Hangers | AR Engineering.

You want the center bolt hole (differential) preferably in same location. If not, your propeller shaft length will change and not sure how it would look not centered in wheel well.
You also want rear bushing hole in same location - but this can be moved providing the rear spring hanger position is moved (forward or rearward) the same distance the rear bushing hole is located - but this might be a bit more difficult to do. To not do this will mess up the spring geometry.

I happen to have a used pair of '69 GTX 440 HP rear leaf springs in my garage - with the "Plymouth B" front spring hangers. Those spring hangers are about (measured attached to spring - so might be off) is 2-3/4 inch from front edge to center of bolt.

The front spring hangers I have (measured on car - so might be off) are 2-3/4 inch from front edge to center of bolt - so might be the same as above (...maybe).

The problem I see is none of the above leaf springs have the rear segment length, of 34 inch. I was wondering what you are going to do for this has been a stumbling block I have. To me, it appears I need to have a new top spring made to fit my car, but with a shorter front segment (using a different (longer) front spring hanger), then either use the lower leafs from my 53 year old '69 440HP springs, or get a new set of either SS or 440HP/Hemi springs and merge the two.
For those who don't know, the 440HP/Hemi leaf springs (B-body or E-body variants) have 5-leafs on one side and 4 full plus 2 half leafs on the other side. I "think" the SS springs are different.

I'd like to hear your thought - for maybe you have other insights I've missed.
BudW

4F670640-BFCF-4296-9F56-052768DFC572.png
 
Back
Top