Author: Matt Webb
Subject: Fiat 124, Converting Rear Ends.
Resources: Mirafiori forum members and first hand experience.
Intro to 124 rear ends:
There are two major styles of 124 rear ends that this write up will cover. These are often referred to as "Early" and "Late". The early torque tube style rear ends (66-68) will not be covered in this write up.
Early are 1978.5 and below, Late are 1978.5 and up. This does not appear to hold true in all cases. The only way to know is to visually ID your rear.
The early rear ends are easily identified as the type that have the removable "pumpkin" or “hog's head", while the late rear ends have a rear inspection cover.
- early, believed to be stronger.
- early, easier to rebuild
- early, easier to swap a new unit in
- early, pinion yoke sits 10mm further toward rear of car than late. Different drive shafts
- panhard rod mount points are different
- panhard rods are different (early straight, late is bent)
- early spring perches sit higher and require shorter springs
- early shock mounts are bolted on, late are welded.
- brake dust shields are different
- caliper carrier bolts are different.
- trailing arm, shock, and spring mounts all in the same location.
- Brakes are the same
- early and late unibodies have the same trailing arm and panhard rod mounting points
Because the pinion sits further forward on the late rears and further back in the early rears, different drive shafts are used with the two styles of rear ends.
The late rear end uses a shorter driveshaft. The early rear end uses a longer driveshaft.
The differences in the driveshaft are simple. The U-Joints, pillow block and bearing, pillow block mount point are the same.
The front sections are dependent on transmission style. They will vary in length between different transmission types. This is especially obvious when comparing automatic and manual transmission driveshafts.
The rear sections depend on rear end style. This is an important detail that should not be overlooked. Late drive shaft rear sections are 10mm SHORTER than early drive shaft rear sections.
Val's summary: use the front section that matches the transmission and the rear section that matches the differential.
There is some confusion on this, but I believe the following is accurate.
The only definitive way to determine what ratio you have is to count the teeth on the ring gear and divide by the number of teeth on the pinion gear.
- 1967-70 4.10:1 (1438cc engine)
- 1971-78.5 4.30:1 (1592cc,1608cc,1756cc engines)
- 1978.5-end of ‘78 4.30:1 (1756cc)
- 1979-on 3.90:1 (1995cc engine)
- 1979-on 3.58:1 (auto transmission cars only)
Note: some late 1978 spiders came with the late style
differential with 4.30:1 gear.
Requirements for the swap:
Early rear install requires:
- longer driveshaft, w/longer rear section
- early rear panhard rod (long and straight)
- early springs, shorter
Late rear install requires:
- shorter driveshaft w/ shorter rear section
- late rear panhard rod (short and bent)
- late springs, taller.
Interchangeable parts include:
- long trailing arms
- short trailing arms
- brake parts
If you're converting to an early rear and have a late rear driveshaft (short), the splines at the front yoke can provoide the extra length (10mm) required, however, this will require that the pillow bearing also moves 10mm towards the back of the car.
This could be achieved either by elongating the mounting holes a LOT, or, from what I have observed but NOT tested:
It appears that the mount bracket for the pillow block can be rotated 180 degrees, which would maintain mounting hole location but move the pillow block approximately 10mm towards the rear of the car. Slight hole modification/elongation may be required, but probabally not much.
The other option in this scenario is to machine a 10mm thick spacer plate to go between the driveshaft and the pinion yoke. DO NOT force them to fit. You must properly address the spacing problems.
If you are converting to a late rear and have an early rear driveshaft (long), the splines are not going to be able to help you cheat. Get the correct short driveshaft. AGAIN, DO NOT force them to fit. You must run the proper driveshaft.
If you need to maintain front section length on your driveshaft, you can mix and match front and rear shafts however you want. Balance issues from doing this seem to be a myth, as the shafts are balanced independently and many have reported doing this without problems. If you do have balance problems, Allen Lofland had good luck and fair prices @
As mentioned, early springs are shorter and late springs are longer. The difference does not appear to be on the body of the car, rather in the heights of the spring perches on the rear end.
To maintain correct vehicle height it is recommended that you run the correct springs for your rear. It is possible that late springs are taller only for US governmental ride/bumper height regulations. I do not know.
It is worth mentioning that aftermarket performance springs seem to be available in one size fits all.
A word about rear Sway Bars:
If you have an early rear end and swaybar setup and you wish to convert to a later rear end, be aware that the earlier axle has thicker tubes than the later. This means that some of the swaybar mounting hardware will be too big when adding it to a
later rear end. Aluminum spacer blocks can be used to take up the slack.
I bought one of the popular Addco rear swaybar that the vendors sell for the late rear ends. It fit my late rear end fine, and it bolted right up to my early rear end as well.
Leaks, Seals, and Preload:
The pinion yokes get leaky after time. There's a seal in there, and the only way to get to it is to remove the yoke. If you go down this path, be advised that you are messing with pinion pre-load, a very important and sensitive adjustment. It's been rumored, but never really confirmed, that part of what makes the late rear ends weak is that this adjustment drifts over time, causing poor gear lash and accelerated pinion bearing wear. It's a good idea to at least check for play in this area.
Ideally when replacing the pinion seal a new crush sleeve would be installed and the procedure in the manual would be followed. This is not an option for us, as these sleeves are NLA. The best method is to mark the nut relative to the shaft and count the number of turns as you remove it. Replace the seal, and put it back together the way you found it. Check pinion turning torque. It should feel snug with no play, but it should not feel tight. It should turn easily.
The seal is not the only source of leaks in the pinion area. Oil will also seep past the threads on the pinion shaft/nut. Before bolting the driveshaft to the rear end, apply some RTV to the joint. This will help prevent those annoying drips.
Swapping Internal Parts:
As far as I know, parts from early rear ends with the pumpkin units swap right over from one to the other.
Thanks Csaba for the following info on the late rears:
On the later style axle it may be of note that the internals changed twice, so there are three types of ring and pinions, even though the axles look the same externally. I think the divisions are (per Fiat Mechanical Parts Book, ie. the black book):
1/78 to 8/79
9/79 to chassis#1966226 [around 1982]
from chassis#1966227 on"
Important! I'm 99% sure that 'chassis #' is a misprint, and what they mean is
'spares number' located on the metal tag on the firewall. At least that's what
I deduced looking at my list of chassis and spares numbers.
This means that trying to find the correct ring and pinion for a 'later' axle
when the date of manufacture or spares number for the car is unknown may well
turn into a nightmare.
The earlier types of the 'late' rearend came with axleshafts with ~24mm diameter
splined ends. The later cars (after chassis or spares #1966226) ones had ~26mm
About Torque Tube rear ends:
Thanks Pete Angel for this info on the early torque tube rear ends:
“not much to mention about the 66 up to early 68 torque tube rear except that
they used no upper trailing arms. the panhard rod, lower trailing arms and
axles are interchangeable with the early type (with no removable rear cover).
they came with 14mm dia rear sway bars. the housing, ring & pinion, and drive
shaft are NOT interchangeable with anything but the torque tube type drive. the
tt was found in the early 124 coupes, 124 spiders, and 124 sedans.”
Limited Slip Options:
Csaba @ Vick's Autosports has/had a line on some $1400 units from overseas that he has had good luck with.
Phantom Grip sells a unit that fits our diffs. Not really a limited slip, but an OK cheat. Affordable too. It reminds me of the VW CV cup stiffer spring washer trick.
I went down this path with Quaife a while back, and started to get a deal worked out. Then it hit me. Our ring and pinion gears are weak. If you need a limited slip, IMO, you need a stronger diff, or at least stronger gears. Why spend big $ on a rear end that's still going to use stock (weak) gears??
There was someone on the now gone FiatSpider.com forum that announced availability of a clutch type limited slip. It was a big name LSD manufacturer that he had ties with. I can't remember anything but that.
Bits Of Italy sells a heavy duty diff replacement, which I believe has a limited slip as well.