70 rearend swap to an 83

[tima01864]

Becoming more complicated for me. I have a 1970 rear end 40:10 ratio on my 1983. I want to change it back to having a newer rear end, differential geared 3.98. There are a few reasons, My speedometer is way out of calibration, the axles do not center the rims (maybe the steel rims were centered on the alignment studs) and Pan hard rod rubs on the fill plug. Guess that is all.
Possible I could swap out the axles(?), Figure out why the pan hard rod is rubbing, swap correct speedometer box at the transmission(?) and buy a new 3:98 differential from a vendor. Or just swap out the rear end as a whole, would be cheaper to swap it out. Problem solved(?), there are members who think the 40 is a better choice for the car, more robust and better in the lower gears. It would be easiest to not do anything and just live with it as is, but I want to do what is best.
My question is what are your thoughts, Help.

[azruss]

I did such a swap last summer. I have an 80 FI with 70 rear end. I took out the 4.30 diffy and replaced it with a 3.90 LSD.
Probably the simplest swap would be the entire axle. Changing out the diffy requires some specialty tools or some tricky work-arounds. I posted pics and explanations of what i did. If you have a panhard rod rubbing it could be to having the wrong rod or the rod has been bent. If you are going to switch to a newer style axle, you will need the newer style panhard rod as well. The sheetmetal backing for the brake rotors are different as well. Everything else should switch out ok.

[azruss]

Here is some more on the results. I liked the quickness off the line from the 4.30. Made the car feel more sporty, like the early models. It gave me awkward gear ranges, specifically at highway speeds. The 3.90 makes the car feel more like a cruiser and less hyper and feeling more relaxed at highway speeds. the 2L FI handles the highway just fine with the lower RPMs. My speedo went from reading 5% high to 5% low. I keep my car in the mid rpms and seldom get above 5k.

[DieselSpider]

A set of these hubcentric spacers from AutoRicambi could get er done too:
https://autoricambi.us/collections/perf ... r-set-16mm



Then you have the best of both worlds where you can change third members and gear ratios without the need for special tools or skills.

This is how the OEM spacers should have been made from day one.

[tima01864]

I have seen these at Midwest as well, I think that the standard spacer is 8 mm. I dont know how 16 mm would rest on the car. 144 plus per axle too. What rearend are you running on Diesel? I agree on that, maybe the steel rims located on the alignment pins? When Fiat started to offer Alloys they had new problems? AR has a note "For racing use only"

[DieselSpider]

I am running the old style with the 4.30:1 ratio with the OEM steel rims and am tempted to get a 3.90:1 third member to get the highway RPM down. Even though the overhead cam diesel redlines at 5,400 rpm that's still lower than the 1.8 L gasoline engine original to the car. The 4FB1 1.8L overhead cam diesel usually is mated to a 3.30 to 1 rear axle with a more highly overdriven 5th gear in the transmission.

With the 4.30 the torquey Turbo Diesel it pulls more like a tractor than a car and at times you are tempted to skip first gear altogether.

People have commented that when the believe they hear what sounds like a garbage truck pull into their driveway they know I am there.

There are many companies that make spacers of all different thicknesses that are hubcentric so if the Recambi are too thick and the aluminum not suitable for street use there are other sources for hubcentric wheel spacers.

US Wheel Adapters start at $59 for 4 bolt units but might have a 6mm minimum thickness for duplicating the lip:
https://www.uswheeladapters.com/shop/4-lug-slip-on/

At times I am even tempted to have a center ring made up to be pressed into the existing spacers. There is also the option to get 0 offset rims for use in the back to eliminate the spacers and run directly on the lipped hubs built into the old style axle.

[Nut124]

... I took out the 4.30 diffy and replaced it with a 3.90 LSD.

What kind of LSD?

[tima01864]

A machine shop could make 8 mm 10 mm spacers with the centering boss, out of SS. Proto trak the bolt pattern. Worth pricing it out? Reading Azruss posts on his swap, From what I understand Azruss is satisfied with the 3:90 from the 4:30? AR sells an LSD for pre 78, Which would work on my car as it is. Leaning more to swapping out the whole unit. Timothy Leary is dead.

[85redpini]

Becoming more complicated for me. I have a 1970 rear end 40:10 ratio on my 1983. I want to change it back to having a newer rear end, differential geared 3.98. There are a few reasons, My speedometer is way out of calibration, the axles do not center the rims (maybe the steel rims were centered on the alignment studs) and Pan hard rod rubs on the fill plug. Guess that is all.
Possible I could swap out the axles(?), Figure out why the pan hard rod is rubbing, swap correct speedometer box at the transmission(?) and buy a new 3:98 differential from a vendor. Or just swap out the rear end as a whole, would be cheaper to swap it out. Problem solved(?), there are members who think the 40 is a better choice for the car, more robust and better in the lower gears. It would be easiest to not do anything and just live with it as is, but I want to do what is best.
My question is what are your thoughts, Help.

sounds like a complete used rear end might be the answer.

you can try Matthews parts, they have taken apart many spiders recently

Tim @ timmatthews1@gmail.com

[tima01864]

That is the plan, We have Fun Imports nearest to me. I bought and tired 15" rims, EXIP with General Altimax tires. The weather has not been good enough for a drive, A long drive, maybe in a few weeks. Thanks for the response!