7.25 to 8.25

prowler · Mar 3, 2025

77 LeBaron 2 door coupe,....Going to swapping out rear, 7.25 to 8.25. Will be complete rebuild, new gears and carrier. Question: Seen two different style replacement carriers. One from Spicer and the other a Yukon unit. Any thing to a aware of in differences between the two??

Second question: Since the drive shaft will have to be modified or replaced, any difference in length between 4 door and 2 door models??

What about rear u-joint?? Same size as 7.25 or larger for 8.25??

Thanks.

Sub03 · Mar 5, 2025

BudW gathered a lot of great info in this thread:

FMJ Differentials

And yes, same U-joint.

AMC Diplomat · Mar 5, 2025

Different length drive shafts between 7 1/4 and 8 1/4 in both 2 and 4 doors

Are you using 27 spline axles? You have to make sure the carrier is for the older 27 splines

Mikes5thAve · Mar 5, 2025

Driveshaft is shorter for 8.25.
70s LeBaron coupe is the same wheelbase as the 4 door.
The axle tubes are smaller on those 7 1/4 so you'll need different shock plate and axle spring brackets also.
Different gearing means different speedometer gear. 77 should still use the older style speedo gear which is good because they're more readily available.

prowler · Mar 8, 2025

Thanks for the reply's. Car has 2.76 ratio and found a new 8.25 set, probably the only new set left as I've been told gear manufactures don't make any more ratios in the 2's. Have new 27 spline carrier coming as well. Spent a great deal of time going over all the info on this site regarding diff swaps.

AHBguru · Mar 14, 2025

Somewhere in mid-1980, maybe beginning in 1981, they added some width to the rear ends, but a newer assembly will work just fine in an older car.

prowler · Mar 17, 2025

Got the 8.25 torn all apart. Looks to be never have been apart since the 70's when new. Didn't come apart in what I'd call a friendly manor. 2.45 pinion is huge. Noticed one thing I'd never seen before regarding the pinion's pressed on bearing. I've never seen a pinion bearing(rear larger) that wasn't bottomed out on the pinion face. This one has an almost 1/8" gap between the pinion collar and the face of the pinion itself. The adjustment spacer is between the pinion collar and housing. Not sure since I haven't removed the pinion bearing if it's pressed on to a machined lip that will only allow it to be pressed on just so far. That would create that gap.

One issue I haven't seen discussed is the brake back plates. Any one know if they're the same for all diff's, 7.25 included? In other words will 7.25 back plates bolt onto the 8.25?? 8.25 have totally different axel seals and bearings. No where's near the 7.25's in terms of any compatibility.

AMC Diplomat · Mar 17, 2025

prowler said:
One issue I haven't seen discussed is the brake back plates. Any one know if they're the same for all diff's, 7.25 included? In other words will 7.25 back plates bolt onto the 8.25?? 8.25 have totally different axel seals and bearings. No where's near the 7.25's in terms of any compatibility.

I think the late 80's 7 1/4 backing plates off M bodies will fit an 8 1/4, but not the late 70s 7 1/4 and absolutely not for the small bolt pattern 7 1/4.

You can use 8 3/4 backing plates on an 8 1/4, but you have to use the right drums and shoes to go with whatever backing plate you go with.

This is why some guys just buy them off ebay or the other forums as whole assemblies rebuilt by another enthusiast. It's is cheaper to do it yourself, but an absolute pain to hunt it all down.

Mikes5thAve · Mar 17, 2025

Thats a good question on backing plates. The 80s 7 1/4 with the bigger axle tubes swap over, dint know if the smaller tube 7 1/4 had smaller flanges for the backing plate or not.

AMC Diplomat · Mar 22, 2025

Speaking of, there are sets for sale again: FOR SALE - brake kits

SRTMirada · Mar 24, 2025

The 7 1/4 does not have smaller axle flanges. I swapped the brakes from my 7 1/4 to an 8 3/4.

AMC Diplomat · Mar 25, 2025

SRTMirada said:
The 7 1/4 does not have smaller axle flanges. I swapped the brakes from my 7 1/4 to an 8 3/4.

Which year 7 1/4?

DCAspen · Mar 25, 2025

1980-89

Mikes5thAve · Mar 25, 2025

At least some 1980 had the older style 7 1/4. I thought it was 81 or 82 that switched over.

prowler · Mar 25, 2025

So far I'm finding in nothing common between 7.25 and 8.25. Axles, backing plates, bearings, seals,
attaching points. You name it.

Mikes5thAve · Mar 25, 2025

prowler said:
So far I'm finding in nothing common between 7.25 and 8.25. Axles, backing plates, bearings, seals,
attaching points. You name it.

With those 70s ones yes. The 80s ones have more in common.

DCAspen · Mar 25, 2025

Mike,1980 the axles changed to all of them having 3 inch tubes. The length also changed from 58.5 inches to 59.5 inches

Mikes5thAve · Mar 25, 2025

DCAspen said:
Mike,1980 the axles changed to all of them having 3 inch tubes. The length also changed from 58.5 inches to 59.5 inches

Yes, like I said previously 1980 was the transition year and could have either tube size. There was even a variant briefly in 1980 that had the new style cover but smaller tubes.