3-bolt LSD vs 4-bolt LSD

[Capt Obvious]

My '91 has the 3-bolt LSD rear differential. I also have a complete 4-bolt swap. Aside from the axles, are there any notable differences between the differentials themselves? I haven't tried yet, but it appears that I can just swap the cups and use the 4-bolt axles with my 3-bolt rear end as the splines appear to be the same. I've searched quite a bit and couldn't find anything discussing this option, just a bunch of thread about the non-LSD 3-bolt vs the 4-bolt.

If possible, I like this option because my car has really low mileage and is super clean and I'd prefer to keep the original diff with the car instead of installing a 4-bolt with unknown mileage.

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[dwb]

Your 91 3-bolt rear diff is PN MB597594 or MB597591 (one is LSD but I can't tell via ASA)
A 92-94 4-bolt rear diff is PN MB861261 (all 4-bolts are LSD).
You must use 4-bolt axles with a 4-bolt rear and 3-bolt axles with 3-bolt rear. 4-bolt axles are larger diameter and the splines do not mate up to a 3-bolt rear end.
This may also help: http://vfaq.com/mods/4bolt.html

 

[Capt Obvious]

Does anyone know the actual differences between the two styles (aside from the axles)? What is different internally between the 3-bolt and 4-bolt LSD?

 

[DSMPT]

as the splines appear to be the same.

You should measure them again at several spots, not only at one spot. They look the same but they are not the same. Some spot on the 4 bolt spline part diameter is a few MM larger than the 3 bolt. Also the 4 bolt shaft diameter is larger, too. That few MM larger diameter is the reason why people swap from the 3 bolt. Because that part is the location that you usually break first.