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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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There were many changes made to the diff/transmission housing and internals from the 91-93 to 94 and 95 in order to strengthen/stiffen these units, according to the parts disc. Evidently Alfa also recognized the inherent weaknesses of the earlier units.

Obviously you would benefit from installing as late a version as possible. I personally would have liked to have the Q transmission JJ had for sale for a while to eventually install in my 91S when it finally gets a new clutch.
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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Be sure to carefully check all cases for cracks, using some sort of dye check if possible. Any resultant cracking could further weaken the housings. Alfa did make many changes in these cases, as well as the internals, in order to increase the stiffness/strength.
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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You have to trace your way through the parts eper disc for 91-95, checking effectivities and the applicable models. In the parts descriptions they note why the changes were made, such as increased stiffening, etc.

I did this once but I don't know where I left my notes. If I find them, I'll report.
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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I would think that internal circlips (if there is room) would be a satisfactory backup failsafe solution to pin migration, in addition to an interference press fit.

I would imagine a good machine shop could cut the required grooves in the housing.
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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According to the parts eper cd, for the 94's and continuing on for the 95's there were many changes to the gearbox and diff housing as well as the internal parts in order to stiffen/strengthen the drivetrain. I was able to follow the changes through the eper effectivities, but have lost my notes. The effectivities usually said only that "part was modified for increased stiffness".

They evidently knew there were potential problems.

I suspect the casings can be changed, but don't know for sure. You would have to be sure the "ring and pinion" gears match as well.
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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Personally, I wouldn't weld anything onto a heat treated high alloy shaft like that. It takes pretty high loads, and I wouldn't want to compromise the strength or fatigue life. and I sure wouldn't drill or tap any more holes into the diff cage. That would just shorten the fatigue life of that highly loaded piece.

I think the problem is how to better ensure the roll pin stays in the cage, although the Chrysler style retaining brackets show promise. I think the shaft working back and forth as the diff works can slowly work the pin out of it's hole in the cage, thus releasing the shaft.
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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Be sure to check other carriers such as DHL, Greyhound, etc before shipping on "Oops". I've never had a lot of luck with them myself. Once took 4 days to get an overnight high priority shipment. No apologies, no nothing
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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When the time eventually comes to finally rebuild the 91S engine, I will definitely have the diff checked for any pin migration, etc.
 

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2015 Chevy (Holden) SS, 1989 Milano (Shankle Sport), 1991 164S
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"I think it’s fair to say these diffs don’t hold up well to abuse"

I would surmise that is the reason Alfa continually stiffened the casing for the diff, if nothing else, according to the explanations stated in the eper. As the power increased in the various models, from the B and L, to the S, then the LS, and then the Q, the parts listed in the eper had mods referenced, stated as stiffening the casings, etc.

I think Jason (Alfissimo) installed a Quaife in a 164. You might ask him about his experience with that.

I have 198k miles on the trans/diff of my 91S, and I always hope that if it hasn't gone by this time, it isn't going to. Of course, I may drive fast sometimes, but I don't do burnouts in 1st gear or things like that. No track time. No fierce spinning of a wheel. The car is simply not a dragster. Hondas can pull away with little trouble, lol.
 
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