Wow, I attempted to do research on the different gm diffs out there and there are a ton of variables. I am hoping that someone can help me shed some light on compatibility. My 71 cutlass has the stock open 8.5" 10 bolt rear end and I would like to put a posi unit in there. I have a 30 spline 10 bolt posi unit and an opportunity to obtain 30 spline axle shafts from a 70 chevelle 12 bolt. I was under the impression that 30 spline axles will work in the 10 bolt diff but I don't know enough of all the differences to know if the chevelle axle shafts are a direct swap. I realize I will need to replace the seals and bearings. Thoughts?

 

This is doable,but not worth the effert.The biggest obstacle is the housing end/bearing style(pressed in axle tube vs. pressed onto the axle).Another obstacle can be at the button end/"C" clip area of the axle/diff relationship,including button heigth,diff cross shaft diameter and depth of "C" clip recess in side gear.If the posi you obtained is fact.truck unit it is most likely a Gov-lok ,also know as Whiz-bang pos.Why not just put in a factory correct unit and save a ton of $ and aggravation

 

Would it be possible to swap the carrier side gears, as in put your 28 spline side gears in place of the 30 spline posi side gears? Then use your existing axle shafts.

 

That is good thinking on your part.I.If he has the gov-lok unit,the GM special tool to disassemble those units is around $300,so you would have to be able to do a bunch of these units to make it worthwhile.Also that tool is ring gear diameter specific,meaning different tool for 7.5 8.5 9.5 10.5 units.To be honest with you I hate these units as much as I hate auburn units .I don't use either of these units,as I mostly do Muscle car & performance rears.Tom from Tom's differentials used to sell me 8.5 Eaton posi's and a set of 12 bolt Eaton side gears.I would disassemble the posi unit and install the 12 bolt side gears.While It's apart,I would blueprint the clutch stacks with a differential of no more than .001".Now you have a 10 bolt rear that is as strong as a 12 bolt rear.Unfortunately Tom is now gone.He was an infinite wealth of info on rearends.His slogan is (Home of made in the USA).Quality man,Quality parts.Most rearend parts are now South Korea & China made and I won't use it.Just so everyone knows,Tom's Differentials is still in business and online.It is being run by his Wife and family and are awesome people and great to do business with