Okay so i bought an 8 3/4 rear. its a non suregrip (posi). what is the best and cheapest way to make it a posi. ive heard you can just upgrade the spider gears but im not sure.

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1969 dart 2 door 2bbl 225

Cheapest is often to find a Sure-Grip 3rd member in the ratio you want. Best? That's dependent upon use. Locking differentials tend to be a bit harsh. Clutch type without preload will still spin one tire on snow or ice. Preloaded clutch type (Auburn) aren't rebuildable. If not drag racing on slicks or prioritizing best Winter traction I like the bevel gear type as they are smooth and put the power down well: https://www.summitracing.com/parts/dtl-913a572

The entire differential case generally gets replaced unless you use a Lock-Right. https://www.summitracing.com/parts/pwt- ... plications

See many options here: https://www.ringpinion.com/DiffWizard.aspx

Select Chrysler and then the 8.75" with your case casting number '41, '42 or '89 and then Traction devices to see what is available.

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Joshua

im guessing these options can hold around 300hp?

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1969 dart 2 door 2bbl 225

You're highly unlikely to break any of them unless you're drag racing with a stick, running slicks and dropping the clutch at high RPM.

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Joshua