I have a 12 bolt non-posi out of a 71 chevelle. I would like to make it a posi. what is the best way? I see places like summit sell posi units. If I am not mistaken, wasn't there a horsepower TV episode that talked about adding something to the rear end that stopped short of completely changing it?

I would get an Eaton posi unit and an installation kit, then reuse the G.M. gears if they are the ratio you want and in good condition. If Chuck did a show on this, you can call him on the ACES hot line and ask him.

You might want to be prepared with what series of "single pull" or "open" rear end that you have in your car (a 3 series or a 4 series) so you can find out the choices without changing the whole rear end. Do you know what ratio rear end you have now?

A powertrax/lock right locker is the cheapest.
Jegs sell them for $209 for a 12 bolt
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=2006&prmenbr=361

A good friend is running one of these exact units in his 12 bolt w/3.07 gears..we installed it back in January..works great.

my rear axle is an original RUK for a 71. my book shows 3.31 gears. that power trax sounds like what I remeber seeing on HP TV. does anybody know the difference between the power trax and a posi unit? thanks

Let us know what you end up doing, I've got the same situation here. I was considering just having a local shop swap in new axles, gears, and posi. It was a bit pricey.

Read it all here http://www.powertrax.com/noslip.html

Post this question in the transmission/rear section and post it for BigGearHead. He is a PRO at this stuff.