So, I have a 72 cutlass with the 8.5" 10 bolt (scalloped cover) open rear end. I'm looking to make this a 3.42 rear end with posi on the cheap. Is there a way to pull parts from a more modern junk yard car that would be able to swap in? If not, is there a rear end that will work with few modifications?

 

Have a look at Richmond GEar's web site for the 8.5" 10 bolt applications. There are a lot of newer vehicles you can pull the posi from:
http://richmondgear.com/pdf/Richmond...n1.pdf#page=31

 

Call Jim Mitschke (monzaz on this site) he specializes in diffs and rears. Owns J.D.Race and Restorations. Google that.

 

Thanks for the info!

 

3.42 gears easily found in late 80-s thru 90's 1500 series pick ups and suburbans etc.. even later into the 2000's became an 8.6" gear but still interchangeable. For a 28 spline Posi to utilize your stock 72 axles your limited to 2nd gen TA and Z28 71-81 or a 71-77 Cutlass , Buick , A body.. etc. Also keep an eye on some late 70's leaf spring Cadillacs that use an 8.5 with disc brakes may have a 28 spline posi avoid the 2.41-2.56 gear units as you would need a ring gear spacer. Stick with a Posi off a 2.73-3.73 gear. If you upgrade to a 30 spline alloy aftermarket axle but retaining the 72 olds style bearing and seals you can get a posi out of a mid 90's Roadmaster wagon or Caprice Classic , Impala SS style car. A lot of the taxi-cop car optioned sedans with a rear swap bar often have the limited slip lube tag and also look for G80 on the RPO sticker. Mind u the switchover year is 89/90 for 28 spline going to 30 spline across the board. Do not get a Gov-Lock unit out of a truck as they are junk. The 30 spline units in the Roadmaster,Impala,Caprice cars are the Auburn style with cone clutches , just do a break away torque test in the yard and see if it is still tight enough to be usable. The mid 70 's F body 28 spline units are disc clutch eaton style. My Dad has an old beater 92-93 ish Buick Roadmaster wagon with a common towing package and the old 87-95 TBI 350 not an LT1 and it has a quite desireable 3.23 30 spline 8.5 Posi rear and at 230k miles it actually seems quite tight when I did some recent testing with car after doing some shock and sway bar upgrades on it.

 

Here is the problem with cheap. ALL the posi units yu are seeking are well over 30 years old and these units are likely used up and will need rebuilding which the 10 bolt 8.5 S spring units are not very good as parts are NOT available anymore... I have a few clutch packs left and they sell for 225.00 a set. Spider gears are totally gone in the dealers etc. 5-7 years ago I found 2 spider gear sets for the 8.5 units and the dealer want 359.00 for a set.

SAVE your money and BUY NEW parts. Gears are readily available in all the pick-up trucks 1/2 ton 3.42 3.73 are plentiful for sure. They will work good to keep that part of the money down. Posi unit BUY new...

My opinion but you can keep trying ...Maybe someone on this sight will sell a good used one.... But remember you will not know how much life is left till yo get it in and drive it. Jim

J D

Pretty sure I have a good set of used 3.42 gears right now.

 

Not knocking new but if someone can can grab his own parts and assemble his own stuff and have something going that still works for under $200 total its alot cheaper and maybe not everyone can afford all new everytime and go for a $1000 build. Heard this argument a hundred times but yet I will still install a 40 plus year old posi unit and it will perform well over and over again. Yes some stuff is wore out but you inspect it and test it if possible. If its tight and still grabs well it can work fine. If its just a street car on street tires occasionally doing a burnout it is not much strain on the P unit. If your going to drag race frequently at the track on slicks doing many slick burnouts it can be too much for an old unit after a while but then a spool or a new HD unit wouldnt have been the smarter choice then. But for Joe blow enjoys his car and lights em up here and there its fine.