kirks74
Member
I have a open with 2.73s. can i install a posi setup with this? thanx,kirk
change to posi
- Thread starter runnin350
- Start date
red78z
Veteran Member
how do i know what rear i have in the car?big gear head said:I'm assuming that you have a 8.5 10 bolt rear end. If you have something else then some of this will not be correct. Remove the cross shaft and c clips and remove the axles. Before you remove anything else from inside the housing you must use a dial indicator to check the backlash on the ring gear. If you don't know what the backlash is then you will not know what to set it to when you install the posi. To check the backlash you must wrap something around the yoke and housing to hold the pinion perfectly still. If it moves ANY you will get an inaccurate reading. Set the indicator up so that it measures rotation of the ring gear. What you will be measuring is the space between the ring gear teeth and the pinion teeth. This is the backlash. This should be around .010 to .012 inch if the gears are pretty good. Now that you know what the backlash is you can mark the bearing caps and remove them. You must mark them so that you know how thay came off and you can put them back exactly the way they were. Now pull the differential out of the housing. There is no reason to mark the bearing races and shims because you shouldn't be using them again anyway. Remove the ring gear from the differential and install it on the new posi. Remember that these bolts have left hand threads. Torque the ring gear bolts to 65 foot pounds and use some Lock Tight on the threads. You should have new Timken bearings installed on the posi. Be sure that the bearings are seated completely against the case. You will need a good shim set to get the backlash set correctly. Install the posi with the bearing races and slip about .225 shim into the left side. Be sure that this doesn't jam the ring gear into the pinion. Now start adding shim to the right side until it is tight. When you get the shims tight install the bearing caps and check the backlash. There is no need to torque the cap bolts at this time, just get them snug. Make adjustments to the shims until the backlash is correct. When you have the backlash correct then you should add about .003 to .005 to each side until the shims are very tight and the differential can not be pulled out by hand. Check the backlash again to be sure it didn't change. Now torque the cap bolts to 60 foot pounds and finish assembling the rear end. If you use a 80W90 mineral based oil you will need to use a bottle of GM additive 1052358 in the oil. I prefer Amsoil 75W90 Series 2000 synthetic oil. This oil doesn't need any additive.
Checking the backlash with a dial indicator.
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Byars Performance LLC
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead
big gear head
Veteran Member
kirks74 said:
Yes, the 8.5 10 bolt uses the same 3 series differential for the 2.73 all the way through 5.13.
ProStreet78z28
Banned
I know old post,
but when you put the differential back into the housing, do the shims go in before or after for preload? (Once I have the backlash right, and going to install the final time)
but when you put the differential back into the housing, do the shims go in before or after for preload? (Once I have the backlash right, and going to install the final time)
big gear head
Veteran Member
I put the left shim in along with the differential, then I use a screwdriver to push everything tight to the left side. I get the right shim started in and then use a small hammer and this tool that I made to tap the right shim in.
ProStreet78z28
Banned
Dang!! Another tool I don't have....I did get the super shim kit, does that make a difference?
big gear head
Veteran Member
The super shim kit is easier to deal with. They will also hold up to tapping with a hammer better than the original cast iron shims. Just be carefull not to hammer flat spots on the shims as you drive them in.
ProStreet78z28
Banned
The super shims looks great, too bad they didnt come with instructions.
2ndgenls1
Veteran Member
I have a 8.5 3 series posi with 3.23 gears. just pulled from a 79 trans am.
