change to posi

big gear head

big gear head

Veteran Member
Jul 30, 2001
6,443
Ky.
This is for the 8.5 10 bolt. Remove the cross shaft retaining bolt, 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 .008 to .012 inch if the gears are pretty good. Now that you know what the backlash is you can mark the right bearing cap and remove the caps. Do not mark the left cap with a punch or chisel. This will cause a weak place in the cap and it could cause it to break. There is no need to mark both caps, and the left cap carries more load than the right cap, so only mark the right cap. You must mark the cap so that you know how they 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 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.
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Byars Performance LLC
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead
 
Last edited:
jeff81

jeff81

Veteran Member
Lifetime Gold Member
Jul 7, 2002
632
Thomson, IL USA
BGH is the MAN. One thing you might consider, is a LOCKRIGHT setup. If you have an "open" differential, then for about $200 you could install one of these in your existing diff, and WHALLAHH!! POSI!! Did this myself, and it was pretty straight-forward.
 
R

runnin350

Veteran Member
Nov 24, 2002
602
Cedar City, UT
Hmm.. this lockright sounds interesting. No problems? And it wasn't too hard? I'll look into that...Oh, and I found out the rear end is from a 75 Nova. Surprised me just a tad!
 
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