'80 K5 4x4 Blazer 12 bolt rear with POSI - is this a golden find ??

[old gyal]

I scored a 12 bolt rear end out of an 1980-81 K5 4x4 Blazer Silverado  - rotting away in a field - a real shame

My plan is to keep it as a back up rear end for my 86 GMC C1500 2WD pickup

I opened it up this weekend to find what looks a POSI  so I thought I had struck gold

having read a bit on this forum - is this a good backup for my C1500 ?

[old gyal]

Also - is the steering box from a K5 Blazer worth taking off and keeping or swapping into my C1500 truck

my truck seems to have a bit of the boating steer issue - I changed the coupling rubber disc on the steering shaft and did some bushings on the front end and new shocks  and alignment - tightened it up a bit but there is always this play before something happens

wondered if trying a different box might be worth it ??

[Jason S]

The Blazer 12-bolt looks like it has what is called a Gov-lock differential.  It is a governor type differential that mechanically unlocks above certain speeds. 

The Gov-lock, if it is still operational, will work until it quits...  I had one in my '74 K10 (i.e., 12-bolt with Gov-lock) and it worked well until one day it quit working and I went from having a posi-trac rear to essentially a spool rear.  I've also heard of them referred to as Timex timebombs... 

I wound up rebuilding my 12-bolt with an Eaton clutch type limited slip rear differential.

As for the steering gear, I don't think the 4x4 steering gear will work with your 2 wheel drive pitman arm.  These trucks never had the best steering, but a rebuild of your current box could help, especially if it has a lot of internal wear.  Additionally, you might verify the other steering components (tie rods, idler arm, ball joints are in good shape). 

[old gyal]

  oH well - got the 12 bolt rear off and cover opened

16 : 41     what are the odds

The gears are worse than my stock 2WD 10 bolt which came with 2 : 73 

Why are these trucks so high geared when they are so heavy ??

[square81]

 Probably geared that was because it had a 3 speed auto or 4 speed standard and keeps the RPMs down when driving. Also depends on tire size too.
 Yeah like mentioned before the gov loc works great till it explodes. Had one till it locked up going down the road not a good time. 

[zieg85]

Unless you are building a hot rod there is nothing wrong with a 2.56 gear and a TH350 behind a low torque built 350.  You should be able to get decent MPG out of it on the highway cruising at 70

[Irish_Alley]

kind of like what zieg said those rears are built more for mpgs than power and jason is dead on about the steering boxes

[old gyal]

My  4.3 V6 was a dog with the 2.73 gears when pulling car trailer even behind the 400 tranny - especially on hills

made the tranny and motor generate a lot of heat through the floorboards by time I backed the car trailer up an incline into my garage

we don't have highways here - but I knew this could not be good for the life of my drivetrain

So I took the 2.73 out of the stock 10 bolt in my C1500 truck and swapped used posi, used 4:56 gears in it with a spacer ring

She hauls anything now anywhere without complaint

I have noticed that rear makes clicking sounds sometimes on slow turns - so I think the posi or gears days may be numbered

Now I just put in a hopped up 355ci - this 12 bolt was going to be a back up in case the posi dies under the new torque

For the few hills where I live - and the hauling duties - I will need some gears

whats the lowest gearset  a 12 bolt series stock posi with 2:56 gears would take ?  to run without a spacer ??

was lo