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Differential questions

Green Monster

Green Monster

1/2 ton status
Joined
Aug 10, 2013
Posts
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Location
Thousand Oaks, CA
76 K5, stock 12 bolt/Dana 44 combo. Seems like the gears are really low. Tach isn't accurate but I'm guessing 3200-3500 rpms at 60-65. I don't push it over that. I was told it was a posi, as the PO (cousin) blew up the rearend right before I bought it and had a shop install a new unit. Cost $1,200 if I remember. So two questions. First, I chalked the pinion on the rear and spun one drum. Took a little over two turns on the pinion to turn the drum once. Other drum didn't spin. According to what I understand, my gear ratio would be a little over 2.00. That doesn't make sense. Then I had my son spin the left drum backward while I was spinning the right forward, and vice versa. We could go back and forth, with the pinion changing directions. I thought on posi units, when you spin one wheel, the other spins the same way. And on open diff, one spins the opposite direction. Neither one applies in this case. And when we both spin the wheels in the same direction, the pinion spins faster than just one of us spinning one wheel. Same case on the front diff. So do I have a posi, or an open diff? And how do I figure my gear ratio? Just FYI, the drums/rotors are dragging a bit on the brakes, so they're not spinning freely.
 
And I'll add one experience. Before I parked it and started tearing everything apart, I took it to the beach. In 2wd. Pulled onto the shoulder and the right rear was in the sand, left rear on the pavement. Right rear started spinning, but left rear had traction and we slowly pulled back onto the pavement. So it acted exactly how I thought limited slip was supposed to work.
 
When you were turning the one side to check the ratio, you just needed to turn the wheel/drum a total of 2 revolutions.
That makes up for the gearing of the spider gears.
And if you can turn one side that easily, I would say that there is not a limited slip differential in there.
I believe that it even a gov-loc would give you more feel that that.
 
6872xtc said:
When you were turning the one side to check the ratio, you just needed to turn the wheel/drum a total of 2 revolutions.
That makes up for the gearing of the spider gears.
And if you can turn one side that easily, I would say that there is not a limited slip differential in there.
I believe that it even a gov-loc would give you more feel that that.
Click to expand...
Thanks. So that would mean they're probably 4:10s. Were open 4:10s standard on mid to late 70s K5's? I thought they were all posi units.
 
limited slips were optional (G80 code), and probably most common gearing on 1/2 tons and K5's were 3.07's.
 
I can't find the RPO codes. So I thought open diffs spin the wheels in the opposite direction, and posi units spin in the same direction. Is this not true?
 
USUALLY, yes, but brake adjustments and physical condition can play a role. Just pull the cover. You'll want to change the gear oil anyway. Take a pic and post it if you are still unsure if you have an open diff or not.
 
this is the G80 gov bomb limited slip...

229779d1139980998-12-bolt-w-g80-axle-removal-help-axle-removal.jpg



this is an open (non posi/locker/limited slip) diff....

1007cct_05_z%2Bchevrolet_c10_rear_axle_upgrade%2Bcross_shaft.jpg
 
there are other aftermarket locking diffs out there as well....pull the cover, take a pic and do a search to see exactly what you have. You can also check the numbers on the ring gear for the ratio....something like 41-10
 
76zimmer said:
there are other aftermarket locking diffs out there as well....pull the cover, take a pic and do a search to see exactly what you have. You can also check the numbers on the ring gear for the ratio....something like 41-10
Click to expand...
I rebuilt the front end, and it looks like the open diff. I was trying to avoid digging into the rearend as it was just rebuilt. But I may because now I'm curious.
 
true Chris...gov locks don't "lock" until about a 100rpm difference is noticed between the two sides..then the govenor weights lock in the clutches and hence the "bomb" part of the govbomb happens
 
As mentioned above, the factory Gov-Lock will behave just like an open when spinning a wheel by hand. I've played with the factory one in my K5 and '03 2500HD in the past and could never get one tire spinning fast enough by hand to get it to lock.

Along with that, when spinning the tire by hand and counting the revolutions to calculate gear ratio with an open diff you have to either times the number by two or spin the tire twice. If you counted just over 2 revs. than that would lead to something around a 4.10 gear ratio.
 
I learned the 10 bolt gov-loc diffs can blow up pretty easy if you have the vehicle jacked up or on a lift and for some reason had to put it in gear and rev it up,like to diagnose a noise or see if the driveshaft is dented or bent..

One day at my friends shop he had a customer sit in his pickup while it was up on the lift,and he had him put it in drive and give it some gas,so he could see if it had a bad tire with a bubble or shifted belt,or a bad driveshaft--truck was shaking and "thumping"above 30 mph--the guy goosed it too hard and got the tires spinning about 60 mph almost instantly--the "BOOM" we heard as the diff came apart inside was pretty spectacular!...the diff cover had a gash in it,with something trying to escape,it flung gear oil everywhere,including all over my friend...

It was probably ready to fail anyways,but that finished it off in a hurry!..the driveshaft came to a complete halt in about 1 millisecond..the truck would not roll when we let the lift down either,we had to raise it back up and pull the diff cover off and dig out all the shrapnel in order to be able to push it outside..
 
I installed the new driveshafts, so I started her up on jack stands. Put it into 4wd high. Both rear wheels were spinning. Grabbed one rear wheel to stop it, and a second later, BOOM......ripped it I out of my hands. Same thing on other side. Tried it on the font, and nothing. Wheel would just hold while the other one spun. So I guess that means I have a posi unit in the back, open in the front.
 
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