69 C10 12 bolt factory posi questions

[Jinxy81]

I am in the middle of getting the rolling chassis going on my 69 c10 and I wanted to get some input on my factory 12 bolt rear end from the experts.

Based on the numbers on the housing and gear I am pretty positive I have 3.73 gears and a factory posi unit.






I will be using an LS 6.0 engine with a mild cam that will be pushing around 450-470 hp at the crank so maybe 370-380 at the wheels? The gears appear to be in very good shape. We will be pulling out the stock axles and upgrading with new 5 lug axles and axle bearings/seals from Early Classic.

I do have some concerns about the factory plugs that are cracked and weep oil. I know you cannot use just any welder when welding these since pumpkin is cast and the tubes are not. I believe you have to stick weld with nickel rod?







1) Should these cracks in plug be cause for concern or just fill them with some jb weld to stop leaking?

2) With sensible driving not getting on it or peeling out all the time how will this factory unit handle the HP?

3)What all should we look for before using this rear end?

*I have talked to a few local mechanics and they all just say "run it and see how it works before spending money on it" not sure how sound of advice that is.

[trac209]

That diff should handle that level of power without issue. Traction is what breaks parts so unless you're running a drag radial or something you'll be fine,even then it would most likely hold up well. As for the plugs I would scuff them up and lay some jb well on there and they will be fine.

[Jinxy81]

Originally Posted by trac209

That diff should handle that level of power without issue. Traction is what breaks parts so unless you're running a drag radial or something you'll be fine,even then it would most likely hold up well. As for the plugs I would scuff them up and lay some jb well on there and they will be fine.

Thanks for the reply! I hope the friction plates inside are still in good shape so they lock up and actually make it a "posi"

[Coley]

Yep...thats a 3.73
Here is the equation to determine that:
Do you see the (2) numbers stamped/listed 11:41?
Those are the tooth quantities on the 2 primary gears (pinion and ring). So simply divide one into the other as follows: 41/11 = 3.727 (....ie: 3.73's)
...and you have the posi unit with it.

All Good
Coley

[Jinxy81]

Originally Posted by Coley

Yep...thats a 3.73
Here is the equation to determine that:
Do you see the (2) numbers stamped/listed 11:41?
Those are the tooth quantities on the 2 primary gears (pinion and ring). So simply divide one into the other as follows: 41/11 = 3.727 (....ie: 3.73's)
...and you have the posi unit with it.

All Good
Coley

Thats what I thought! I took those two numbers that day and divided them and got a familiar looking # I was pleasantly surprised, save me a little money!

[Jinxy81]

Today we pulled the rear axles out of this rear end to do the 5 lug swap. the C clips do not look like they are in too bad of shape, but I will probably order a couple of new Yukon c- clips.

When we pulled the c clips out a shim came out with each one. The shim is fairly chewed up and I would like to replace them but I cannot find what part # or what they are even called. I have searched for a couple of hours LOL

[BLT gmc]

Try Tom's Differentials for your 12 bolt part needs. Also, watch his you-tube video about "tuning" your posi. It will make all the difference. Bruce
Posted via Mobile Device

[Jinxy81]

Originally Posted by BLT gmc

Try Tom's Differentials for your 12 bolt part needs. Also, watch his you-tube video about "tuning" your posi. It will make all the difference. Bruce
Posted via Mobile Device

I will check them out but I still need to know what these shims are called or if they are necessary with new C clips and new axles. Are these considered side shims for bearing load?

[Mike C]

Do you have pics of these shims? I've taken apart and reassembled probably a hundred different GM axles and have yet to see a c-clip shim. You can get thicker shims to reduce end play and GM spec'd them for the JL-8 and HD axle assemblies with the Camaro 4 wheel disc rear because of pad knock back. But this would be highly unusual in a pickup rear.

[Jinxy81]

Originally Posted by Mike C

Do you have pics of these shims? I've taken apart and reassembled probably a hundred different GM axles and have yet to see a c-clip shim. You can get thicker shims to reduce end play and GM spec'd them for the JL-8 and HD axle assemblies with the Camaro 4 wheel disc rear because of pad knock back. But this would be highly unusual in a pickup rear.

I will get pics of the shims soon, but they were directly behind the c clip ON the axle end itself (in the groove with the c clip I assume). These were the original 6 lug axles we pulled out.

[Mike C]

Will be interesting to see. My C10 factory posit had no such shims in it on the factory 6 lug axles. I pulled the posi and put it in a 72 housing with 5 lug axles. No shims in it either.

Like I said before, it it's not a hundred of these I've been in, it's close. And I've never seen anything in the groove in addition to the c-clip.

[Jinxy81]

Originally Posted by Mike C

Will be interesting to see. My C10 factory posit had no such shims in it on the factory 6 lug axles. I pulled the posi and put it in a 72 housing with 5 lug axles. No shims in it either.

Like I said before, it it's not a hundred of these I've been in, it's close. And I've never seen anything in the groove in addition to the c-clip.

Here are some pictures. Each side was like this. I have new Yukon 5 lug axles, and yukon c clips. I am going to see how tight they are without these shims. How much if any play should be there?

[BLT gmc]

I have never seen shims in that application. Tom has different thickness c clips to choose from. Bruce
Posted via Mobile Device

[Jinxy81]

Originally Posted by BLT gmc

I have never seen shims in that application. Tom has different thickness c clips to choose from. Bruce
Posted via Mobile Device

Thanks, I measured the original c-clips vs the new ones and they are the same thickness. So I assume I need to find a c clip with the thickness of the original and shim or find sims to go with new c clips.

[Mike C]

Axle end play on the semi-floater isn't super critical in a drum brake environment. In a disc brake setup it is because of pad knock back. That's usually the reason for the thicker c-clips.

Fascinating to see those shims. They look to be home made but fairly precision cut.

[Jinxy81]

Originally Posted by Mike C

Axle end play on the semi-floater isn't super critical in a drum brake environment. In a disc brake setup it is because of pad knock back. That's usually the reason for the thicker c-clips.

Fascinating to see those shims. They look to be home made but fairly precision cut.

So what do you think? Install same thickness c-clips minus the shims?

[Mike C]

Drum brakes in rear, correct? I wouldn't worry about the shims if that is the case in my previous experience.

The only question I would have is why were they in there in the 1st place?

[Jinxy81]

Originally Posted by Mike C

Drum brakes in rear, correct? I wouldn't worry about the shims if that is the case in my previous experience.

The only question I would have is why were they in there in the 1st place?

Yes drum brakes rear with 5 lug conversion. I dunno...my dad said he had never seen them before either. I am hoping their is no 70's voodoo going on with this rear end from a previous owner