1970 C10 Rear End ID/Help/Advice

[ShortHaul]

Hey all... bought this ‘70 C10 a few months ago and am going through the truck part by part to get her running and driving properly.

I’m trying to ID the rear end with ultimate goal of swapping the gears for a steeper ratio. I have searched plenty, and found the rear end ID thread, but none of the numbers I’ve found stamped on the rear diff match any of the numbers of the ID’s. I know the truck has been swapped to a 5 lug front and rear, but other than that I have no info about the rear. It’s non-posi, and seems to be a very “Highway friendly” set, so I’m assuming stock 3.07 (3.08?) but I have no way of telling.

The numbers I’ve found stamped on the rear diff are.

“GM 36” on the pass. Side by the fill plug
“A151” with a little sundial looking stamping on the driver’s Side front
“1254333” on the drivers side.

And it’s got 10 bolts to it...

Help!?



And the next thing I need help with is how can I swap to a 3.73 (or so) ratio on the cheap? The motor is cammed, and I just installed a 2400 Torque converter to help it idle in drive, but with these gears it rarely sees the RPM it needs to make power.

My local rear end shop quoted 1200 for non posi gear swap, 1700 for posi. Really hoping there’s a cheaper option out there! Thanks!!

Pics to follow
Untitled by awesome_adawson, on Flickr
Untitled by awesome_adawson, on Flickr
Untitled by awesome_adawson, on Flickr
Untitled by awesome_adawson, on Flickr
Untitled by awesome_adawson, on Flickr

Untitled by awesome_adawson, on Flickr

[LockDoc]

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I think you would be looking for a stamped number not a cast in one. If you looked at the thread I posted with the ratio charts, scroll down to the two posts with pictures of the code number locations.

LockDoc

[Redneck Rydes]

I think I would be more worried about those brake lines first.

[Redneck Rydes]

Appears it may be out of a 1981-87 C/10.

[cla511]

It's a 10 bolt out of a later model. No telling what's inside. Pop the cover off and look for the number on the ring gear. 2.76 and 3.42 were very common in these if you think it highway friendly.

[ShortHaul]

Originally Posted by LockDoc

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I think you would be looking for a stamped number not a cast in one. If you looked at the thread I posted with the ratio charts, scroll down to the two posts with pictures of the code number locations.

LockDoc

Copy, yeah I’ve looked all over the axle tubes with no sign of a stamped #. I’ve hit it with a wire brush pretty good to clean paint and crud... I’ll keep searching tho.

Originally Posted by Redneck Rydes

Appears it may be out of a 1981-87 C/10.

Thanks! What leads you that conclusion?

Originally Posted by Redneck Rydes

I think I would be more worried about those brake lines first.

Lol... you think those are bad... wait till you see the wiring! This truck is a mess.

Originally Posted by cla511

It's a 10 bolt out of a later model. No telling what's inside. Pop the cover off and look for the number on the ring gear. 2.76 and 3.42 were very common in these if you think it highway friendly.

Thank you! What lead you to that conclusion? (Trying to learn from you knowledgeable folk here!) When I pop the cover off the diff, the Ring gear will have a # indicating the ratio? I’d guess the 2.76, it runs at 75 on the freeway and barely cracks 3k rpm (15” wheels)

If it is a later model (81-87) rear is it possible to just swap the gears off a similar model?

Thank you all for your help! Much appreciated.

[Knight72]

Originally Posted by ShortHaul

Thank you! What lead you to that conclusion? (Trying to learn from you knowledgeable folk here!) When I pop the cover off the diff, the Ring gear will have a # indicating the ratio? I’d guess the 2.76, it runs at 75 on the freeway and barely cracks 3k rpm (15” wheels)

Thank you all for your help! Much appreciated.

The ring gear will have a number stamped into it. Mine had 11:41. 41/11 is 3.73 gearing.

[LockDoc]

Originally Posted by ShortHaul

Copy, yeah I’ve looked all over the axle tubes with no sign of a stamped #. I’ve hit it with a wire brush pretty good to clean paint and crud... I’ll keep searching tho.

Thank you all for your help! Much appreciated.

The second post in that thread with a picture of the stamped code location is not on the axle tube..... It is on the cast center section.

LockDoc

[stsalvage]

looks to me to be a Dana rear end dose not look like a spicier rear end

[BCOWANWHEELS]

def. a newer gm 10 bolt

[ShortHaul]

Ok I’m reviving this help thread, since I have identified it as a 1981-1987 pickup rear, I need to know what my options are for rear drum brakes? It has the massive rear drum that won’t allow me to fit my 15x10 Wheel Vintiques Steelies.

New question:
Can I run 1972 5 lug rear drums on that axle? Is the backing plate the same? Is there a better fix to be able to run my wheels? I’m pretty set on running them, I really think there’s no better look than a fat steelie in the back.

Other than returning the wheels and starting over, any suggestions?

[68 TT]

I put the square body truck 12-bolt into my 69 stepside and had to deal with the huge brakes.

The mounting flange on the axle tubes is the same and the face of axle is in the same location relative to the housing mounting flange but the backing plate is slightly different to accommodate the wider brake shoes and matching drum width so you have to put the entire assembly from a light duty rear brake application truck onto the rear end.

You should just have to find a light duty rear brake application 2wd truck and get the drum assembly from backing plate out to make the conversion. If I remember right the Astro van is 5 on 5 but I don't know what the backing plate mounting flange style is. It would make sense that it matched the same vintage 5 on 5 trucks so they might be a source for the brakes you need.

Just a little FYI from my swap:
With the square body truck 12-bolt the 6-lug 4x4 housing and the 5-lug 2wd housings and axles were the same width. I would ASSume that the later 81-87 (91) square body truck 10-bolt would be the same way. Spline counts are different between the 12-bolt and 10-bolt so they will not interchange but you can put 5-lug axles into a housing that had 6-lug originally with no other changes to the rear end but the matching brakes. Not like the 67-72 housing width changes between 6-lug and 5-lug.

Originally Posted by ShortHaul

Ok I’m reviving this help thread, since I have identified it as a 1981-1987 pickup rear, I need to know what my options are for rear drum brakes? It has the massive rear drum that won’t allow me to fit my 15x10 Wheel Vintiques Steelies.

New question:
Can I run 1972 5 lug rear drums on that axle? Is the backing plate the same? Is there a better fix to be able to run my wheels? I’m pretty set on running them, I really think there’s no better look than a fat steelie in the back.

Other than returning the wheels and starting over, any suggestions?

[ShortHaul]

Originally Posted by 68 TT

I put the square body truck 12-bolt into my 69 stepside and had to deal with the huge brakes.

The mounting flange on the axle tubes is the same and the face of axle is in the same location relative to the housing mounting flange but the backing plate is slightly different to accommodate the wider brake shoes and matching drum width so you have to put the entire assembly from a light duty rear brake application truck onto the rear end.

You should just have to find a light duty rear brake application 2wd truck and get the drum assembly from backing plate out to make the conversion. If I remember right the Astro van is 5 on 5 but I don't know what the backing plate mounting flange style is. It would make sense that it matched the same vintage 5 on 5 trucks so they might be a source for the brakes you need.

Just a little FYI from my swap:
With the square body truck 12-bolt the 6-lug 4x4 housing and the 5-lug 2wd housings and axles were the same width. I would ASSume that the later 81-87 (91) square body truck 10-bolt would be the same way. Spline counts are different between the 12-bolt and 10-bolt so they will not interchange but you can put 5-lug axles into a housing that had 6-lug originally with no other changes to the rear end but the matching brakes. Not like the 67-72 housing width changes between 6-lug and 5-lug.

Thank you! This is the info I need! So to put it in even simpler terms, I’m planning on buying new parts rather than searching junkyards, so I could buy the entire rear brake assembly from a light duty (aka 1/2 ton aka c10 and maybe Astro) from 1972-1987 as long as they are the 11”x2” drums, they should bolt up?

Thanks!