Rear end ID

[Mindl3ss]

The truck I bought came with this extra rear end. I'm not familiar enough to ID, so hopefully someone out there knows by looking.

Here's what I have identified so far:
3.42 gears (based on GM 1235128 12 41 10 86) stamped on ring gear

Is this an 8.2 or 8.5 10 bolt? Is it a locker/limited slip/open? Is it worth using over what I can only assume is a stock rear end (has discs on it already)

Here are some pictures of the gears (click to enlarge)




And what are these pieces that were loose in the housing?


Thanks for any assistance.

[truckdude239]

looks like posi and those c clips hold the axle shafts in i woukd not run it till you check that the axles havec clips holding them in

[Mindl3ss]

Excellent. Thanks for the ID and move. I'll check the shafts to see if they are being held in. I'm a ways off running it, just working out what I've got myself in to.

[Captainfab]

That does appear to be a 8.5" 10 bolt, and it has a gov lock, commonly known as a gov bomb. That won't stand up to much abuse.

[Mindl3ss]

Originally Posted by Captainfab

That does appear to be a 8.5" 10 bolt, and it has a gov lock, commonly known as a gov bomb. That won't stand up to much abuse.

This is kind of what I was afraid of.

I haven't opened up the differential that's already on the truck, so maybe it's a better setup.

Really, I just got this one included because it has the discs on it already. I think I don't think I paid too much.

[Captainfab]

Are they aftermarket discs or factory discs?

[Mindl3ss]

Originally Posted by Captainfab

Are they aftermarket discs or factory discs?

The calipers have GM stamped on them, so I'm assuming factory. I haven't measured them yet, but they aren't tiny.

[Mindl3ss]

Eh, decided to go snap some now.

Click to enlarge:








This one is upside down, didn't realize it until now. Part number 14055251

[Mindl3ss]

Well, looks like the rear end already on it may have more potential. It is a 12 bolt housing. I haven't opened it up yet so no clue what is inside.

Planning on taking the bed off this weekend, so I will try and get at it at that point.

Any chance the discs will bolt up to it?

[Captainfab]

Someone has converted that differential to discs, using the backing plate off of a K10 front differential and what appears to be '76-'78 Eldorado calipers. Not sure just what the rotors are, but it appears another wheel stud pattern has been drilled. I'm not sure just what you would have to do to bolt those up to your 12 bolt. Those backing plates have been modified once already. You might have to weld up some holes. Yes a truck 12 bolt is a better differential than the one you have with the discs on it.

I wouldn't get rid of those calipers.

[Mindl3ss]

Well, I'm a little bummed that the "extra" axle isn't a direct bolt on like the guy said it was. But that's what I get for not doing my research before picking it up. I rolled it in to the price of the truck anyway, so it's not a huge loss if I can't use anything off it.

A quick glance at the 12 bolt shows a similar stud pattern for the axle, so I may get lucky. I'll know more this weekend. And I'll be posting pics of the gearing in the 12 bolt to get an ID on it too. Regardless, I'm guessing filling and then drilling new holes would be cheaper than a whole new kit, so I suppose I'm ahead at least a little.

Thanks for your help on this Captainfab. I really appreciate the info.

[Mindl3ss]

Well, sorry it took me so long to get back to this. Been busy tearing other stuff apart and haven't bothered to open the diff up until now.

Anyway, near as I can tell it's a 3.08 open diff in the 12 bolt. Both tires off the ground driver spins free passenger sticks.

So, from my limited searching and knowledge I figure this one is more of a cruising gear.

Anyone care to chime in? I'm interested to hear more opinions on this rear end. Anyone think I could sell both diffs I have and buy one that's much better without having to spend a fortune?

[dfrank]

It look a little rusty in there kinda like some water got in it. I would rebuild that axle at least and probably put a eaton posi in and I like 4.10 gears but it depends on what you want to do with the truck.

[Mindl3ss]

Well, I also just noticed this in the shot


I'm not sure I'll go as low as a 4.10 (or is it high? Been a long time since I did ratios)

It's my C10 I'm mostly interested in cruising around. Not going to be racing or anything, but I do like putting my foot down from time to time.

I noticed the rust spots too, but I didn't notice any water when I emptied it. I think the seal on the back was bad. Looked like it had leaked on one side a bit. The diff fluid was thick. Never opened a diff before, though, so I'm not sure what to expect.

[Captainfab]

It looks to me like a lot of the ring gear teeth are damaged. Apparently something came apart in there at one time. To answer your question,yes 3.08's are a good cruising gear. The size of your rear tires play a part in what is a good differential gear for your application.

[Mindl3ss]

Based on the condition of the gear I'm probably going to be hunting down a replacement anyway.

Looks like it's time to research some factory gearing so I can know what to look for at the junk yards. I haven't been to a junk yard in years, and never since I've moved from Washington... This should be fun

I'll continue research on my own, but if anyone has some suggestions for a good donor I'm open. Right now I have no idea what to look for on the surface, so it would be a lot of unnecessary tearing things apart at the yard to find what I need.

Mostly stuff like what year did the 12 bolt come out, what years are the best to look for, etc.

I may just buy new pieces, but my budget for that won't allow until much later. So I'd like to get something to hold me over.

[Captainfab]

The truck 12 bolt was used in the C10's from '63-'82 and the K5 and K10's from '63-'79. The '63 and '64 12 bolts have 17 spline axles which are not desireable. The passenger side axle tube has the original build code stamped into it, but who knows what has been done to the differential in X number of years. The only way to know for certain is to pull the cover and get the tooth count off of the ring gear. Myself, I like the 3.42 gears. You get decent off the line accelleration and the rpm's are a bit down from what a 3.73 would be. But if you aren't towing anything heavy and don't care so much about off the line performance, then the 3.07/308 gears would fit the bill.

[Mindl3ss]

That's good to know. I would guess this one is from a mid-late 70's pickup, but I'll be counting teeth anyway just to be sure.

I think about the same for gear. My 1994 K1500 has the 3.73 in it, and it's a great rear end for that truck, but a bit low for the highway and cruising around town. It does get up and go okay, but at 65-70 it has trouble maintaining a steady RPM. I drove up to Seattle a few weeks back and tried using cruise control, but it would constantly bounce between gears while going up even a slight incline.

This pickup won't be used for towing. It will be strictly a driver just to enjoy. I need to hang on to my K1500 for a while because it's my proven reliable pickup. After I get the '68 at least running I'm thinking about trading the K1500 for an older 1 ton if I can find one that doesn't need a whole lot of body work. My ideal situation would be an 86 K3500 or a pre-72 K30. I can't afford two projects, though.

[Captainfab]

I forgot to mention that the '63-'70 12 bolts in the C10's will have the 6 lug wheel stud pattern. The '71-'82 will be 5 lug and also 1.5" wider. And of course the '71-'72 will mostly be for the trailing arms.

[Mindl3ss]

Originally Posted by Captainfab

I forgot to mention that the '63-'70 12 bolts in the C10's will have the 6 lug wheel stud pattern. The '71-'82 will be 5 lug and also 1.5" wider. And of course the '71-'72 will mostly be for the trailing arms.

Thanks for that. This one is 5 lug.

Seems I have quite the Frankenstein truck

[Mindl3ss]

Well, I measured the studs on the 10-bolt rear end last night and found the guy was pretty much completely dishonest with me. It's a 5x4.75" pattern, which would be great for a car.

I saw a few of the gears and lockers on eBay sell for $259.99 plus shipping, which I figure isn't too bad. I should be able to get my money out of it if I split everything up.

I'm kind of upset, but I'll get over it. I just need to clean it up a bit and sell it to make room in my garage. Had I been "smart" I would have measured it there in the shop when I was negotiating him throwing it in with the truck. That could have saved me some dough.

It's just sad that people can't be more honest about stuff.