12 bolt rear end

[Sawlog]

I'm trying to figure out what rear end I have in my 69 c10.
I guess GM made a type 2, 3, and 4 housing?
I would like to get a posi unit and different gears for my truck sometime.
How do I know what's what?
I have found 4 different sets of numbers on the housing.
HB0909 IS ON top of the passenger side axle tube.
NF3896838 is on the passenger side pumpkin facing the front.
GM58 is 9n the driver side pumpkin facing the front.
H228 is on the driver side pumpkin facing the rear.
I do know I have a 3.07 or 3.08 gear and open rear end. I had the cover off and c9unted the teeth last year.
Thanks

[hjewell2]

if it is a 3;08 you have a series three carrier

[Sawlog]

Originally Posted by hjewell2

if it is a 3;08 you have a series three carrier

Does that mean when I go to buy a posi unit and gears that I have to make sure it's a series 3 unit?

[mongocanfly]

("Does that mean when I go to buy a pork unit and gears that I have to make sure it's a series 3 unit?")

Pork unit?
Just make sure your new posi carrier and gears are compatible....

[Sawlog]

Thank you

[Captainfab]

The 2, 3 or 4 series, refers to the carrier. It has nothing to do with the housing other than fitting a truck 12 bolt. Each series of carrier is compatible with only a certain range of gear ratios. The lowest gear ratio available for a 3 series carrier is a 3.42. If you are wanting 3.73's or lower, you will need a 4 series carrier.

[Sawlog]

Originally Posted by Captainfab

The 2, 3 or 4 series, refers to the carrier. It has nothing to do with the housing other than fitting a truck 12 bolt. Each series of carrier is compatible with only a certain range of gear ratios. The lowest gear ratio available for a 3 series carrier is a 3.42. If you are wanting 3.73's or lower, you will need a 4 series carrier.

So if I want to go from my series 3 carrier to a series 4 so I can have 3.73 gears, then I need to buy a new carrier? Or is a new posi unit with 3.73 gears all I need?

[A1971Blazer]

there is a gear set available so that you don't have to change the carrier.....this one from Yukon Gear will change your 3.08 to a 3.73 by utilizing a "thick" gear...with your existing carrier
https://www.randysworldwide.com/product/yg-gm12t-373t/

[mongocanfly]

But hes wanting posi so he may as well change the carrier.....depending on which one he goes with...
Just make sure it's a 4 series

[A1971Blazer]

Originally Posted by mongocanfly

But hes wanting posi so he may as well change the carrier.....depending on which one he goes with...
Just make sure it's a 4 series

Sorry....I missed that part....for sure that would require a new/different carrier....I use TrueTrac myself....

[mongocanfly]

Thumbs up for trutrac...

[Rich72C10]

Sorry to dig up an old thread thread. I am wanting to go Overdrive, so I need to sort out my 3.07 rear end.

But if getting one of these "thick" sets, like a High performance Yukon Ring & Pinion "thick" gear set for GM 12 bolt truck in a 3.73 ratio (YG GM12T-373T) - is one setting themselves up for killing the rear end? Meaning should one change out their carrier instead? I am not really thinking to have Posi, even though that would be fun to have.

Also any recommendations between Yukon, SVL Drivetrain, US Standard, Excel, Richmond, or any others?

[garyd1961]

Just so you know changing anything in a rear is not as simple as swapping parts. To me it's easier to rebuild a motor than to set up a rear differential. Not saying you can't do it, just that it is a very technical thing to do.

[Captainfab]

I don't think running a thick ring gear is going to be any less dependable than a standard ring gear. Yukon is about the best quality gears you can get.

[Rich72C10]

@garyd1961 - this is not something I would do myself but would look to have done, professionally - same with having my TH350 being swapped with a OD transmission (which as of now I am looking at a 2004R from bowties overdrives). I know what I can / cannot do and I haven't any problems with paying for something to be done right.

@Captainfab - thank you!

Basically, I want to be an informed buyer - aka researching options. I already have a shop in San Marcos that I fully trust after having my TH350 worked on to fix all my leaks (they actually had parts in stock in their attic for it of all things). Which they actually brought up options on switch to OD when I picked up my truck. They are an older couple and are wonderful people (Riley's Transmissions).

[Rick Bollinger]

If you want 3:73 gears and don't care if it's posi I am sure you could find one here for $100 or so. A lot of 6 cylinder trucks came with this gear ratio. To pay a professional and buy a new set of gears and not be posi will still cost you $700.00+. Just get a used rear change the oil brakes and go.

[Rich72C10]

That's the problem of me wanting to go OD, its is a bit of a rabbit hole. I doubt I'd ever get a ROI on swapping out the TH350. Darn my 3.07 rear end!

Though, I have been searching for a rear end but nothing really comes up around here locally. Plus, if I get a rear end then it seems smart to have it gone through - then if it needs work well then I am right back up at the price of just having my changed out. So the upside, if there is one, of me having my changed out is that I know it's been refreshed. Or that is how I am think at the moment.

My rear end now is all greasy, I image it needs to be drained, sealed up, and filled back up at the very least.

Thank you for your insight Rick, it does make me think about the various options I need to weigh. This is likely a summer project, since I pay as I go on my projects. I have at least my frontend to deal with first. Right now the TH350 was freshly rebuilt and there is no indication my rear end is in trouble (aside than its cover looking greasy).

[garyd1961]

I like my 308 (307) rear with my th350 trans. I have my L31-R crate motor set up for low end torque. That's why I stayed with the factory cam also. With the low end torque it gets off the line really quick and I can cruise the interstate with no problem.
The bigger the cam the more gear you need in the rear.

[AcampoDave]

I'm with Gary on this one. I think a Turbo 350 and a 3.08 are a sweet combo. But then, I've always been one who likes to keep it simple.

Edit: I know others will disagree, but I feel a carburated engine is really not getting full benefit from an OD anyhow. I think carbs like that bit of extra rpm at speed to keep their air velocity up, especially when trying to overcome the poor areodynamics of a classic truck.

[Rich72C10]

I am thinking a LOT of staying with the TH350 / 3.08. I'd say 90~95% of my driving will be around town roads, even though I am right next to I35. Even when I plan to go to East Texas, I'd be going the back roads. Granted a lot of back roads are 65~70 for the most part (in between little towns).

So, it seems I might be shooting myself in the foot to go a wee bit faster for better gas mileage on the rare long distance travel but actually get less MPG under OD speeds around town. Or at least I think that is what I'd end up doing to myself with a 3:73 around town roads.

[AcampoDave]

In the interest of further discussion about freeway rpm, I can tell you that my '66 shrotbed used to run a 2.76 rear end with a 283 and a non-overdrive 4 speed. The 2.76 was a nice mate to the 4 speed I had at the time because an extra wide gear spacing made for nice even shifts. Later when I went with a different non-OD 4 speed that had a closer ratio, I went to a 3.08 rear gear. I have noticed no increase in fuel consumption and my engine just feels happier cruising along at 2200-2300 than it did at 2000 rpm. With the gear change, it simply takes a curl of the toe on the gas pedal and the engine willingly responds. With the 2.76, it was always a little dogged in high gear and many times I actually drove in 3rd gear around town. Every time traffic slowed below 35 mph or I came to an incline, a downshift was a reflexive action to keep the old small block in its sweet spot. Now, I find the engine is quite fine in 4th gear in many of those same instances. Even though my highway speed only went up by a few hundred rpm, my motor feels more powerful in almost every situation.

[Rich72C10]

I have to say, my truck is very happy at the 23k~25k spot (something like 60~65). The way you put it: "it simply takes a curl of the toe on the gas pedal and the engine willingly responds". That is how I could put my SBC 350/TH350/3.08 right now, exactly.

The more I think about the pain to switch and the actual ROI (if any, ever) the more I lean towards staying with the TH350. If I baby the truck (which is really hard and no fun at all) I can get right around 14~16 MPG in mixed street / Tx Farm Roads or on a straight 50 Mile I35 65 MPH drive.

Now if a 2004R and new Posi 3.73 shows up for free on my door step, perhaps that would be a better deal. Which to be honest, my brother could easily come up with those. It is amazing the stuff he gets for free or nearly free. It's like give him a cooler and a few weeks later he'll have a boat after all the trading LOL!

[garyd1961]

A stock factory motor with a stock factory stall converter works really well with a th350 and a 308 rear. If you are running a big cam and high stall converter you will need a bigger gear in the rear.

[Rich72C10]

On my rebuild, only a mild RV Cam was installed, other wise stock.