'63 12 bolt rear

[J1MMY]

Will it physically take a 2:73 diff and if so are there any outlets that can supply this? Stock gearing is killing me, even on 28.5" tyres!

[vince1]

My understanding is no, unless the carrier is also changed. Something about series 3 or 4.

I had a 64 3.73 in my 66 for a while. Took it out and put in a 3.07 from a 71, which still has the six studs but is 1 1/2 wider.

You might get better results with putting in an overdrive transmission.

[The1960APE]

I don't have an answer but google Randy's Ring gear, I always use them for rear end parts, great products at good prices! And they are VERY knowledgeable

[J1MMY]

Originally Posted by vince1

My understanding is no, unless the carrier is also changed. Something about series 3 or 4.

I had a 64 3.73 in my 66 for a while. Took it out and put in a 3.07 from a 71, which still has the six studs but is 1 1/2 wider.

You might get better results with putting in an overdrive transmission.

I don't want to go with an O/D trans as the SM420 is in fine working order and id rather not cut holes in the hump.

How have you found the 3.07? What trans are you running? I took the cover off
The axle on my '79 donor this evening and it has a 3.07 in it. I am aware of the whole series thing with the 12 bolts but don't understand it.

[Brad54]

Originally Posted by J1MMY

I don't want to go with an O/D trans as the SM420 is in fine working order and id rather not cut holes in the hump.

How have you found the 3.07? What trans are you running? I took the cover off
The axle on my '79 donor this evening and it has a 3.07 in it. I am aware of the whole series thing with the 12 bolts but don't understand it.

Basically, the "series" thing works like this:
The thickness of the ring gear is different, depending on what the gear ratio is. It's in the manufacturing process.
The "case break" (series) are specific to those different ring gear thicknesses. If you put the wrong ring gear on a case, it'll either be too far or too close to the pinion gear.

I don't know what the actual "case breaks" are on the 12-bolt... On the Dana 44, it's 3.90-up, and 3.73 down (and there might be a third case break farther down).

Oddly, Mopar 8 3/4 and Ford 9-inch drop-out rear ends don't have case-breaks--they just make the ring gear itself thicker or thinner so they all ride on the same differential case.

-Brad

[vince1]

The 3.07 is sort of OK I guess as far as rolling down the highway goes. I have the 4sp manual and hate changing gears so I see a 700R4 in the future. Might have to go back to the 3.73 though if I do that.

[Genuine GM]

The more teeth on the ring gear, the thicker it is, so the carrier has a thinner flange. 2.56-2.73 is Series 2. 3.08-3.73 is Series 3, and 3.90 up is Series 4.

If you don't need posi you can find plentiful series 2 carriers with 2.73 gears in late 70's square bodies. Sometime in the early eighties they dropped the 12 bolt, '81 maybe?

If you want posi you'll be looking at Auburn or Eaton aftermarket.

[Captainfab]

On the truck 12 bolt if you wan to switch to a gear higher than 3.73, which is what you likely have, you will have to change the carrier. Since the '63 & '64 12 bolts have 17 spline axles, you will have to either install your 17 spline spider gears into the new carrier, or switch to 30 spline axles. In my opinion the 3.42 gear is a good all around gear, but your rpm's will still be a little high for highway driving. If you go with a 3.07 gear or higher, you will likely have to use 1st gear for everyday driving. This is of course dependent on what engine you have.

Originally Posted by J1MMY

Will it physically take a 2:73 diff and if so are there any outlets that can supply this? Stock gearing is killing me, even on 28.5" tyres!

Those are the carrier splits for the car 12 bolt. The truck 12 bolt is 2.56-2.73 is 2 series 2.76-3.42 is 3 series and 3.73 and lower is 4 series.

Originally Posted by Genuine GM

The more teeth on the ring gear, the thicker it is, so the carrier has a thinner flange. 2.56-2.73 is Series 2. 3.08-3.73 is Series 3, and 3.90 up is Series 4.

If you don't need posi you can find plentiful series 2 carriers with 2.73 gears in late 70's square bodies. Sometime in the early eighties they dropped the 12 bolt, '81 maybe?

If you want posi you'll be looking at Auburn or Eaton aftermarket.

[Genuine GM]

Originally Posted by Captainfab

Those are the carrier splits for the car 12 bolt. The truck 12 bolt is 2.56-2.73 is 2 series 2.76-3.42 is 3 series and 3.73 and lower is 4 series.

Yep, got those mixed up in my mind.

[blackbart63mule]

BACKING PLATE WELDED TO AXLE TUBE. While on the subject of 63 12 bolts- I have a 63 1/2 ton panel, built in December 62, the backing plates are attached/WELDED to the AXLE tubes from the factory. Anybody know of a disc brake set-up that adapts to this? After hearing about the 17 spline versus 31 spline, and having a better choice of custom wheels, I may go with 5 lug aftermarket axles. I've even thought of cutting the attached DRUM backing plate down and making adapters to fit a disc brake setup, I would definitely need/keep the emergency brake and original handle as is in the cab. I've never heard of any aftermarket kit or cure for this, or have been able to find mention of it in this in forum. I've also thought I may have to replace the whole axle assembly with new/used unit because the current one is an open, no posi one. I have'nt figured cost into this yet, either. Any thoughts, ideas, comments is greatly appreciated. Dog..........over and out

[Captainfab]

The only way to convert a '63 12 bolt to disc brakes is to cut the original backing plate off and weld a caliper mount onto the axle tube.

Originally Posted by blackbart63mule

BACKING PLATE WELDED TO AXLE TUBE. While on the subject of 63 12 bolts- I have a 63 1/2 ton panel, built in December 62, the backing plates are attached/WELDED to the AXLE tubes from the factory. Anybody know of a disc brake set-up that adapts to this? After hearing about the 17 spline versus 31 spline, and having a better choice of custom wheels, I may go with 5 lug aftermarket axles. I've even thought of cutting the attached DRUM backing plate down and making adapters to fit a disc brake setup, I would definitely need/keep the emergency brake and original handle as is in the cab. I've never heard of any aftermarket kit or cure for this, or have been able to find mention of it in this in forum. I've also thought I may have to replace the whole axle assembly with new/used unit because the current one is an open, no posi one. I have'nt figured cost into this yet, either. Any thoughts, ideas, comments is greatly appreciated. Dog..........over and out

[BigDan3131]

The whole reason for those gears is because of that tiny little engine that doesn't make any power and putting 2.73 gears in it is going to kill any acceleration you have now. Since you don't want to switch trans buy 33" tall tires instead.

[vince1]

Yes, IMO 273 is too high.

With 3.07 I still don't get it into 4th very much when driving in the city. A lot of it is done in third. 3.73 would be better for on the farm or in the city at 30 - 40 MPH. On the highway though the 3.07 lets you cruise nicely at 60 - 65 MPH. And it will go still go faster if you want it to.

[Evoracer]

I have 3.42's with a granny 4spd, 400sbc. It's ok around town, still not enough gear for freeway speeds, imo. 3.73's were awesome around town. 3.07 in my future.