Rearend swap

[64shortbed]

Ok so i have been thinking when i do the front disc brake swap i think i may just stay with the 5 lug pattern. My 64 gmc has the factory rearend in it now. It's a dana 44 on leafsprings with 3:54 gears. Im running a 350/350 combo and at 65 mph the engine is turning around 2800 rpm. I would like to lower my rpms and maybe in the prosses gain a few mpgs. So my quetion is which rear ends will bolt in and i can run a set of ralley wheels for now? Is the 70-72 one or can i put a 73-87? A friend of mine has a rearend out oa a 73-87 suburban he thinks has around 2:70s in it.
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[66redw/white]

My understanding is that they are wider than the rear axles in our trucks so you might have an issue with tire fitment. Also if your truck is not a leaf sprung truck then you would have to do some cutting and welding to get the perches for the trailing arms on your new axle. Easily accomplished if you or someone you know can weld. You'll also need a place to mount a panhard bar to. I think if it were me I would just move for an overdrive trans. But that's because I like to have some good get up and go aswell
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[64shortbed]

My 64 gmc is a leaf spring truck and i have no plans as to go any more wild with wheels then a 15x8. That is what size of wheels i have now.
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[Chevy Fleetside]

they are only 3/4 wider on each side. YOu should be ok and I think you can use the rear end from a 73 - 87. Just pull out your tape measure and measure twice.

[64shortbed]

Originally Posted by Chevy Fleetside

they are only 3/4 wider on each side. YOu should be ok and I think you can use the rear end from a 73 - 87. Just pull out your tape measure and measure twice.

Thats about what we measured. And the spring mounts seem to be close to the same. About 42 1/2 center to center. Just wasn't sure which ones were the best fit. Thanks for the reply and help.
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[BurlyCrew]

Yeah I was just thinking any 12 bolt would be the way to go....

[64shortbed]

I went and looked at it tonight it is a 12 bolt so it should be good.
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[Captainfab]

I have used a '73-'82 12 bolt under a GMC before. If I remember correctly, the spring perches are slightly off, but there is typically enough movement in the spring bushings, that they will bolt up. The one thing you will have to address is the different position of the shocks. The '73 and up trucks have staggered shocks, whereas the earlier trucks have both shocks on the front.

[64shortbed]

Originally Posted by Captainfab

I have used a '73-'82 12 bolt under a GMC before. If I remember correctly, the spring perches are slightly off, but there is typically enough movement in the spring bushings, that they will bolt up. The one thing you will have to address is the different position of the shocks. The '73 and up trucks have staggered shocks, whereas the earlier trucks have both shocks on the front.

So would it be easyer to cut the shock mount off and turn it around or relocate the upper mount. i have the classic parts relocate gas tank upder my truck.

[Chevy Fleetside]

I dought the rear end has 2.70's in it. Take the cover off and look for the stamped # on the ring gear. It should tell you how many teeth it has and then count the teeth on the Pinion gear and divide the two and it will give you the ratio. There also might be a tag on the housing that just tells you.

[64shortbed]

Yeah im thinking more along the lines of maybe 3:08's most likely? I plan on looking it over good and counting the teeth first before i jump on it.

[robber grin]

I just got a 86 C10 1/2 ton 5 lug rear and it is 2.73:1 gear ratio.

I thought they all were 3.08's, but we double checked the GM OEM build sticker and it's a 2.73:1 gear.

I'm told I won't be burning it up between the lights in town but will likely not notice otherwise and save some rev's on the highway.

rg

[gabe2000]

I used the front-end and rear-end from 1987 suburban with no problems. I did remove the shock bracket and order some new one and weld at the correct place.
http://67-72chevytrucks.com/vboard/s...d.php?t=562943

[64shortbed]

Very kool guys thanks for the info. Like i said before i tell him ill take it we will remove cover and count teeth. I'll post what we find out and let you guys know. Either way sounds like a easy enough swap. Now when i do every thing i may take the bed off so i can run all new brake lines. I'm going to use the 71 master cylinder and prop valve i have it only has 2 lines in from master cylinder and 2 lines out. So i'm thinking of just running brake line for the rear down the drives side of the frame and T it in the rear like factory did. Then all i have to do is T the front. Right?

Oh by the way i'm putting disc off of a 85 c10 on the front.

[64shortbed]

Well i went out tonight and we pulled the rearend in his shop. I removed the cover and shes clean. The ring gear was stamped 43:14. We counted the teeth and came out to 43 teeth on the ring and 14 on the pinion. So 3:08's it is. So we loaded it in the back of the pickup and i brought it home. So now i found a rpm calculator on line and it looks like ill at least drop around 400 to 500 rpm at 65 mph. And i can live with that.
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[BurlyCrew]

I'm not very fluent with rear end gears, but what would be a good overall average Ratio for some get up and go, with also some higher mph freeway speeds?? I'm sure the Trans comes into play, and I have a 700R4....

[64shortbed]

Originally Posted by BurlyCrew

I'm not very fluent with rear end gears, but what would be a good overall average Ratio for some get up and go, with also some higher mph freeway speeds?? I'm sure the Trans comes into play, and I have a 700R4....

Not sure but i do know trans and tire size plays a lot. If i was to stay with my original rearend (it has 3:54s) and i have 255 60r 15 rear tires and i was to put a 700r4 in i would turn 2000 rpm at 65 mph. And i think to me thats about as perfect as yo can get?
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[Captainfab]

If your truck has the 12 bolt, I would suggest a set of 3.42 gears. You will need a 3 series carrier for those though.

Originally Posted by BurlyCrew

I'm not very fluent with rear end gears, but what would be a good overall average Ratio for some get up and go, with also some higher mph freeway speeds?? I'm sure the Trans comes into play, and I have a 700R4....

[Jeff La]

Originally Posted by Captainfab

If your truck has the 12 bolt, I would suggest a set of 3.42 gears. You will need a 3 series carrier for those though.

Ok so educate me, what's a three series carrier?? I'm trying to decide whether to go with 3.42's or 3.54's. Gonna run 350, 700r4 and tire size not decided yet

[Clyde65]

I found this on Mark Williams website.

TIP #12 -- CARRIER CONFUSION: When shopping for 12-bolt differential parts, keep this in mind: Chevrolet generally used three different types of differentials for the 12-bolt -- the two series carrier, the three series carrier and the four series carrier. The "two series" has little use in a hi-po application and should be avoided if at all possible. The three series carrier accepted ring and pinions that ranged from 3.08:1 to 3.73:1 while the four series carrier accepted ring and pinions ranging from 3.90:1 to 6.14:1. While there are a number of subtle variations between the three types of carriers, a major difference is the thickness of the gear flange. Two series carriers are the thinnest while three series flanges are thinner than four series flanges. Because of the size of the flange, the carriers cannot be randomly used with ring and pinion sets from the trio of "families".

It also showed a chart with the thickness' of the flange that the ring gear sits on. I'll measure the 3.73 carrier to make sure it is indeed a 4 series carrier.

[64shortbed]

Would like some help to ID the 12 bolt rearend that i got for my truck. I'd like to know what year it is most of all. So i can get brakes and brake hardwear.

Axle shaft code KAC W023 2

These numbers are on the housing 3896839N, GM69, and A166.

These were on the ring gear 3817779 14:43 GM 01762 (I know 14:43 means it has 3.07 ratio)
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[Captainfab]

All of this is talking about the 12 bolt car differentials. Those are not the same as the 12 bolt truck differential. The truck 12 bolt has only 2 series of carriers, a 3 series and a 4 series

Originally Posted by Clyde65

I found this on Mark Williams website.

TIP #12 -- CARRIER CONFUSION: When shopping for 12-bolt differential parts, keep this in mind: Chevrolet generally used three different types of differentials for the 12-bolt -- the two series carrier, the three series carrier and the four series carrier. The "two series" has little use in a hi-po application and should be avoided if at all possible. The three series carrier accepted ring and pinions that ranged from 3.08:1 to 3.73:1 while the four series carrier accepted ring and pinions ranging from 3.90:1 to 6.14:1. While there are a number of subtle variations between the three types of carriers, a major difference is the thickness of the gear flange. Two series carriers are the thinnest while three series flanges are thinner than four series flanges. Because of the size of the flange, the carriers cannot be randomly used with ring and pinion sets from the trio of "families".

It also showed a chart with the thickness' of the flange that the ring gear sits on. I'll measure the 3.73 carrier to make sure it is indeed a 4 series carrier.

[Captainfab]

The 3 series carrier has the ring gear flange farther from center than a 4 series carrier.....kind of hard to describe. I don't happen to have one handy to measure. If I can find time tomorrow, I'll see if I can get a measurement and post here. The 3 series carrier will only work with 3.42 gears and higher. The 4 series carrier will only work with 3.73 gears and lower. There are no 3.54 gears for the 12 bolt, just 3.42's. The 3.54 gears are available for the Dana differentials.

Originally Posted by Jeff La

Ok so educate me, what's a three series carrier?? I'm trying to decide whether to go with 3.42's or 3.54's. Gonna run 350, 700r4 and tire size not decided yet

[Captainfab]

The brake drums, shoes and hardware is basically the same for all 12 bolt truck differentials, or at least interchangeable, as long as you stay with the width of shoe the drum and backing plate is designed for. In '66 the self adjusters were first available. More than likely you will need the 2" x 11" shoes. The wider 2-3/4" x 11-5/32 shoes are more common in the mid to late 70's.

Somewhere here on the forum is a listing of the axle ID codes. You might check in the FAQ index.

Originally Posted by 64shortbed

Would like some help to ID the 12 bolt rearend that i got for my truck. I'd like to know what year it is most of all. So i can get brakes and brake hardwear.

Axle shaft code KAC W023 2

These numbers are on the housing 3896839N, GM69, and A166.

These were on the ring gear 3817779 14:43 GM 01762 (I know 14:43 means it has 3.07 ratio)
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[64shortbed]

Thanks for the help captainfab. It has the bigger brakes. I ran up to work today on my day off andpicked up new shoes and cylinders. Then rebuilt the brakes. I don't has a E-brake on my truck so i remove all the stuff for that. But i saved it because i do plan on some day putting on in. I got it all bqck together and painted it black then built new brake lines for it. It uses smaller lines the my 64 does. Is that because it has split brake lines instead of the single.

So now it's ready to put in. I'm having the new bushings put into the control arm for the front monday night i hope. Then it's time for new wheel bearings and brake pads for the front. After that I'll be ready to tear in to the truck and start removing the old brake system and install all the new!!

Oh and thank you captainfab my brake booster bracket showed up in the mail this morning! Looks great and I'll be sure to post some pics when i get to do the install.
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