12 bolt rear end w/ 2.76 gears

[Fastest32]

I found a 12 bolt rear end on cl with 2.76 gears in it. The guy says he pulled it from a 50's truck and it has 6 lug axles. I want to buy it, but I'm scared it wont fit my 64. Did the 50's trucks have 6 lug 12 bolts? I dont mind swapping the whole carrier and ring and pinion into mine if thats easier, but I wanna make sure Im not buying a car rear end or something. I told him Id go back and get it tomorrow. Any ideas?

[1963c-10]

Chevy has used the 6 lug in trucks since the 30's. Do you have any of the #'s of the axle tubes? Can run the numbers and see exactly what it is. Also...chevelle and camaro used a 12 bolt rearend that was super tough. Had one in my 69 Chevelle SS w/factory 396 BBC.

[Fastest32]

Well if it came out of a truck I should be able to swap the carrier and ring and pinion then? I'm gonna go back tomorrow and look at the #'s stamped on it. It had some but I didnt write them down.

[fleetsidelarry]

I don't think they put 12 bolts in trucks 'till '63

[Fastest32]

Originally Posted by fleetsidelarry

I don't think they put 12 bolts in trucks 'till '63

Do you think its out of something newer? also, did trucks ever come with 2.76 gears? I took off the cover and counted the teeth so I know its a 2.76 ratio. I wish I wouldve taken a picture of it now. I'll go back tomorrow and take pictures of it and maybe that'll help.

[fleetsidelarry]

well, that gear ratio was another thing that didn't sound right for a truck 12 bolt from the mid 60's. does the cover look like the one in your '64?

the trucks switched to 5-lug in '71 and stayed that way for several years and the passenger car 12 bolts mentioned earlier would have been 5 lug.

My guess is that you'll get a definitive answer in about 2 hours

a picture would certainly help, though

[Fastest32]

The cover looked a little different. I feel like mine has a more pronounced ridge, like for the ring clearance. The drums on it were huge. I'll post pics tomorrow after I take pics.
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[Captainfab]

The truck 12 bolts didn't come out until the '63 model year.

As far as I know, the 2.76 gears weren't available until sometime in the mid 70's. I picked up a 12 bolt a year or so ago that had 2.76 gears, but I don't remember just what year it was. It is entirely possible that this differential has been put together by someone with those gears for some reason. If the housing is the '71 and newer version, then it will be 1.5" wider than the one under your '64.

[Fastest32]

Ok I got some Pictures!!!

[Fastest32]

Is that pitting on the ring too bad to even mess with? Also the number on the other side of the axle said gm82. I think it was 56 1/2 inches from backing plate to backing plate. It was hard to measure bymyself and around the diff.

[Fastest32]

So, apparantly those numbers are just casting numbers and are worthless.

[Rich 5150 69]

Top right side a little to the back on the axle tube will be a series of letters / numbers, those are the numbers we need to determine what you have...use a wire brush to clear the rust off....

[Fastest32]

I think I'm just gonna buy it. Its gotta be a truck rear end right? Im thinking its out of a 70s truck. From what I hear if it's a truck 12 bolt the insides should fit in mine right?
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[Captainfab]

Measuring backing plate to backing plate is not an accurate way to measure a differential. The correct measurement is WMS to WMS (Wheel Mounting Surface) If that differential is from a '71-'82 truck, it will be 1.5" wider overall than the original differential in your '64. The '63 to early '70 truck differentials measured 62" WMS to WMS. Also if this differential was installed in a 50's truck, it likely has leaf spring mounting pads on it. If your '64 is a Chevy it should have trailing arms with coil springs. Installation of this differential may require some cutting and welding, but thay depends on just what year of differential this one is. Is there a reason you are wanting 2.76 gears? IMO those are just too tall of gears for a truck. If you're after better mileage, your mpg's may actually suffer from too tall of gears. Higher gears doesn't automatically mean better mpg's.

[Fastest32]

..

[Fastest32]

Captain, I don't plan on using the whole rear end. I was planning on only using the carrier and ring and pinion. My truck right now has a 250 with the sm420 and is screaming going down the highway. Second gear won't even get you out an intersection. I figured the 2.76 gears would work, do you think it's too drastic of a change? I don't have a tach so i don't know how high I'm revved going down the highway right now, but i don't even like doing it cause it feels way too high. I measured my ratio just by spinning the wheel on a jack and came up with 3.73's. Also the guy wants $100 bucks for the whole rear end. The cheapest I could find a ring and pinion is like 200 a that's for 3.42or 3.08s. Maybe id be better off finding a junkyard set? I wouldn't know what trucks to start looking in though.
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[koda360]

I figure $100 ain't bad for a 12 bolt, given that it's not seized up... If you wanna do all the work, swap in the 2.76s and try it. If too tall at least you can make an educated guess as to what you want. Then you'll have a 12 bolt for a later project, some 2.76s for a lighter vehicle, and a set of extra axles for something...... You could always part it out and you might make a few bucks. Only thing you'll be out is time. Just my 3 cents!

[tincan1966]

Maybe I approach things differently, but you can effectively change the gear ratio as simply as changing rear tire size, too. If, for example you originally have 3.73 gears with original tires that are say, 30" tall, installing 32-33" tall tires changes the effective ratio. Somewhere I have a formula for figuring all of the numbers. If memory serves,my Suburban has 3.73 gears, but by going with the 275/70/15 tire, my final ratio is closer to the 3.40's or maybe even 3.20's. I chose my tire size, by inputting my desired engine RPM(@ cruise speed-65 mph) and original gear ratio into the formula, and then let the math tell me the tires size I needed.

[Fastest32]

So I went ahead and bought the rear end. The code on the axle tube is raa 276w81. The w is fainter and a little smaller?
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[Captainfab]

In my opinion, you need an engine that develops a good amnount of torque right off of idle to be able to pull a high gear like a 2.76. But then again the actual diameter of the rear tires will have an affect. I don't think the 250 makes enough low end torque to make this work like it should, but you may be happy with it. My choice of a gear would be the 3.42's, or an overdrive trans, to keep the highway rpm's down.

[Fastest32]

Well they were pretty cheap. Alot cheaper than changing the motor or transmission. If they're too high I'll start looking at junkyards or somewhere else. Eventually this truck will get a small block and an automatic transmission and the 3.73 will go back in it. Until then I'd like to be able to drive it to work at a decent rpm. I will update after the install. Hopefully they'll work out.
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[Fastest32]

Update: I installed the gears in my truck today!! Huge improvement. On my way to work I was running 3000rpm @ 62mph, on my way home, after gears, i was doing 70mph @ 2500. I had to use my old spider gears so that my axles would still fit, and i had to run to the junkyard to Pick up a pinion flange that would work with the new pinion. i went into a panic when i realized my old pinion flange wouldn't work and nobody had a pinion flange. I went to the pick and pull and found a 12 bolt with the right flange under the first truck i looked under! Oh and i got it for $10 too!
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[Captainfab]

The highway rpm's sound good. How is it on take off from a stop? Can you still take off in 2nd gear?

[Fastest32]

Originally Posted by Captainfab

The highway rpm's sound good. How is it on take off from a stop? Can you still take off in 2nd gear?

I can still take off in second gear. Still drives good around town. I only drove it today,but so far so good.
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[LostMy65]

Thanks for posting an update.
I'm looking at possible 3.07 for my 65.
Right now with my 25" tall tires, a 3.90 rear, and the sm420, I can start off in 3rd, and I keep wanting to shift above 4th when only doing 45mph.