12 bolt rear end w/ 2.76 gears

[rideblue00]

Just a quick note: RAA decodes as 1972 C10 3.73 from my chev trk ID numbers book

So, since you obviously counted the teeth and did the math, someone has changed the gear set.

This is cool that you did this cause this is some good info. I was wondering how low you could go and it still be drivable. I think this gear set would work with the SM420 trans and you would have a usable 1st gear. Taller tires would also drop the rpms more, but drivability may suffer.

[Fastest32]

I got my gears out of a 79 rear end. Lost, you should definitely do it. I live about 30 miles from work and the first time I got on the highway to take it to work those gears became my priority for that truck. I usually do most of the work to it when I'm slow at work and I couldn't even drive it to work before. now I can do my p/s install with the Captains brackets. I can't wait. I have all the parts just need the time.
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[Fastest32]

Originally Posted by rideblue00

Just a quick note: RAA decodes as 1972 C10 3.73 from my chev trk ID numbers book

So, since you obviously counted the teeth and did the math, someone has changed the gear set.

The parts guy at work looked it up and he came back with the 2.76 gears i don't member the year he said but he printed it out. I might still have the paper at work. Either way I think they worked out just right.
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[Fastest32]

This was me driving home @ 70mph. Haha. So much better.
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[MikeB]

I know this is an old thread, but wanted to say my 82 C10's 2.73 axle works great with my new 350/TH350 and 255-70x15 tires. No problem at all to get moving.

But the 2.73 pretty much sucked with the 250 six and 3-speed manual trans. Getting started with the 3.50:1 first gear wasn't too bad, but RPMs dropped off like crazy after shifting. I literally had to drive 40-45 before I could shift into 3rd if I didn't want to lug the engine. However, the OE tires were were nearly 2" shorter (they cross to 205-75x15).

An 81 truck I once owned had a 305 and 2.56 axle. It was pretty much a slug at any road speed in any gear. I ended up disabling the TH350C lock-up T/C, which helped a lot, especially since the highway speed limit back then was only 55 mph.

[BabyBlue64]

Hello everyone,

I have a '65 12 bolt rear end with 3.73 gears in it under my '64 truck. I have seen an 12 bolt rear end for sale with 3.08 gears out of an '55. Can I put the gears in to mine?

[vince1]

Like has been said 12 bolts weren't around in 55 so that is irrelevant. If I remember right my 66 had smooth drums and the 71 replacement had drums that look like those in the picture above. That rear end is probably 1 1/2" wider than yours. I think you also have to make sure the carrier comes across to yours too. You would be re-using your 17 spline axles and spider gears, and drums too I think.

My truck is now 3.07. I thought 3.08 was for cars but maybe they had both in trucks. If it is a car differential I don't think they will work. I am not talking from experience here, just on what I have read.

[Captainfab]

The truck 12 bolt was used from '63-'82. As was said above a 12 bolt did not come in a '55. Now someone could have put a 12 bolt under that '55, but it could be either a car 12 bolt or a truck 12 bolt. If it is indeed a truck 12 bolt, then yes you can put those 3.08 gears into your housing. You will need to also use the carrier that the 3.08 gears are bolted to. Before you get too carried away, do some measuring and see if it may be easier to just change the entire differential.

Originally Posted by BabyBlue64

Hello everyone,

I have a '65 12 bolt rear end with 3.73 gears in it under my '64 truck. I have seen an 12 bolt rear end for sale with 3.08 gears out of an '55. Can I put the gears in to mine?

[vince1]

I missed the part where you wrote you have a 65 diff in your 64. 64 had 17 spline axles but I think by 65 they had gone to 30.

Yes, why not use the whole thing?

[BabyBlue64]

Thank you all for the feedback. I will ask the guy if he exactly knows what kind of differential it is and if he can measure some things to compare it with mine.

When I put the '65 rearend under my truck I made a little bend in the panhardbar. I wasn't able to find a used 17 spline axle to replace a worn out one of my original rear end.

[andyh1956]

On the car 12 bolt rear axle assys the pinion gear shaft diam is larger than for the truck pinions. Go figure! But you can mount the pinion up in a lathe w/ a grinding attachment & turn it down to fit. Seen it done for dirt track cars here before.
I once tried to put a 2.73 gear set were a 3.42 came from & the gear on the pinion wouldn't fit into the housing, it was too large! Went with 3.07s instead.
I know of an Oldsmobile w/ a 2.41 gearset in it!
I had an '83 C10 w/ a 6.2 Diesel w/ 2.73 axle & a 700r4 trans. That truck LEAKED more fuel than it used! But it kept the direct clutches burnt up also!
On top of all this info there were 12 bolt axles in GM BOP A body cars w/ 10 bolt gears in them!

[Sgtfluffy16]

Originally Posted by BabyBlue64

Hello everyone,

I have a '65 12 bolt rear end with 3.73 gears in it under my '64 truck. I have seen an 12 bolt rear end for sale with 3.08 gears out of an '55. Can I put the gears in to mine?

any idea where to purchase backing plates for these 12 bolt axles? I have one that i think came out of a 80 k10. it has 276 gears (strange ratio) it will be mated to a 700r4 trans and a 305 with a mild cam

[Captainfab]

What is wrong with the backing plates you have, or do you not have any? A 2.76 gear is definitely not a good gear ratio to run with a 305 and a 700R4.A set of 3.73 gears is what I would run. The lower the rpm does not necessarily mean you will get better MPG's. The engine needs to run in it's 'sweet spot' RPM wise.

Originally Posted by Sgtfluffy16

any idea where to purchase backing plates for these 12 bolt axles? I have one that i think came out of a 80 k10. it has 276 gears (strange ratio) it will be mated to a 700r4 trans and a 305 with a mild cam

[Sgtfluffy16]

Originally Posted by Captainfab

What is wrong with the backing plates you have, or do you not have any? A 2.76 gear is definitely not a good gear ratio to run with a 305 and a 700R4.A set of 3.73 gears is what I would run. The lower the rpm does not necessarily mean you will get better MPG's. The engine needs to run in it's 'sweet spot' RPM wise.

well, I do agree that it's not that good of a gear ratio. It's what's already installed in the rear axle and it seems to be a good gear set. And I'm looking for backing plates because I believe my passenger side, is rusted out, it actually has holes rusted in the backing plate and I need to put a new one on

[Captainfab]

A backing plate from any '64-'82 truck 12 bolt or '79-'87 truck 10 bolt will work as long as it is for the same width of brake shoes that you currently have. The shoes will be either 2" wide or 2-3/4" wide.

[Sgtfluffy16]

I pulled one of the drums off this evening. From what I can guess without a tape measure they are two and three quarter of an inch wide. I know you are telling me applications for the backing plates which I do appreciate. But do you happen to know a part number of a new one that I can purchase locally? Or even from a website?