Swapping from 4.56's to 3.73's

[Classic69]

Im not very knowledgeable in this area so I thought id ask some people who might be able to give me some information. This fall/winter id like to swap the rear end gears in my '69 C10 from 4.56's to 3.73's, but im not sure what all is needed or how hard it would be for myself to do?

I like my 4.56 gears around town, but they aren't the least bit highway friendly with my TH350 (about 3900 RPM at 70 MPH), so id like to buy a new 3.73 gear set and install em in my 12 bolt rear end. What all needs to be done when swapping? Can anybody shed some light for me? Thanks!

[Jparks]

I think other people will chime in shortly and they'll all tell you this is not something you want to do yourself unless you know what you are doing. I asked the same question a couple months back and that's the reply I got. Take it to a reputable shop and let them do it for you.

[treveiger]

It's very difficult to get it right if you don't know exactly what your doing. Find a rear end with the gear you want, it might be cheaper....
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[Classic69]

Originally Posted by Jparks

I think other people will chime in shortly and they'll all tell you this is not something you want to do yourself unless you know what you are doing. I asked the same question a couple months back and that's the reply I got. Take it to a reputable shop and let them do it for you.

Ya...im considering calling around to a few shops and seeing what it'd cost me to get done professionally...but im sure it wont be too cheap! What did it cost you if I may ask? What did you get done?

Originally Posted by treveiger

Find a rear end with the gear you want, it might be cheaper....
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True...but I dunno what kinda shape a used rear end would be in and does it matter if its a 5-bolt or 6-bolt axle with 3.73 gears...even though my '69 is a 6-bolt?

[cdowns]

changing gears really depends on what rearend you have// 12bolt can b changed but best done by a pro larger rears 8lug pattern i dont think 373 is available easily or cheaply

[Fitz]

Originally Posted by Classic69

Im not very knowledgeable in this area so I thought id ask some people who might be able to give me some information. This fall/winter id like to swap the rear end gears in my '69 C10 from 4.56's to 3.73's, but im not sure what all is needed or how hard it would be for myself to do?

I like my 4.56 gears around town, but they aren't the least bit highway friendly with my TH350 (about 3900 RPM at 70 MPH), so id like to buy a new 3.73 gear set and install em in my 12 bolt rear end. What all needs to be done when swapping? Can anybody shed some light for me? Thanks!

The GM 12 bolt rear end is a straightforward rebuild. If you have a Dial Indicator with a magnetic base, a torque wrench, a 6-10 ton press and a half day you have almost everything you need.

The Chassis Overhaul manual devotes 4 or 5 pages on how to do what you want to do. if you don't have one go here: http://www.themotorbookstore.com/fac...e-manuals.html
Just be sure you get the OVERHAUL manual, not the service manual. the 12 bolt diff for 67-72 trucks are the same internally with different length axle tubes so don't panic if you have to pick another year.

The new gearset (if you go to a place like DTS) will be about $200.00 and you will need the complete install kit for somewhere around $125.00. There are two diff shops here in Tucson that will charge over $800.00 to do this so you decide if your skills are up to it or not. There are TWO critical adjustments you must make....engagement depth and backlash. The engagement depth is set with the pinion shims, backlash is adjusted with carrier shims. The first two pictures show engagement depth which was too deep in the first then just right in the second. The second picture also shows the contact pattern with the backlash set to 0.008 in. which was in the middle of the allowable range. It took about 4 hours to set this diff up correctly so if you decide to do this take your time.
One trick I use to set engagement depth is an old pinion bearing that a machine shop has turned on a lathe to enlarge the inner hole until it is a slip fit on the pinion shaft. This will let you test fit and change shims without ruining the new bearing and crush sleeve in the assembly kit by attempting to remove it to change shim sizes. if you can't get a machine shop to turn it dod for you you can always do this by hand using crocus cloth but that's going to take some tiome as those bearings are hard.

[leddzepp]

take it to a shop....that way you get a warranty and guarantee that it won't howl at the moon.
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[rsavage]

Another option, rather than changing gear set, would be to change the 350 to a 200 4R overdrive transmission. Same length as your current transmission. I would only do this if you have a V8 with a 4 bbl carb (quadrajet or holley) where you can get the proper throttle valve cable bracket and carb linkage swing. If you have a 6, I'd go with the gear change as the throttle valve cable adjustment and the degree of arc and position at 1/4 throttle are critical to its proper operation. Not easy to do if you are fabbing bracket and linkage on a six, but easy to do with a quadrajet as that is what they mostly came with or with a Holley where aftermarketers make a setup (BowTie Overdrives for example) that will work.

[Classic69]

Originally Posted by leddzepp

Take it to a shop....that way you get a warranty and guarantee that it won't howl at the moon

Agreed! That's what I think ill probably end up doing....

Soo...I called around to a few different shops in town and got some quotes: $750, $850, $1100, and as high as $1500!!! Every place offers a 1 year / 20,000 km warranty which is good...

[Classic69]

Huh...so I guess I had 3.73's in my '69 C/10, but I dunno why it screams so high on the highway, it should be about 600-700 RPM lower!! Because right now 65 MPH im at about 3600 and at 70 MPH im around 3900 RPM, when it should be closer to or just over 3000 RPM's....

I got new bearings and seals, along with an Eaton Posi (which wasnt cheap!), and I guess one of my axles needed to be replaced cuz it was worn pretty bad. Total cost will end up being about $1500. I pick it up in 2 days, so we'll see if it'll be worth it lol

[treveiger]

They ant cheap keep us informed!
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[LONGHAIR]

Originally Posted by Jparks

I think other people will chime in shortly and they'll all tell you this is not something you want to do yourself unless you know what you are doing. I asked the same question a couple months back and that's the reply I got. Take it to a reputable shop and let them do it for you.

Yup^^^

[Classic69]

Ya....I met a guy at a show n shine the other weekend who is doing it for me. He had a '70s Camaro and his old man had a '60s Chevy pickup. Him and his dad run "R & S Driveline", which does differential work, restoration, bodywork, etc.

ntm....they had the best prices & are close by lol

[treveiger]

^^^ can't beat that!
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[Classic69]

Soo...I picked up the truck last night after work and I gotta say it was well worth the $1580 to get the rear end rebuilt...The guy even did a few "extras" for me...I got a new bearing/seal kit, along with an Eaton posi unit, 3.73 gears, replaced one axle and the steady bearing/rubber because they were worn out, as well, he adjusted my brakes because they were "grabbing" a bit

It stops alot better, takes off alot better, and drives alot better!! And I had to test out that new positraction by leaving 2 short black stripes lol