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10-16-2022, 02:39 PM
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#1 |
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Registered User
Join Date: Apr 2022
Location: Moorpark, CA
Posts: 775
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72 12 bolt 4 series to 3 series swap?
Have a 12 bolt with 3:73 and would like to put in 3:08. Not finding 3:08 for a 4 series
Do I need to sap out entire rear end housing or? |
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10-16-2022, 03:08 PM
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#2 |
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Registered User
 
Join Date: Jan 2003
Location: Safford,AZ
Posts: 3,625
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Re: 72 12 bolt 4 series to 3 series swap?
Just the carrier
__________________
1969  307, t3501950 Chevy Wagon 1978 Big 10 1967 C10, 250,3-OTT |
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10-16-2022, 03:18 PM
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#3 | |
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Registered User
Join Date: Apr 2022
Location: Moorpark, CA
Posts: 775
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Re: 72 12 bolt 4 series to 3 series swap?
Quote:
Are the stock axles, 72 6 cyl, 30 spline? If so this 3 series Eaton and a 3 series gear set would work https://www.summitracing.com/parts/etn-19587-010 |
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10-16-2022, 03:46 PM
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#4 |
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Registered User
Join Date: Jan 2003
Location: Safford,AZ
Posts: 3,625
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Re: 72 12 bolt 4 series to 3 series swap?
I believe so, but wait for someone else to chime in.
__________________
1969 307, t3501950 Chevy Wagon 1978 Big 10 1967 C10, 250,3-OTT |
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10-16-2022, 07:36 PM
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#5 |
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Registered User
Join Date: Apr 2022
Location: Moorpark, CA
Posts: 775
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Re: 72 12 bolt 4 series to 3 series swap?
30 spline
GM 12 BOLT SPECS Applications: 1964 - 1982 Chevrolet & GMC 1/2 ton to 3/4 ton trucks Full/Semi Float: Semi floating rear axle Ring Gear: 8.875" Axle Spline Count: 30 spline Axle Tube Diameter: 3.00" Pinion Diameter: 1.625" Pinion Spline: 30 spline U Joints: 1310 series Ratios: 2.50 - 4.56 : 1 Carrier Break: 2.50 - 2.76 | 3.42 -4.56 Identification: 12 bolt rear cover, passenger car version has a different cover and is easily distinguishable GM 12 Bolt axle identification |
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10-16-2022, 07:43 PM
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#6 |
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Registered User
Join Date: Mar 2014
Location: Lincoln Park, NJ
Posts: 192
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Re: 72 12 bolt 4 series to 3 series swap?
but which side of the carrier break is 3.08(3.07)?
__________________
 1970 C10 LWB 350SBC, SM330 Muncie (not saginaw SM326) (on tree), 3.07 open (assumed), PS, PB, Idiot lights
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10-16-2022, 08:42 PM
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#7 |
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Registered User
Join Date: Apr 2022
Location: Moorpark, CA
Posts: 775
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Re: 72 12 bolt 4 series to 3 series swap?
My understanding is 3:08 gears only are for 3 series carrier. 4 series starts at 3:73 and goes numerically higher
Truck 12 Bolt Series 2 = 2.73 - down (smaller numerically) Series 3 = 2.76 - 3.42 Series 4 = 3.73 - up (larger numerically) |
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10-17-2022, 07:35 PM
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#8 |
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Senior Member
Join Date: Jul 2000
Location: North Texas
Posts: 3,599
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Re: 72 12 bolt 4 series to 3 series swap?
GM 12 Bolt Truck carrier breaks:
2.73 & Down 2.76–3.42 3.73 & Up
__________________
Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! |
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