There are several ways to tell what the type/size/application of a rearend. Besides visually checking and comparing rearends to one another, there are casting numbers, build codes and dates displayed or cast/stamped in various locations on the rearend and elsewhere. These all can be used to help ID a rearend.
 Casting numbers
Many if not all rear end housings will have some sort of casting number or ID cues to identify it. If the RPO decal is present (newer vehicles) or the broadcast or "build" sheet is available, it can show what was originally in the vehicle. Sometimes body/fender/cowl tags contain coded info that may list specifics on the rearend.
 Axle codes
Some sites with ratio and axle code info are:
- Axle Codes from novaresource.org
- Chevrolet Rear Axle Code Identification and Decoding from thecamaro.com
- Chevrolet Full Size and Impala Rear End/Axle Assembly Codes from from thecamaro.com
- Various tid-bits, et cetera from aplsweb.com
- GM A-body codes from tech.oldsgmail.com
- Gear ratio info from gnttype.org
- H-body axle codes from v8monza.com
Ford rear ends sometimes had tags under one of the carrier bolts that had the ratio and a code for being a LSD or not. Ford also used an axle code on the body tag on some models.
 Date codes
The date code format can vary, but sometimes the date code is a letter followed by two or three numbers. The letter is the month. The next number or two numbers will be the day of the month. The last number is the last digit of the year.
- Example: C124
- C = March (A = Jan., B = Feb., etc.)
- 12 = day of the month
- 4 = year ending in 4, i.e. 1964, 1974, 1984, etc.
 Axle dimensions
 Axle flange dimensions
 GM removable carrier type
The removable carrier rear end was used by GM for a number of years in passenger cars before GM started using the integral carrier rear end. The Pontiac/Olds versions from the later '50s were a drag racing mainstay for many years- even in front engined Top Fuel dragsters and Altereds.
 1955-'64 carrier casting numbers
3 series (2.73:1- up): 410408N
2 series (2.56:1- down):410409N The Chevy version used various tags to designate the ratio.
 Casting numbers and casting codes
- The casting number for a 1956 case should be 3707306
- The casting number for a 1957-'61 non-posi center case should be 3725899
- The casting number for a 1957-'61 posi center case is 3743833
- The casting number for a 1962-'64 posi center case is 3789812
The above are all casting numbers for passenger cars. These casting numbers are the same for Corvette rearends because the rearends were cast and assembled at just two facilities: the Detroit Gear and Axle plant and the Buffalo, NY plant. The only thing that would be different between the same year Corvette and a passenger car rearend center section would be the stamped (not cast) two letter code on the passenger side of the case. The code designates the gear ratio.
- Ford Rear Axle Assembly Identification Stamped axle codes
Ford used a vast array of rear ends over the years. Besides the codes stamped into the axle tubes, some carried ratio and/or posi tags as well as the vehicle having a plate containing coded info on the rearend. Below are photos of some of the more common rear ends for visual reference and comparison:
 Removable carrier
An easy way to tell the difference between the 8-inch and the 9-inch Ford differential is that on an 8-inch, a socket will fit on the lower nuts seen in this picture. Since the pinion centerline is lower on a 9-inch, a wrench is needed to gain clearance.
 8 inch
 9 inch
One of the strongest Ford rear end was the 9". Still one of the best rearends to modify or use in stock form to handle big horsepower.
 9-3/8 inch
Removable carrier. Found in some Lincoln, Mercury and Thunderbird.
 Non-removable carrier (Salisbury-style rearend)
 8.8 inch
 8.7 inch
Non-removable carrier. Found in Granada, Monarch 1977 to 1978.
 7.75 inch
Non-removable carrier. Found in Fairlane.
 7.5 inch
Non-removable carrier. Found in light duty applications such as six cylinder Falcon, Fairlane and 170 cid Maverick (Mavericks powered by 200 cid, 250 cid and 302 cid engines had the 8-inch differential).
 7.25 inch
Non-removable carrier. Found in Falcon, Econoline, Fairlane, Comet.
 7 inch
Non-removable carrier. Found in Falcon.
 6.75 inch
Non-removable carrier. Found in some Pinto and Mustang II.
 Columbia 2-speed rear end
More of an overdrive unit than an actual rear end. Included in the Ford section since it was offered as an option on '30's to '40's Fords.
 General Motors
 GM diff covers
 10 bolt
 10 bolt ID tips
If you don't have the codes on hand to identify if a 10 bolt rear end is an 8.5" or not, you can measure the distance between the very bottom bolt for the cover and the next adjacent bolt (either right of left), it will be 3-3/4" for a 8.5" and 3-1/4" for a 7.5".
Note: Some installation kits for the 8.5" and 7.5" use ring gear bolts with a 5/8" head, so if someone has rebuilt the rearend with one of these kits the bolts will have smaller heads than the original bolts.
Another tip is to check the size of the pinion nut. The 8.2" uses a 1-1/8 inch socket on the nut and the 7.5" and 8.5" use a 1-1/4 inch socket. That is as long as they have a OEM type nut on them.
There are usually two distinctive lugs cast into the bottom of the 10 bolt rear end housing. These lugs are arrowed in the image above.
The ring gear of the corporate 10 bolt is secured by 7/16-20 left hand thread bolts having a 3/4" hex head on them; there will be ten bolts holding the ring gear to the differential. The ring gear is (as mentioned) 8.5" diameter and the pinion shaft diameter is the same as the 12 bolt at 1.625". The 8.5" pinion has 30 splines.
The 8.2" diff will have 10 bolts holding the ring gear on the diff, the bolts will have a 9/16" hex head and be right hand threaded. The 8.2" uses ten 3/8-24 right hand thread bolts to hold the ring gear on the diff. The pinion diameter is 1.438", having 25 splines.
The 7.5" diff will have the ring gear secured with 10 bolts that are 7/16-20 LH thread, like the 8.5" corporate rear end. However, the ring gear diameter is smaller than the 8.5" differential.
 8.75 inch 10-bolt
This rearend was used in some mid-'70s Chevrolet station wagons, Impala, Caprice, some Chevelles, El Caminos, usually with larger engines and/or heavy duty options (towing, etc.). Also possibly found in some Pontiac LeMans, some 1977 Buick Regals and Cadillac models (needs confirmation).
Can be found with either 28 or 31 spline axles. It has a 10-bolt cover with indentions next to the axle tubes and 12 bolts holding the ring gear to the carrier (the bolts have a 3/4" hex head, right hand threads). The pinion nut is 1-1/4", it has 3" axle tubes.
This is a strong rearend, but unfortunately it was only used briefly and aftermarket support is poor.
 8.5 inch 10-bolt
Sometimes referred to as the "corporate 10-bolt", the GM 8.5" 10-bolt rearend began production in 1971 to replace the 8.2" and 12-bolt rearends. There was simultaneous use of the 8.2" and 8.5" rearends during the changeover period, like in the Nova. Has a 10 bolt cover as well as 10 bolts securing the ring gear.
The 8.5" 10-bolt GM rearend has a 30 spline pinion, uses C-clips, and could have 28 or 30 spline axles depending on the year. It uses a 1-1/4" pinion nut.
|Apollo: 1973 - 1975||Bel Air: 1970 - 1975|
|Blazer: 1979 - 1994||Bonneville: 1970 - 1986|
|C10 1/2 Ton (2WD): 1979 - 1987||C15/ C1500 1/2 Ton (2WD): 1979 - 1990|
|C1500 1/2 Ton (2WD): 1988 - 1990||Camaro: 1970 - 1981|
|Caprice: 1970 - 1996||Catalina: 1970 - 1981|
|Chevelle/Malibu: 1973 - 1973||Cutlas: 1970 - 1977|
|Cutlas: 1985 - 1987||El Camino: 1971 - 1976|
|Electra: 1971 - 1989||Firebird: 1970 - 1981|
|G10 1/2 Ton: 1979 - 1989||G15/ G1500 1/2 Ton: 1979 - 1988|
|G20 3/4 Ton: 1975 - 1989||G25/ G2500 3/4 Ton: 1978 - 1988|
|Grand Prix: 1971 - 1987||GTO: 1970 - 1977|
|Impala: 1971 - 1988||K10/ K1500 1/2 Ton (4WD): 1979 - 1988|
|K15/ K1500 1/2 Ton (4WD): 1979 - 1988||K5 Blazer (4WD): 1979 - 1991|
|LeMans: 1971 - 1983||Malibu: 1973 - 1983|
|Monte Carlo: 1973 - 1978||Nova: 1970 - 1979|
|Omega: 1973 - 1979||Phoenix: 1978 - 1979|
|Regal: 1973 - 1987||Regal: 1985 - 1987|
|Riviera: 1975 - 1978||Skylark: 1970 - 1972|
|Skylark: 1975 - 1979||Sonoma: 1994 - 1998|
|Suburban 1/2 Ton: 1979 - 1991||Suburban 1/2 Ton: 1979 - 1991|
|Suburban 1/2 Ton: 1979 - 1991||Suburban 1/2 Ton: 1979 - 1991|
|Trans Am: 1970 - 1981||Ventura: 1976 - 1977|
 8.2 inch 10-bolt
The 8.2" rearend was used through 1970 in the B-body (Impala) and F-body (Camaro) cars. The 8.2" was used through 1971 in the X-body (Nova) and through 1972 in the A-body (Chevelle).
|Bel Air: 1964 - 1972||Biscayne: 1964 - 1972|
|Camaro: 1967 - 1970||Caprice: 1965 - 1971|
|Chevelle/ Malibu: 1964 - 1972||Chevy II: 1964 - 1968|
|El Camino: 1964 - 1972||Impala: 1964 - 1972|
|Monte Carlo: 1970 - 1972||Nomad: 1968 - 1972|
|Nova: 1964 - 1972||Skylark: 1964 - 1969|
|Tempest: 1964 - 1970||Trans Am: 1969 - 1972|
|Ventura: 1971 - 1971||Wildcat: 1964 - 1972|
 10 bolt carriers
The corporate 10 bolt 8.5 inch rear end uses two different carrier series depending on the rear gear ratio:
- 2.56 and higher gears ("higher" gears have lower numbers, like 2.41, 2.29, etc.) go on the 2 series carrier
- 2.73 and lower ("lower" gears = higher numbers, like 3.08, 3.23, etc.) go on the 3 series carrier
A 3-series gear can be installed on a 2-series carrier by using a ring gear spacer, but this is not advised for stick shift applications, or if there's a lot of traction, HP and/or weight involved.
 8.5" non C-clip rearend
The 1971-'72 Buick GS/Skylark and Olds Cutlass A-body 8.5 10 bolt did not have C-clip axles.
The non C-clip 8.5" 10 bolt, type "C" rearend was used in some 1969-'72 Pontiac GP and 1970-'72 Monte Carlo.
There have been non C-clip rearends found in Chevelle wagons through at least 1974.
 12-bolt cover/10-bolt ring gear
This "O" (Olds) rearend was used in the Olds Cutlass A-body from 1968-'70. It was the rearend that had a 12 bolt cover, but with a 10 bolt 8.5" ring gear. This is not the same as the corporate 8.5 rearend, and there is no parts interchange between the two rearends.
 12-bolt car
- Ring gear diameter: 8.875"
- Cover bolts: 12
- Ring gear bolts: 12
- Axle spline count: 30
- Pinion spline count: 30
- Carrier Breaks: 2.76-down / 3.07 / 3.90 / 4.10-up
- Pinion Nut Size: 1-1/8"
Pinion yokes interchange between the 12-bolt car and truck. Originally the 12-bolt rearend used U-joint sizes 1310, 1330 or GM 3R series.
|Bel Air: 1964 - 1972 (Rear)||Biscayne: 1964 - 1972 (Rear)|
|Camaro: 1967 - 1972 (Rear)||Caprice: 1966 - 1970 (Rear)|
|Chevelle/ Malibu: 1964 - 1972 (Rear)||Chevy II: 1964 - 1968 (Rear)|
|El Camino: 1965 - 1972 (Rear)||GTO: 1970 - 1972 (Rear)|
|Impala: 1964 - 1972 (Rear)||Monte Carlo: 1970 - 1972 (Rear)|
|Nomad: 1968 - 1972 (Rear)||Nova: 1964 - 1972 (Rear)|
|Trans Am: 1969 - 1972 (Rear)||Wildcat: 1964 - 1972 (Rear)|
- 1310 series, (aka "small Chevy") has 1.063" diameter caps, 3.219" measured across to lock ring points (1-1/16" x 3-7/32"), uses outside clips. Was OEM for many 1967-74 GM cars.
- 1330 series pinion yokes use locating tabs to center the U-joint, has 1.063" diameter caps, 3.625" measured across to lock ring points (1-1/16" x 3-5/8"), uses outside clips. Used on heavy-duty/high performance/big engine applications.
- GM 3R series is the "inside C-clip", or "inside lock" style. Used ~1975-up. It does not use locating tabs. It has 1.125" diameter caps, 2.5625" measured across caps (1-1/8" x 2-9/16"), aftermarket/replacement U-joints uses inside clips, originals use plastic injection.
On the 12-bolt, the hardware used to hold the U-joint into the pinion yoke was either two 5/16" U-bolts, or 4 bolts and 2 metal straps. Both styles were used on original equipment applications. The U-bolts are the preferred method, if for no other reason than if either were to fail the U-bolt type can be easily replaced while the strap type might break off the bolt inside the yoke, making extraction difficult.
The yokes may be different for straps or U-bolts. The strap yoke (if smaller) can be drilled out to use U-bolts, but in some cases this leaves too little metal around the holes. A better approach is to use a yoke made for U-bolts in the first place.
U-joint girdles (right) can be used to replace the original hardware. Be sure to match the diameter of the fasteners to the hole size of the yoke.
AMC axle codes are stamped on the differential casting between the cover and the tube weld hole on the passenger side) beginning in 1970 for the AMC 20 series rearend, 1972 for the AMC 15 series. First four letters/ratios are 1970 rear axle codes stamped on the differential housing cover flange at about 3 o'clock:
A = 3.54:1 B = 3.15:1 C = 2.87:1 D = 3.91:1 E = 3.58 (12/43) F = 3.08 (13/40) G = 3.31 (13/43) H = 2.73 (15/41) J = 2.37 (19/45)
The 15 series had 10 bolt oval cover with drain plug center right.
 15 series rearend
The AMC 15 series rearend has a 7-9/16" ring gear. It was used in 1965 and earlier models with 196 ci or 199 ci inline 6 engines. The small cars (cars other than those listed below as "big car") used the AMC 15 rearend with inline 6 engines, AMC 20 rearend was used with V8 engines. The AMC 20 rearend could be ordered in fleet cars with inline 6 engines.
 20 series rearend
The big cars (Ambassador, Classic, Rebel, Matador and Marlin) used the AMC 20 rearend with 232 ci inline 6 and bigger engines.
The series 20 was used in Jeeps as well as AMC passenger cars. A Jeep 20 series rearend would have a 5 on 5" bolt pattern. Passenger cars used the 5 on 4-1/2" wheel bolt pattern.
 GM RPO axle codes
|G80||Axle Positraction, Limited Slip||
|G81||Positraction Rear Axle||
|G86||Axle , Rear, Limited Slip||
|G87||Ring Gear, 8.50 Inch||
|G89||Ring Gear, 7.50 Inch||
|G91||Special Highway Rear Axle||
|GH4||Axle Rear (2LS V6 Camaro)||
|GT4||Axle Rear (DUP WITH 5 X 1)||
|GT5||Axle Rear (DUP WITH GT8)||
|GT8||Axle Rear (DUP WITH GT5)||
|GW9||Axle Rear (DUP WITH GU3)||
|GX3||TransAxle Final Drive||
|GX8||TransAxle Final Drive||
|GY3||TransAxle Final Drive||
|GY7||TransAxle Final Drive||
 GM ratio to tooth count
|Axle Ratio||Tooth Combination|