The first letter is the month of the year; A is January, B is February, and so on. These should be square blocks, each with the outer side 90 degrees (vertical) to the road and the bottom-side surface horizontal to the road. M-21 close ratio 66-70 . Axles for high-performance or heavy-duty applications commonly used higher spline-count axles while common passenger car axles use lower spline counts. This design was used in all GM rear-drive cars from 1964 through 1972. Big-block Chevelles, Camaros, and other GM high-performance vehicles were fitted with the 12-bolt limited-slip axle to maximize torque transfer and traction. You can copy and paste this link to share: https://www.chevydiy.com/history-and-identification-of-chevy-10-and-12-bolt-chevy-differentials/. It is driven by the input shaft with 21 teeth. The axles are retained by a set of C-clips on the inner end of the axle shaft inside the carrier. These are the numbers on my ring gear. Camaros, Novas, and 1968 and later trucks used leaf springs like these. The nomenclature actually refers to the number of ring gear bolts. A pair of long flat areas on the front side of each axle tube is a clear indicator of an 8.5-inch Chevy 10-bolt. Note the thicker head on the bottom axle where the C-clip rides. Trucks have a smaller inner pinion shaft (1.438 inches versus 1.675 inches) and bearing, and the pinion rides lower on the ring gear. The Chevy’s rear axle code contains information on the vehicle’s gear ratio, date of manufacture and the rear axle assembly plant. The identification code will tell you the gear ratio, if it is a posi, and in some cases what engine and transmission the car is equipped with. The front bearing retainer index is 5.60”, and has a … M-21 close ratio 63-65 . Option code list. Ford Rear Axle Assembly Identification Stamped axle codes Ford used a vast array of rear ends over the years. Essentially, the housing is machined to clear the larger gears and carrier, and it’s not a job for the novice. The axles are retained by a set of C-clips on the inner end of the axle shaft inside the carrier. The M-22 is distinguished from an M-21 by the angle of the gear teeth. As you can see, the Chevy 12-bolt differential is one stout axle, and it was the rear axle of choice for GM muscle cars and many GM competition cars. The 8.5-inch differentials have 103/4-inch hex head bolts with 7/16-20-inch left-hand thread or reverse-thread bolts that hold the ring gear to the carrier. For 1969 and later, the code typically features six to eight digits, including three letters, three numbers, and sometimes an additional number and letter. Click the button below and we will send you an exclusive deal on this book. The aftermarket fully supports the 8.5. General Motors has been using this axle assembly in cars for 16 years and in 1/2-ton trucks for 30 years. Looking up the GT4 code via Google, or any GM reference literature, the code shows your truck has a 3.73 gear ratio. Most 8.5-inch 10-bolts have two lugs on the bottom of the housing at the 5 and 7 o’clock positions. The casting numbers for 10-bolt rear differentials are typically located either on the forward side of the passenger-side axle tube or on the driver’s side. The 1969-1976 Camaros, 1964-1976 Chevelles, Malibus and Monte Carlos used an optional quick ratio 12.7:1 unit. For example, a 12-bolt axle that was built on March 28, 1967, is C287. There is some crossover on the trucks, as some earlier trucks had leaves and some later trucks had the trailing arms. There are temporary fixes for the 8.2, such as a carrier girdle, but they don’t provide a reliable and suitably strong solution. In addition, the trucks typically have larger axles and brakes. Chevy Muscle Car Tech Tips, Guides, Articles & How-To. A better choice for handling is the 12.7:1 ratio boxes found on some of the higher performance factory muscle cars from the 70s and 80s. It is more difficult to swap these housings from car to car if they do not share the same suspension design. The rear axle identification code consists of a two letter prefix indicating the gear ratio ~ four digits indicating the assembly date as month/day ~ and a single letter suffix indicating the manufacturing location. Locate a Sierra Gear & Axle Distributor near you. The casting numbers are simple to decode. It remained in the 1/2-ton trucks until 1999, when the 8.6 replaced it, using the same basic design. 2429 Mercantile Drive Suite A Gear Ratio 1st, 2nd, 3rd, 4th: Spline Input: Grooves Input: Cluster Pin : Tooth Count Input: Tooth Count 4th Cluster: Output Spline: M-20 wide ratio 63-65 M-20 wide ratio 66-70 . The 2010-up Camaro uses a similar design (8.6 10-bolt) in the center section of its independent rear suspension. From 1973 - 1980 Chevy trucks had a 13 digit VIN#. A Yukon aftermarket clutch-type limited-slip differential is on the left; the GM Posi-Traction differential from a 1971 Buick Gran Sport 8.5 10-bolt is on the right. In this example (Figure 3), GT4 is the axle ratio code. A variant of this axle assembly was used in 1971–1972 Buick GSs and Skylarks, Oldsmobile Cutlasses, and some 1969–1972 Pontiac Grand Prixs, as well as the 1970–1972 Monte Carlos. When it comes to GM muscle cars and sports cars, the 12-bolt axle has been the top high-performance axle assembly for decades. Also the 55-62 Corvette standard ratio was 3.70 as the 55-56, probably because of … The M-22 was generally installed behind severely powerful (high torque) big-block engines. Need help with your differential identification? If you take off the center section cover and discover this kind of damage, you need to identify the cause so you don’t repeat this type of failure. These are considered highway gears that are good for top speed, not for off-the-line performance. This is specific to the carrier. (Accurate identification using the GM RPO code assumes that the rear end has not been previously modified) The prefix is usually F, G or H when referring to Axle Identification. You need to machine the housing to accept a 12-bolt carrier, which also requires custom axles. The 8.2-inch differential does not have these lugs. All GM 12-bolts use C-clip–style axles. The passenger car 12-bolt has an oval-shaped differential cover, and it measures 1015/16 x 105/8 inches. However, the smaller 10-bolt unfairly gained the reputation as a weak and inadequate rear end for high-performance applications. Thanks Location: Main Index » Forum Index » 55-57 Central » 1955-1957 Chevys (Stock / restored / show) » Gear Ratio Identification FusionBB™ Version 2.1 ©2003-2006 InteractivePHP, Inc. In this version, the axles bolt to the housing ends just as on a Ford 8- or 9-inch. Buick and Oldsmobile bolt-in axles mount at the bearing flanges on the housing ends. The spline of the input is 1-1/4” diameter 10 spline and protrudes approximately 7-1/2” from the face of the transmission. Most passenger car 12-bolts used a four-bar trailing arm mounting system, with the exception of the Camaro and Nova, which used leaf springs. Once the C-clip has been removed, the axle slides out of the housing. The truck housings are not as durable as the passenger car housings due to a narrower carrier and a smaller inner pinion bearing. Therefore, the specified pinion and ring gears must be used together. Was introduced in 1964 and installed in cars and trucks for 30 years installed severely. 10-Bolt housings ( numerically lower ) such as 2.41 more difficult to swap housings. 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