Staring at an unknown transmission and wondering what you’ve got? Whether you’re restoring a classic Camaro, swapping a crate engine, or just trying to buy the right rebuild kit, correct Chevrolet transmission identification saves you time, money, and a whole lot of frustration. This guide walks you through every major GM gearbox — automatic and manual — from the Powerglide to the 10-speed. Stick around, because the details matter.
Start Here: The Fastest Way to ID Any GM Automatic
The quickest starting point for Chevrolet transmission identification is always the fluid pan. GM changed pan shapes constantly to fit new valve bodies, overdrive gearsets, and electronic solenoids. Flip it over, count the bolts, and note the shape. That one step narrows your options fast.
Here’s a quick-reference pan ID chart:
| Transmission | Pan Bolts | Pan Shape |
|---|---|---|
| Powerglide (Aluminum) | 14 | Sideways/irregular |
| TH350 | 13 | Nearly square, one angled corner |
| TH400 | 13 | Asymmetrical “Texas” shape |
| 700R4 / 4L60E | 16 | Rectangular |
| TH200-4R | 16 | Long “boat” shape |
| 4L80E | 17 | Oval |
| 6L80 / 6L90 | 18 | Rectangular, ribbed |
| Allison 1000 | 12 | Deep, metric bolts |
Pan shape gets you close. Stamped codes and casting numbers confirm it.
Classic Two-Speed and Three-Speed Automatics
Powerglide
GM switched the Powerglide from cast iron to a lightweight aluminum case in 1962. That aluminum unit is still a drag racing staple today because it’s light, simple, and tough under shock loads. The 14-bolt pan is your visual tell. Pre-1964 units use a 16-spline output shaft, while later ones upgraded to 27 splines. Overall length runs 25.37 inches (short shaft) or 27.56 inches (long shaft).
TH350
The TH350 is one of the most common GM transmissions ever built. Its 13-bolt, nearly square pan has one distinctly angled corner — that’s your signature feature. It uses a vacuum modulator at the rear of the case and a kickdown cable on the passenger side near the dipstick tube. Output shaft is 27 spline, and length ranges from 27.68 to over 33 inches depending on the tailshaft housing.
TH400
The TH400 also uses a 13-bolt pan, but the shape is where it differs. That pan looks irregular and bulgy on the passenger side — some people call it the “Texas” shape. Instead of a pull cable, the TH400 uses an electronic kickdown switch. The vacuum modulator sits on the side of the case, not the rear. Internally, it has a heavy 32-spline output shaft, which makes it a favorite for high-horsepower builds.
Watch out for the TH375 variant — it’s a TH400 internals built to fit a smaller driveshaft yoke. It has a 27-spline output and a tailhousing cast with “375-THM.”
TH250 vs. TH350: Easy Mix-Up
These two look similar at a glance, but the TH250 has a band adjustment stud and locknut on the passenger side of the case. The TH350 doesn’t. That single detail settles the debate every time.
Overdrive Era Automatics: 700R4 Through 4L80E
700R4 and 4L60/4L60E
The 700R4, introduced in 1982, brought a steep 3.06:1 first gear and a 0.70:1 overdrive. It uses a 16-bolt rectangular pan and relies entirely on a mechanical Throttle Valve (TV) cable — no vacuum modulator anywhere. Getting that TV cable adjustment wrong destroys the transmission, so identify it correctly before you touch anything.
In 1990, GM renamed it the 4L60 (4 speeds, longitudinal, 60-series rating). The transmission stayed mechanically the same.
Then in 1993, the 4L60E arrived with full electronic control. No TV cable at all. Instead, look for a large circular electrical connector on the right side of the case and a vehicle speed sensor on the tailshaft. A 1996 update moved the stamped ID number to the bottom of the case.
The biggest visual change came in 1997 — GM split the case into a two-piece design with a removable bellhousing. Earlier 4L60E units have a one-piece case. That detail matters when you’re sourcing a replacement.
Variants like the 4L65E, 4L70E, and 4L75E look identical on the outside. Internal inspection or VIN decoding is the only way to tell them apart.
4L80E
The 4L80E is the electronic successor to the TH400 — bigger, heavier, and built for serious torque. It measures 26.4 inches long and weighs 236 pounds dry. The 17-bolt oval pan is unmistakable. It uses a 32-spline output shaft and has two speed sensors — one on the input, one on the output — so the TCM can monitor clutch slip in real time.
Modern Six-Speed to Ten-Speed Automatics
6L80 and 6L90
The 6L80 ditched traditional bands entirely and uses clutch-to-clutch shifting for all gear changes. Its 18-bolt ribbed pan is the visual ID. The standout feature is the Transmission Electro-Hydraulic Control Module (TEHCM) — it lives inside the pan on the valve body, not outside on the case. That internal computer learns your driving habits and adapts shift timing over time.
The 6L90 shares the same exterior pan but runs about 1.4 inches longer. Output shaft splines differ too: the 6L80 uses 32 splines, the 6L90 uses 29 splines in 4WD or 36 splines in 2WD. That spline count difference matters significantly when you’re matching a driveshaft yoke.
8L90E and 10L80/10L90
These late-model units are deeply integrated into modern chassis electronics. Visual identification isn’t practical here. Your best move is decoding the Regular Production Option (RPO) codes in the glovebox or on the door jamb sticker. One important maintenance note: the pan bolts on the 8L90 and 10L90 require grade 10.9 steel fasteners. Don’t substitute standard hardware — internal hydraulic pressures will push past it.
Allison 1000
The Allison 1000 uses a deep 12-bolt pan with M10x1.5 metric fasteners. It was a five-speed from the factory, upgraded to six speeds in 2006. The interesting part? The internal clutch packs didn’t change — the six-speed conversion happened entirely through valve body revisions and TCM reprogramming. It also features PTO provisions on the case for auxiliary equipment like hydraulic pumps.
Gear Ratio Reference: GM Automatic Transmissions
Gear ratios matter when matching a transmission to an engine and rear axle. Here’s the full breakdown:
| Transmission | 1st | 2nd | 3rd | 4th | 5th | 6th | Reverse |
|---|---|---|---|---|---|---|---|
| Powerglide | 1.82 | 1.00 | — | — | — | — | 1.82 |
| TH350 | 2.52 | 1.52 | 1.00 | — | — | — | 1.94 |
| TH400 | 2.48 | 1.48 | 1.00 | — | — | — | 2.07 |
| TH200-4R | 2.74 | 1.57 | 1.00 | 0.67 | — | — | 2.06 |
| 700R4 / 4L60E | 3.06 | 1.62 | 1.00 | 0.70 | — | — | 2.29 |
| 4L80E | 2.48 | 1.48 | 1.00 | 0.75 | — | — | 2.07 |
| 6L80 / 6L90 | 4.02 | 2.36 | 1.53 | 1.15 | 0.85 | 0.67 | 3.06 |
Decoding Automatic Transmission Stamp Codes
Pan shape gets you in the ballpark. Stamped codes get you to the exact unit.
Powerglide: Early units use a format like “C531D” — plant code (C = Cleveland), date (May 31), and shift (D = day). Post-1967 units shift to a format like “P9E31” — Powerglide (P), 1969 (9), May (E, using A–M and skipping I), 31st day.
TH350: Codes appear on the passenger side of the oil pan flange. A string like “16B123456” means Chevrolet division (1), model year 1976 or 1986 (6), Baltimore plant (B), and a six-digit serial number.
TH400: The partial VIN stamps onto the driver-side pan rail. A riveted metal tag on the passenger side carries a two-letter calibration code (like “CR” for a 1970 454), a Julian date (001–365), and the transmission serial number.
4L60E: Numbers moved to the bottom of the case by 1996. A code like “3KCDY12345678” breaks down as: model year (3), engine (KC = 5.3L), transmission (D = 4L60E), plant (Y = Toledo), serial number.
Manual Transmission Identification: Muncie, Saginaw, and Borg-Warner
Manual transmissions don’t have pans to look at. Instead, you’re checking the side cover bolt count, case material, reverse arm location, and input shaft groove patterns.
Key Visual Differences
The Muncie is all aluminum — case, cover, tailhousing. It doesn’t rust. Seven bolts on the side cover. Pre-1969 covers have a threaded stud for the shifter linkage; 1969 and later use a bolt.
The Saginaw is all cast iron and will be rusty. Also seven bolts on the side cover, but the reverse shifter arm mounts directly on the side cover itself. On the Muncie and Borg-Warner, the reverse arm is on the tailhousing.
The Borg-Warner T10 and Super T10 have a nine-bolt side cover with a curved bottom edge. Materials vary — often a cast-iron case with aluminum side cover and tailhousing.
Reading Input Shaft Grooves
You can’t see gear ratios from outside the case. GM solved this on the factory floor by machining rings into the input shaft splines. Count the grooves and you know the ratios.
Saginaw groove count:
- 0 grooves: 2.84:1 first gear
- 1 groove: 2.54:1 first gear
- 2 grooves: 3.11:1 first gear
- 3 grooves: 3.50:1 first gear
Muncie groove count:
- 2 grooves: M20 wide-ratio (1966+)
- 1 groove: M21 close-ratio
- 0 grooves: M22 “Rock Crusher”
The M22 earns its nickname from its straighter tooth cut — roughly 20 degrees versus 30 degrees on the M20/M21. That angle reduces case flex under extreme torque but creates a mechanical whine during operation.
Borg-Warner Super T10 groove count:
- 2 grooves (T10-X): 2.43:1 first gear
- 3 grooves (T10-W): 2.64:1 first gear
- 5 grooves (T10-U): 3.42:1 first gear
- 6 grooves (T10-Y): 2.88:1 first gear
Decoding Muncie Serial Numbers
Every Muncie serial number starts with “P” for the Muncie, Indiana plant. GM skipped “M” because it already identified Muncie three-speeds. From 1963–1966, the format was plant + four-digit date (P0701 = July 1). From 1967 onward, a year digit and alphabetical month code were added, skipping I, O, and Q. “P8T13” means Muncie plant, 1968 model year, December, 13th day. From 1969 onward, a suffix letter confirms the ratio: A = M20, B = M21, C = M22.
Casting numbers further narrow the production era. Early 1963–1964 cases carry numbers 3831704 or 3851325 and lack a drain plug. The 1965 upgrade to casting 3885010 added a larger one-inch cluster shaft for big-block durability. Final-gen cases from 1968–1974 used 3925660 and 3925661.
Heavy-Duty Truck Manual Transmissions
SM420 and SM465
The SM420 ran from 1947 to 1967. It’s compact at 10.7 inches, with a massive 7.05:1 first gear. Look for the bulbous protrusion on the passenger side housing the reverse idler gear and a PTO port on the driver side. Eight bolts on the cast-iron top cover.
The SM465 replaced it in 1968 and stayed in production until 1991. It’s larger at 12 inches and about 175 pounds. Heavy vertical ribbing covers the iron case, and it has dual PTO ports — one on each side. First gear is 6.55:1. Medium-duty commercial trucks got a heavier 1.5-inch input shaft versus the standard 1.125-inch unit.
NV3500 and NV4500
The NV3500 is easy to spot — the bellhousing is cast directly into the aluminum case. It can’t be removed. Look for the waffle-pattern exterior texture.
The NV4500 is the heavy-duty sibling. Cast-iron main case, aluminum top cover, and a removable bellhousing. It weighs 195 pounds and measures 12.37 inches. Early GM versions (1993–1994) have a 6.34:1 first gear; later units use 5.61:1. The NV4500 uses carbon-fiber synchronizers that require a specific synthetic gear oil — using the wrong fluid will destroy them.
Manual Transmission Spline and Dimension Reference
| Transmission | Input Splines | Output Splines | Case Length | Material |
|---|---|---|---|---|
| Saginaw 3/4-Speed | 10 or 26 | 27 | 9.62 in | Cast iron, 7-bolt cover |
| Muncie M20/M21 | 10 or 26 | 27 or 32 | 10.5 in | Aluminum, 7-bolt cover |
| Borg-Warner T10 | 10 or 26 | 27 or 32 | 10.0 in | Mixed, 9-bolt cover |
| SM420 | 10 | 10 or 35 | 10.7 in | Cast iron, passenger-side bulge |
| SM465 | 10 | 10, 32, or 35 | 12.0 in | Heavy ribbed iron, dual PTO |
| NV3500 | 10 or 26 | 27 or 32 | Varies | Integrated aluminum bellhousing |
| NV4500 | 10 | 32 | 12.37 in | Iron case, aluminum top cover |
| Tremec T56 | 26 | 27 or 32 | 29.8–35.0 in | Aluminum, double overdrive |
| Tremec TR6060 | 26 | 31 | 25.5–33.0 in | Multi-cone synchronizers |
Modern Performance Six-Speeds: T56 and TR6060
The Tremec T56 and TR6060 are the go-to six-speeds for modern LS and LT performance builds. Both use a 26-spline input shaft. The TR6060 steps up to a 31-spline output — that extra spline count eliminates torsional shearing failures common in older 27-spline setups under big horsepower.
Triple-cone synchronizers on gears one through four make these units shift smoothly under load. Double-cone units handle fifth, sixth, and reverse.
Length varies by application. A standard F-body T56 runs 31.8 inches. The GTO and CTS-V applications stretch it to 33 and 35 inches respectively due to remote shifter linkage setups. The CTS-V also uses a fixed output flange instead of a slip yoke, which pairs with a two-piece driveshaft to cut vibration at high speeds.
VIN Decoding for Provenance Verification
Matching the partial VIN stamped on the transmission to the chassis VIN is the final step in confirming a numbers-matching vehicle. GM’s VIN formats changed across decades and platforms.
Tri-Five Chevrolets (1955–1957): The VIN sits on the left front door hinge pillar post. A “V” prefix confirms a V8 engine. Series codes follow — A for 150 models, B for 210, C for Bel Air, D for Sedan Delivery. Then comes the model year (55, 56, or 57), plant code, and a six-digit sequential production number.
Nova (1962–1979): The plate moved from the door jamb to the upper dash panel in 1968. From 1972 onward, the fifth digit is an explicit engine code — D for the 250 cubic inch, H for the 350 two-barrel, L for the 350 four-barrel. That engine code directly points to the transmission it left the factory with.
Camaro and Firebird (1967–2002): Early F-body units used the door jamb; post-1968 units moved to the dash. The third digit in 1967–1971 Camaros tells you engine geometry — 3 for six-cylinder, 4 for eight-cylinder. From 1972 onward, the fifth digit carries specific engine codes. For Firebirds between 1972 and 1980, that digit identifies the exact displacement and tune — an X, for example, identifies the legendary 455 Super Duty. By the third-generation F-body starting in 1981, the eighth digit became the universal GM engine code, locking down exact powertrain configuration.
Accurate Chevrolet transmission identification combines pan geometry, bolt counts, casting numbers, input shaft grooves, and VIN decoding into one clear picture. Whether you’re chasing numbers-matching originality or building a purpose-built drivetrain, getting the identification right first means everything else goes smoothly.
