GM manual transmission casting numbers are factory identifiers stamped into the trans case and related components during production. Reading these numbers accurately helps you verify gear ratios, axle limits, build year, and original equipment fitment details.
This guide walks through how to locate, decode, and document GM transmission casting numbers with a focus on practical accuracy for identification and restoration projects.
| Component | Typical Location | Example Format | Key Information Provided |
|---|---|---|---|
| Transmission Case | Left side, rear extension housing area | 1234567 | Base model, plant code, basic configuration |
| Extension Housing | Front face near bellhousing | AB 7890 | Input shaft length, clutch type, year code |
| Transmission Pump | Front cover plate | P 4567 | Pump family, displacement, match code |
| Torque Converter | Stamped on hub or weldment | TC 890123 | Stall characteristics, spline count, balance status |
Identifying Transmission Case Castings
Locate the main casting number on the transmission case pad, typically found on the driver side just behind the bellhousing flange. Use a stiff wire brush and angled light to reveal faint stampings that may be obscured by rust or paint. Record the full string, including any leading zero, plant letter, and date stamp, without altering the mark.
Cross reference the case number with factory build sheets or GM part numbers to confirm axle ratio, gear set type, and application fitment. Misidentified cases can lead to incorrect rebuild parts, so verify against the transmission tag when available.
Reading Extension Housing and Date Codes
Location and Surface Prep
Clean the extension housing pad using a soft wire wheel and solvent to expose the original metal before attempting to read stamping. Avoid aggressive grinding that can remove critical characters.
Format and Meaning
Extension codes often include a letter for year, a pair for plant, and a numeric sequence for suffix or clutch type. Matching this to GM service information clarifies overdrive, lockup torque converter, and spline details.
Pump and Rotor Markings for Verification
The pump casting and rotor hubs contain family codes that indicate dimensional series, clutch diameter, and internal components. Align pump and case numbers during teardown to ensure rebuild kits match the original design.
Incorrect pump family selection changes pressure, capacity, and shift feel, so confirm part compatibility using both the pump casting and the transmission case casting together.
Torque Converter and Spline Details
Stamped markings on the torque converter hub identify stall range, balance treatment, and spline count. Compare these markings against the transmission input shaft splines to prevent engagement or vibration issues.
Use a dial caliper or pin gauge to confirm spline count visually, then cross check with the stamped torque converter code for full matching before final assembly.
Key Takeaways for GM Manual Transmission Casting Numbers
- Always record the full casting string, including leading zeros and plant letters
- Cross reference case, pump, and extension codes with GM rebuild references
- Verify spline count on the input shaft and torque converter hub before assembly
- Use matched components based on casting numbers to ensure correct shift performance
- Document findings with clear photos and notes for future service or sale
FAQ
Reader questions
How do I differentiate a 1972 TH350 from a 1979 TH350 using casting numbers?
Check the extension housing date code and case pad year identifier; the 1972 unit typically shows an 'F' year prefix and earlier plant codes, while the 1979 version carries an 'H' prefix and updated suffix formats.
Can incorrect pump casting cause shifting problems in a rebuilt GM automatic?
Yes, using a pump from a different family alters porting and governor response, leading to harsh shifts, slip, or delayed engagement that requires pump replacement to resolve.
What should I do if the casting number is partially erased on a used transmission?
Clean gently with a non abrasive brush and apply a dye penetrant, then photograph and compare against GM reference diagrams to recover as much of the original number as possible.
Why does the torque converter stamp not match the case year code?
Torque converters are often assembled from leftover stock, so the stamp may reflect an earlier build date; rely on the case casting number for true vehicle fitment verification.