The tag on your GM transmission tells you exactly what you have. Here's how to decode it.
Where to find the serial tag
- Stamped on case (sometimes)
- Riveted ID tag on bell housing or pan rail
- Some on side of case near solenoid pack
- Read our identify your transmission
Tag format (typical)
- Build date (Julian)
- Plant code
- RPO code
- Serial sequence
- Build line
Example 4L60E tag:
- ABCD-12345 → A=plant, BCD=date, 12345=sequence
RPO codes by trans
- M30 = 4L60E
- M32 = 4L65E
- M70 = 4L70E
- MT1 = 4L80E
- MN8 = 4L85E (HD 4L80E)
- MYC = 6L80
- MYD = 6L90
- M5U = 8L90
- M5N = 10L80
Read our 4L60E vs 4L65E
Build date decoding (Julian)
Julian date = day of year (1-365)
- 001 = Jan 1
- 100 = Apr 10
- 200 = Jul 19
- 365 = Dec 31
Combined with year letter (varies by plant) gives build date.
Why decode matters
Match correct rebuild kit:
- Year-specific frictions
- Read our best 4L60E rebuild kit
Match correct solenoids:
- Pre-OBDII vs OBDII different
Verify swap candidate:
- Right year for your project
- Read our LS swap transmission guide
Tools
- Strong flashlight
- Camera (to photograph tag)
- GM service manual for year (for full decode)
- AT-260 decoder app (Allison)
When tag is missing
- Use casting numbers on case
- Sometimes stamped on housing
- Last resort: count clutches and frictions to ID
Decoded examples
4L60E from 2002 Silverado:
- MYC plant: B (Buffalo)
- Build date: 230 (Aug 18)
- Year code: 2
4L80E from 1998 Suburban HD:
- MT1 plant: T (Toledo)
- Build date: 045 (Feb 14)
- Year code: W (1998)
What to do after decoding
- Order year/RPO-specific parts
- Verify with service manual
- Read our transmission parts buying guide
Need year/RPO-specific GM trans parts? Shop our catalog. Rebuild kits matched to your trans. Free shipping over $70.
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