* Transmission type: Both years used variations of the 700R4 automatic transmission, but there were internal differences across the model years. A direct swap *might* work, but it's not certain without verifying the exact transmission codes.
* Bolt patterns: The transmission case bolt patterns should be the same, but double-checking is crucial.
* Input shaft: The input shaft diameter and spline count must match the 1987 S-10's transfer case and engine.
To know for sure:
1. Find the transmission codes: These are usually stamped on the transmission itself. Compare these codes between the 1984 and 1987 transmissions. A parts catalog for each year will help decipher the codes.
2. Compare physical dimensions: If possible, physically compare the transmissions side-by-side to verify that the bolt patterns and input shaft match.
Without this information, attempting a direct swap is risky. You might find that the transmission won't bolt up, the input shaft doesn't engage properly, or other issues arise. A professional mechanic can assess compatibility, but it is likely more cost-effective to simply source a suitable transmission for the 1987 model year.
