* Wrong drum size: The most common reason. S10s used different size drums over the years and even between models (e.g., different wheelbases or options). You may have a drum intended for a different vehicle or even a different year S10. The diameter and width must match precisely.
* Incorrect backing plate: The drum fits *on* the backing plate. If the backing plate is bent, damaged, or from a different vehicle, the drum won't fit correctly, even if it's the right size. The wheel cylinders and other components need to align perfectly.
* Rust and corrosion: Years of rust and corrosion can warp the backing plate or the drum itself, preventing a proper fit. Cleaning and inspecting for damage is crucial.
* Improper installation: The drum needs to be installed correctly, including proper alignment and seating. Forcing it will damage the brake system.
* Wrong part number: You might have purchased the incorrect drum. Double-check the part number against your vehicle's specifications.
Before attempting anything further:
* Verify the part number: Make absolutely certain the drum you have is the correct part for your specific 1994 Chevy S10. Check your owner's manual, a parts catalog (like AutoZone or Napa online), or a reputable online parts retailer.
* Inspect the backing plate: Carefully examine the backing plate for any damage, bending, or corrosion. This is a critical component for proper drum fit.
* Check for seized components: If parts are seized, you may not be able to install the drum. This may require specialized tools or professional help.
If you've checked all of these and the drum still doesn't fit, seek assistance from a qualified mechanic. Improperly installed brakes are extremely dangerous.
