The factory sealed beam headlights on most OBS trucks are underwhelming. Swapping to a composite housing is one of the most practical upgrades you can do — better output, more bulb options, cleaner look. Here’s how it works.
Sealed Beam vs. Composite – What’s the Difference?
Sealed beam: The entire headlight assembly is one sealed unit — the lens, reflector, and bulb are fused together. When the bulb burns out, you replace the whole unit. Most OBS trucks came with 4x6" rectangular or 7" round sealed beams depending on year and trim.
Composite: The housing is separate from the bulb. You replace just the bulb (usually H4/9003), and the housing stays. This allows for halogen, LED, or HID bulb options.
What Fits GMT400 (1988–1998)
1988–1993 (first gen): Factory 4x6" rectangular sealed beam setup. Many owners swap to aftermarket composite housings or 7" round conversion units.
1994–1998 (second gen): Factory used rectangular composite housings on higher trims, sealed beams on base models. The composite swap is especially clean on these because the mounting provisions are already similar.
Verify before ordering: Pull your current headlight and check the mounting tabs. Some years differ.
Bulb Options After the Swap
| Bulb Type | Notes |
|---|---|
| Standard halogen (H4/9003) | Direct replacement, inexpensive |
| High-output halogen | Better output, no wiring changes needed |
| LED conversion (H4) | Good output — verify beam pattern quality before buying |
| HID retrofit | Best output but requires ballasts and proper projector housing |
Important: Cheap LED H4 bulbs in a reflector housing often scatter light in all directions and reduce visibility rather than improving it. If you’re going LED, use a well-reviewed brand or pair with a projector housing.
The Swap – What It Takes
30–45 minutes per side on most GMT400 trucks.
Steps:
- Open the hood and remove the headlight retainer ring (4 screws on most OBS)
- Pull the sealed beam out, disconnect the plug
- Set the new composite housing in the bucket, connect the wiring
- Secure the retainer ring
- Aim the lights — check beam pattern on a wall before driving
Wiring note: You may need new socket pigtails if the factory plugs don’t match the new housing. Check before you start.
Aiming: Set the hot spot of the beam level with or slightly below headlight center height, measured on a wall 25 feet away with someone in the driver’s seat.
Drop your year and what you’re running or planning to run. Happy to help narrow down what fits and what bulbs actually work.