Windshield and Glass Replacement - GM Square Body Trucks (1973-1987)
Overview
Windshield and glass replacement is a critical restoration and maintenance task for 1973-1987 GM Square Body trucks. These vehicles utilized different glass installation methods throughout their production run, transitioning from rubber gasket systems to urethane adhesive bonding. Proper glass replacement not only ensures visibility and weather sealing but also contributes to the vehicle's structural integrity, particularly important in rollover scenarios for K5 Blazers and other utility models.
This article covers windshield replacement, side glass installation, rear glass (where applicable), and vent window service for all Square Body variants including C10, C20, K10, K20, K5 Blazer, and Suburban models. Understanding the specific requirements for your year and model is essential, as GM made significant changes to glass installation methods during the 1980-1981 model years.
Common Issues
Windshield Problems
- Rubber gasket deterioration (1973-1980 models): Original rubber gaskets become hard, cracked, and lose sealing properties after 40+ years
- Urethane adhesive failure (1981-1987 models): Old urethane becomes brittle, causing leaks and potential windshield detachment
- Rust around windshield frame: Common in A-pillar areas and cowl, particularly on models without factory rust protection
- Incorrect glass installation: Many replacement jobs use wrong adhesive or gasket types for the specific year
Side Glass Issues
- Vent window seal failure: Rubber seals deteriorate, causing wind noise and water intrusion
- Window regulator problems: Worn regulators cause binding or complete failure of window operation
- Glass channel wear: Felt-lined channels become compressed or torn, allowing glass to bind or rattle
- Quarter window problems (Blazer/Suburban): Fixed quarter windows often leak due to gasket failure
Door Glass Challenges
- Improper glass adjustment: Replacement glass often requires careful adjustment to seal properly against weatherstripping
- Division bar issues (vent window models): Alignment problems between vent and main glass sections
Parts & Materials Needed
Essential Tools
- Glass removal tools:
- Cold knife or piano wire for urethane removal
- Gasket installation tools (for rubber gasket systems)
- Suction cup glass lifters (minimum 2)
- Preparation tools:
- Razor blades and scrapers
- Wire brushes
- Primer applicators
- Caulk guns
Windshield Components
1973-1980 Models (Rubber Gasket System)
Primary Parts:
- Windshield glass: GM part varies by model
- Rubber gasket: Steele Rubber #70-0489-90 (standard cab)
- Locking strip: Steele Rubber #70-0490-90
- Reveal molding clips: GM #348315 (where applicable)
1981-1987 Models (Urethane Bonded)
Primary Parts:
- Windshield glass: GM part varies by model/year
- Urethane adhesive: 3M 08609 or Dow Betaseal Express
- Primer: 3M 08682 Glass Primer
- Activator: 3M 08681 (for metal surfaces)
- Reveal molding: Model-specific GM part numbers
Side Glass Components
Door Glass Assembly:
- Main door glass: Year/model specific
- Vent glass (1973-1977): Including frame assembly
- Window regulators: Varies by manual/power windows
- Glass channels: Steele Rubber or Metro Moulded
- Window felts: Complete weatherstrip kits available
Quarter Glass (Blazer/Suburban):
- Fixed quarter glass with gasket
- Gasket: Steele Rubber parts by specific model year
Specialty Materials
- Glass cleaner: Ammonia-free for tinted glass
- Rust treatment: POR-15 or Corroseal for frame prep
- Weatherstrip adhesive: 3M 08008 or equivalent
- Leak sealer: 3M 08579 for minor adjustments
Step-by-Step Guide
Windshield Replacement - Rubber Gasket System (1973-1980)
Removal Process
- Remove interior trim: Carefully remove headliner edge trim and any interior moldings
- Extract locking strip: Use gasket tool to remove rubber locking strip from gasket groove
- Push glass outward: From inside vehicle, carefully push glass and gasket assembly outward
- Clean frame: Remove all old gasket material and treat any rust spots
Installation Process
- Prepare new gasket:
- Install gasket on new windshield glass
- Apply thin bead of weatherstrip adhesive in gasket channel - Position glass assembly:
- Place glass/gasket assembly in frame opening
- Ensure even positioning around entire perimeter - Install locking strip:
- Begin at bottom center, work upward on both sides
- Use gasket tool to press strip into groove
- Overlap ends at top center - Final adjustments:
- Check for even gasket compression
- Install any exterior reveal moldings
Windshield Replacement - Urethane Bonded (1981-1987)
Removal Process
- Remove reveal molding: Carefully extract clips to avoid breakage
- Cut urethane bond:
- Use cold knife or piano wire
- Cut close to glass to leave urethane base on frame - Lift glass: Use suction cups with two-person team
- Clean frame:
- Remove loose urethane (leave 1-2mm base layer)
- Clean with alcohol, allow to dry
Installation Process
- Surface preparation:
- Apply primer to clean metal surfaces
- Apply glass primer to windshield edge
- Allow primers to cure per manufacturer specifications - Apply urethane:
- Use triangular bead approximately 10-12mm high
- Maintain consistent bead around entire perimeter - Set glass:
- Position glass within 15 minutes of urethane application
- Press firmly to achieve proper compression
- Maintain pressure for 2-3 minutes - Finishing:
- Remove excess urethane immediately
- Install reveal molding
- Allow 24-hour cure before driving
Door Glass Replacement
Preparation
- Remove door panel: Disconnect window controls and remove inner panel
- Lower glass: Position glass for access to mounting bolts
- Disconnect regulator: Remove bolts securing glass to regulator mechanism
Glass Installation
- Install new channels: Replace worn glass channels and felts
- Adjust regulator: Ensure smooth operation throughout travel
- Mount glass: Secure to regulator with proper torque specifications
- Adjust alignment:
- Check glass position when fully raised
- Adjust door glass stops as needed
- Verify proper seal against door frame weatherstripping
Year/Model Differences
Major Transition Points
1980-1981 Model Year Change
The most significant change occurred between 1980 and 1981 model years:
- 1973-1980: Rubber gasket system with locking strip
- 1981-1987: Urethane adhesive bonding system
- Glass compatibility: 1980 and earlier glass cannot be used in 1981+ applications due to different edge preparation
Model-Specific Variations
C10/C20 Regular Cab
- Consistent windshield design throughout production
- 1973-1980: Single-stage gasket installation
- 1981-1987: Standard urethane bonding
K5 Blazer
Unique Considerations:
- Removable hardtop models: Special weatherstrip requirements
- 1973-1975: Full convertible top option affects glass installation
- 1976-1987: Fixed hardtop with larger rear glass area
- Structural importance: Glass contributes to rollover protection
Suburban
- 1973-1980: Three-piece rear glass configuration on some models
- 1981-1987: Single-piece rear glass becomes standard
- Extended body: Additional quarter windows require specific gaskets
Crew Cab Models
- Rear door glass: Unique to crew cab, different regulator mechanisms
- Extended weatherstrip: Longer door glass channels required
Glass Part Number Variations
Windshield Glass by Year Range:
1973-1980: Multiple suppliers, verify fitment
1981-1984: GM part numbers change mid-production
1985-1987: Standardized part numbers
Side Glass:
1973-1977: Vent window equipped
1978-1987: Single-pane door glass (most models)
Tips & Best Practices
Professional vs. DIY Considerations
Recommended for Professional Installation
- Urethane bonded windshields: Structural adhesive requires proper technique
- Heated rear glass: Electrical connections require care
- Insurance claims: Many policies require professional installation
Suitable for DIY
- Rubber gasket windshields: More forgiving installation method
- Side glass replacement: Generally straightforward with proper tools
- Vent window restoration: Good learning project for beginners
Quality Considerations
Glass Selection
- OEM equivalent: PPG, Pilkington offer best fit and optical quality
- Aftermarket budget: Acceptable for driver-quality restorations
- Reproduction issues: Some reproduction glass has distortion problems
Gasket and Seal Quality
- Steele Rubber: Highest quality reproduction gaskets
- Metro Moulded: Good alternative, extensive catalog
- Generic seals: Often poor fit, avoid for quality restorations
Installation Best Practices
Weather Conditions
- Temperature range: 65-85°F ideal for urethane applications
- Humidity: Low humidity improves primer adhesion
- Wind protection: Prevent contamination during installation
Safety Protocols
- Eye protection: Always wear safety glasses
- Cut-resistant gloves: Essential for glass handling
- Two-person minimum: Never attempt single-handed glass installation
- Proper lifting: Use suction cups rated for glass weight
Common Mistakes to Avoid
- Using wrong adhesive type: Modern fast-cure urethanes may not be appropriate for vintage applications
- Insufficient surface preparation: Poor cleaning leads to adhesive failure
- Rushing cure times: Allow proper cure time before driving
- Over-tightening regulators: Can crack glass or bind mechanism
- Ignoring frame rust: Must address rust before glass installation
Troubleshooting
Windshield Issues
Water Leaks
Symptoms: Water intrusion around windshield perimeter
Diagnosis:
- Check gasket compression (rubber gasket systems)
- Inspect urethane bond integrity (bonded systems)
- Look for frame distortion or rust damage
Solutions:
- Rubber gasket: Adjust locking strip, replace gasket if hardened
- Urethane bond: Inject additional sealant or complete re-installation
- Frame issues: Repair rust, check for accident damage
Wind Noise
Symptoms: Whistling or air noise at highway speeds
Diagnosis:
- Check reveal molding fit
- Inspect gasket or urethane seal continuity
- Verify proper glass positioning
Solutions:
- Adjust or replace reveal molding clips
- Add weatherstrip adhesive to loose gasket areas
- Re-position glass if recently installed
Side Glass Problems
Window Won't Raise/Lower
Symptoms: Glass binds or stops during operation
Diagnosis:
1. Check regulator mechanism for wear or damage
2. Inspect glass channels for debris or wear
3. Verify proper glass alignment in door frame
Solutions:
- Lubricate regulator with white lithium grease
- Replace worn glass channels and felts
- Adjust glass position and door stops
Glass Falls Into Door
Symptoms: Glass drops when window control is operated
Diagnosis:
- Broken regulator mechanism
- Loose glass mounting bolts
- Worn regulator attachments
Solutions:
- Replace regulator assembly
- Re-secure glass with proper hardware
- Upgrade to heavy-duty regulator if repeatedly failing
Vent Window Issues (1973-1977)
Poor Sealing
Symptoms: Water or air leaks around vent window
Diagnosis:
- Check vent window gaskets and weatherstrips
- Inspect division bar alignment
- Verify vent window adjustment
Solutions:
- Replace vent window seals and gaskets
- Adjust division bar position
- Realign vent window in door frame
Difficult Operation
Symptoms: Vent window hard to open or won't stay positioned
Diagnosis:
- Worn vent window mechanism
- Corroded pivot points
- Bent vent window frame
Solutions:
- Disassemble and clean mechanism
- Replace worn bushings and springs
- Straighten or replace vent window frame
Glass Optical Issues
Distortion
Symptoms: Wavy or distorted view through glass
Diagnosis:
- Poor quality replacement glass
- Improper installation causing stress
- Glass not properly supported during installation
Solutions:
- Replace with higher quality glass
- Re-install with proper support and technique
- Check for frame distortion causing stress points
Scratches or Pitting
Symptoms: Reduced visibility, especially at night
Diagnosis:
- Normal wear from wipers and debris
- Chemical etching from acid rain or cleaners
- Manufacturing defects in glass
Solutions:
- Professional glass polishing for minor scratches
- Complete replacement for severe damage
- Upgrade to modern scratch-resistant coatings where available