LS/LT Engine Swap Guide - Square Body Trucks

Overview

The LS/LT engine swap has become one of the most popular modifications for 1973-1987 GM Square Body trucks, offering modern reliability, fuel efficiency, and performance in a classic platform. These modern General Motors engines provide significant advantages over the original small block and big block options, including electronic fuel injection, improved combustion efficiency, and readily available parts support.

Compatible Square Body Models:
- C10/C20 (2WD trucks)
- K10/K20 (4WD trucks)
- K5 Blazer
- Suburban
- GMC equivalents (Jimmy, Sierra, etc.)

Popular LS/LT Engine Options:
- LS1 (1997-2004): 5.7L, 345-350 HP
- LS2 (2005-2009): 6.0L, 400 HP
- LS3 (2008-2017): 6.2L, 430 HP
- LT1 (2014+): 6.2L, 455+ HP with direct injection
- LS 5.3L/6.0L truck engines: Budget-friendly options from Silverado/Sierra

This swap transforms the driving experience while maintaining the classic Square Body aesthetic.

Common Issues

Fitment Challenges

• Hood clearance: LS engines are taller than original small blocks, particularly with plastic intake manifolds
• Oil pan clearance: Front crossmember interference requires specific oil pan selection
• Accessory drive conflicts: Power steering and A/C routing complications
• Transmission tunnel modifications: May be required for T56/4L80E transmissions

Electrical Integration

• Wiring harness complexity: ECU integration with existing electrical systems
• Gauge compatibility: Modern sensors don't directly interface with original gauges
• Emissions compliance: Varies by state and year of recipient vehicle

Cooling System Modifications

• Radiator capacity: Original radiators may be inadequate for LS power levels
• Fan shroud modifications: Electric fan conversions often necessary
• Hose routing: Custom radiator hoses typically required

Parts & Materials Needed

Engine & Transmission

Engine Options:
- Complete LS engine (5.3L, 6.0L, or 6.2L recommended)
- ECU/PCM with matching calibration
- Engine wiring harness
- Throttle body and intake manifold

Transmission Choices:
- 4L60E/4L80E: Electronic overdrive automatics
- T56/TR6060: 6-speed manual options
- 700R4/4L60: Non-electronic alternatives with standalone ECU

Swap-Specific Components

Motor Mounts:
- Hooker BlackHeart LS swap mounts: 71221001HKR
- ICT Billet LS conversion mounts: Various part numbers by year
- Custom fabricated mounts (advanced builders)

Oil Pan:
- Holley 302-2: Rear sump pan for Square Body chassis
- Moroso 20505: Performance option with increased capacity
- GM OEM truck pan: Budget option requiring minor modifications

Headers:
- Hooker BlackHeart LS headers: 70401301-RHKR (specific to Square Body)
- Hedman Hedders: 69390 (budget option)
- Custom fabricated (best fit, highest cost)

Wiring Solutions:
- Painless Performance LS harness: 60508
- PSI Standalone harness: Custom configured
- Howell Engine Developments: Complete plug-and-play systems

Supporting Modifications

Cooling System:
- Griffin aluminum radiator: 1-25221-X
- Spal electric fans: 30102120 (16" puller)
- Custom radiator hoses or universal kit

Fuel System:
- Electric fuel pump (58+ PSI capability)
- Fuel pressure regulator
- Return line installation kit
- Fuel filter (high-pressure rated)

Exhaust:
- 3" exhaust tubing
- High-flow catalytic converters
- Performance mufflers (Flowmaster, Borla, etc.)

Tools Required

• Engine hoist (2-ton minimum)
• Welding equipment (MIG recommended)
• Basic hand tools (metric and standard)
• Torque wrench set
• Electrical crimping tools
• Cut-off wheel/reciprocating saw

Step-by-Step Guide

Phase 1: Planning & Preparation

1. Choose Engine/Transmission Combination
   - Determine power goals and budget
   - Select compatible ECU and harness
   - Plan transmission crossmember modifications

2. Remove Original Drivetrain
   - Drain all fluids
   - Disconnect wiring, fuel lines, and cooling hoses
   - Remove transmission first, then engine
   - Clean engine bay thoroughly

Phase 2: Chassis Modifications

3. Install Motor Mounts
   - Remove original small block mounts
   - Install LS-specific mount brackets
   - Torque to manufacturer specifications (typically 35-50 ft-lbs)

4. Modify Transmission Crossmember
   - 4L60E/4L80E: May require repositioning or custom fabrication
   - T56: Significant tunnel modifications typically required
   - Maintain structural integrity during modifications

5. Fuel System Upgrades
   - Install electric fuel pump (in-tank or external)
   - Run return line to tank (3/8" supply, 5/16" return)
   - Install fuel filter and pressure regulator

Phase 3: Engine Installation

6. Prepare LS Engine
   - Install appropriate oil pan and pickup tube
   - Mount accessories (alternator, power steering, A/C)
   - Install headers before engine installation

7. Engine Installation
   - Use engine hoist to position engine
   - Align with motor mounts carefully
   - Check hood clearance before final positioning
   - Torque motor mount bolts: 35 ft-lbs

8. Transmission Installation
   - Mate transmission to engine (bellhousing bolts: 37 ft-lbs)
   - Install transmission crossmember
   - Connect driveshaft (may require shortening/lengthening)

Phase 4: Systems Integration

9. Cooling System
   - Install appropriate radiator for LS application
   - Fabricate or install electric fan setup
   - Connect upper/lower radiator hoses
   - Install coolant overflow tank

10. Electrical Integration

    • Install main engine harness
    • Connect ECU in protected location
    • Integrate with existing charging system
    • Install fuel pump relay and fusing

11. Exhaust System

    • Connect headers to exhaust system
    • Ensure proper ground clearance
    • Install oxygen sensors in correct locations

Phase 5: Initial Startup & Tuning

12. Pre-Start Checklist

    • Fill cooling system and check for leaks
    • Prime fuel system
    • Check all electrical connections
    • Verify proper oil level

13. Initial Startup

    • Turn key to "on" position, verify fuel pump operation
    • Crank engine and monitor oil pressure immediately
    • Allow engine to reach operating temperature
    • Check for leaks and proper operation

Year/Model Differences

1973-1980 vs 1981-1987 Differences

1973-1980 Models:
- Larger engine bay provides better clearance
- Simpler electrical systems ease integration
- May require fuel tank modifications for return line
- Original radiators typically adequate with upgrades

1981-1987 Models:
- Tighter engine bay clearances
- More complex factory electrical systems
- Often equipped with better cooling systems
- Computer Command Control (CCC) systems require careful integration

Model-Specific Considerations

C10/C20 (2WD Models):
- Easiest swap application
- No transfer case clearance issues
- Standard transmission tunnel modifications

K10/K20 (4WD Models):
- Transfer case compatibility considerations
- May require NP205/NP208/NP241 adapter
- Additional ground clearance challenges

K5 Blazer:
- Shorter wheelbase affects driveshaft angles
- Removable top considerations for wiring routing
- Limited space behind engine for accessories

Suburban:
- Excellent cooling capacity due to size
- Long driveshaft may require carrier bearing
- Ample space for fuel system modifications

Tips & Best Practices

Planning Phase

• Start with a complete running LS engine - avoid project engines
• Budget 150% of initial estimates - unexpected costs are common
• Choose transmission early - affects many other component choices
• Research local emissions requirements before starting

Installation Tips

• Mock up everything twice before final installation
• Use quality motor mounts - cheap mounts cause vibration issues
• Plan wire routing carefully - avoid heat sources and moving parts
• Install a quality fuel pump - inadequate fuel pressure causes tuning issues

Performance Optimization

• Cold air intake systems provide measurable power gains
• Tune for your specific combination - generic tunes leave power on table
• Consider cam upgrades for naturally aspirated combinations
• Upgrade rear axle if planning significant power increases

Common Mistakes to Avoid

• Don't reuse old motor mount bolts - they may be wrong thread pitch
• Avoid mixing coolant types - use only what's specified for LS engines
• Don't skip the break-in procedure - follow manufacturer recommendations
• Never assume electrical compatibility - verify all connections

Troubleshooting

Engine Performance Issues

Engine Cranks But Won't Start:
- Verify fuel pressure (should be 58-62 PSI)
- Check for spark at all cylinders
- Confirm ECU power and ground connections
- Verify cam/crank sensor signals

Rough Idle/Poor Performance:
- Check for vacuum leaks around intake manifold
- Verify MAF sensor calibration and cleanliness
- Inspect for exhaust leaks affecting O2 sensors
- Confirm proper fuel injector operation

Overheating Issues:
- Verify proper coolant mixture (50/50 recommended)
- Check electric fan operation and thermostat rating
- Ensure adequate radiator capacity for power level
- Inspect for air pockets in cooling system

Electrical Problems

Charging System Issues:
- Verify alternator output (13.5-14.4V at idle)
- Check battery connections and condition
- Ensure proper ECU ground connections
- Inspect alternator wiring for damage

Gauge Incompatibility:
- Install sending unit adapters for oil pressure/temperature
- Consider aftermarket gauge clusters designed for LS swaps
- Use interface modules for factory gauge retention
- Verify proper sensor calibration

ECU Communication Problems:
- Check OBD-II connector wiring and pinout
- Verify ECU part number matches engine combination
- Ensure proper ECU grounding (multiple points recommended)
- Confirm no aftermarket devices interfering with bus communication

Mechanical Issues

Vibration Problems:
- Check motor mount alignment and torque specifications
- Verify driveshaft balance and U-joint condition
- Inspect transmission mount for proper positioning
- Consider harmonic balancer condition on high-mileage engines

Oil Leaks:
- Most common at rear main seal and oil pan gasket
- Verify proper torque on oil pan bolts (106 in-lbs)
- Check valve cover gaskets (106 in-lbs)
- Inspect for cracked or damaged oil cooler lines

Transmission Issues:
- Verify proper fluid level and type (Dexron VI for 4L60E/4L80E)
- Check for proper ECU communication with transmission
- Ensure adequate cooling for electronic transmissions
- Verify torque converter compatibility with engine combination

This comprehensive guide provides the foundation for a successful LS/LT swap in Square Body trucks. Success depends on careful planning, quality components, and attention to detail throughout the process.