March 25, 2025

Assembling a cross head (2 piece) piston for a Detroit Diesel 8V71. This video was filmed during the assembly of an 8V71 long block.

Assembling the Piston Kits for Detroit Diesel 8V71

Whether you’re rebuilding a Detroit Diesel 8V71 for a truck, marine vessel, or industrial power unit, piston assembly is one of the most critical steps in engine reassembly. The 8V71 engine may use either trunk-type (1-piece) or crosshead-type (2-piece) pistons, depending on the specific application.

This comprehensive guide walks through the proper procedures for assembling both piston types, referencing the original Detroit Diesel Series 71 service manual and adapted for modern aftermarket parts use. Ensure you identify the piston type installed in your engine before proceeding.

Trunk-Type vs. Crosshead-Type Pistons: Know the Difference

Trunk-Type Pistons (1-Piece)

Trunk Type (1 Piece Piston)Kit For Detroit Diesel 8V71

Common in highway trucks and light-to-medium marine and industrial use
Piston crown and skirt are forged as a single component
Wrist pin connects piston directly to the connecting rod
Lighter and simpler to assemble

Crosshead-Type Pistons (2-Piece)

Cross Head (2 Piece) Piston Kit For Detroit Diesel 8V71

Common in continuous-duty marine, generator, and locomotive versions
Piston is split into a crown and a skirt (crosshead)
Crown is connected to the skirt with a shouldered piston pin
Reduced side thrust; improved alignment and wear resistance for heavy-duty use

Section 1: Assembly of Trunk-Type (1-Piece) Pistons

Tools and Supplies

Piston ring expander
Piston ring compressor
Assembly lube or clean engine oil
Wrist pin installation tools
Torque wrench (foot-pound scale)
Feeler gauge set
Plastigage for bearing clearance (optional)

Step 1: Pre-Assembly Inspection

Confirm piston part number matches the bank (left or right).
Inspect the ring lands, skirt, and wrist pin bore for wear or damage.
Clean all oil drain-back holes thoroughly.
Inspect connecting rod bushing and ensure it is free from scoring.

Step 2: Install Piston Rings

Each trunk-type piston typically uses:

Two compression rings (top and intermediate)
One three-piece oil control ring

Ring Installation Procedure:

Use a ring expander to carefully install the oil control ring expander into the bottom groove.
Fit the upper and lower oil rails over the expander.
Install the second ring (intermediate) next—this often has a taper or step; verify top side marking.
Install the top compression ring—often keystone-shaped.

Important: Always stagger ring end gaps by 120° to prevent blow-by. Never align a ring end gap over the wrist pin centerline.

Step 3: Install Wrist Pin and Connecting Rod

Lubricate wrist pin bore and small end of the connecting rod.
Insert the rod into the piston.
Slide the wrist pin through until centered.
Install both wrist pin retaining rings securely.

Rod Orientation: The spray hole in the rod must face the camshaft side of the engine.

Step 4: Install Piston into Cylinder

Apply engine oil to the piston, rings, and liner wall.
Use a ring compressor to compress rings fully.
With the crankshaft rotated down and out of the way, insert piston into its cylinder.
Guide the connecting rod to the crank journal.
Tap the piston in with a rubber mallet handle—do not force it.

Step 5: Install Bearing Cap and Torque

Clean and lube new bearing shells.
Align tangs and install bearing in rod and cap.
Install the cap and torque bolts to 90–110 lb-ft (check manual for bolt size).

Section 2: Assembly of Crosshead-Type (2-Piece) Pistons

Tools and Supplies

Piston ring expander
Pin driver or arbor press
Assembly lube or clean engine oil
Feeler gauge
Dial caliper
Crosshead skirt retainer rings (if required)

Key Components

Piston Crown
Piston Skirt (Crosshead)
Shouldered Piston Pin
Retainer Rings
Compression and Oil Rings
Connecting Rod

Crosshead pistons are bulkier and require precise alignment during assembly. The pin bore in the crown is offset and mates to the skirt.

Step 1: Pre-Assembly Inspection

Inspect crown and skirt for scuffing, cracks, and signs of overheating.
Ensure the pin bore and bushing (if applicable) are clean and within spec.
Verify correct matching crown and skirt for the application.
Confirm that the piston is for the correct cylinder bank.

Step 2: Join Crown and Skirt

Lubricate piston pin bore and skirt bore.
Align the skirt to the crown with the oil gallery holes aligned correctly.
Insert the shouldered piston pin—usually a light press fit—through the crown and into the skirt bore.
Install retainer rings on both ends of the pin if applicable.

Orientation Tip: The deeper combustion bowl side or the identification mark should face toward the intake port (per Detroit Diesel manual orientation specs).

Step 3: Install Piston Rings

Follow the same procedure as trunk-type pistons:
- Install oil control ring first
- Follow with intermediate and top compression rings
Stagger the gaps evenly
Use correct ring markings to ensure proper facing direction

Crosshead pistons typically have slightly higher ring land spacing and larger skirts, so ensure you’re using a compatible ring set.

Step 4: Install Wrist Pin and Connecting Rod

On crosshead types, the wrist pin is retained in the piston crown, not in the rod. Some marine configurations use a fully floating pin, while others retain the pin via retainer rings.

Lubricate pin bore and connecting rod end.
Insert the connecting rod into the assembled piston.
Drive or press the pin through until centered.
Install circlips or other pin retainers if specified.

Rod Spray Hole Orientation: Must face the camshaft as with trunk pistons.

Step 5: Install Piston into Cylinder

Lubricate the liner with clean engine oil.
Compress rings and insert the piston into the cylinder.
Guide the connecting rod to the crankshaft journal.
Seat the piston and verify crown-to-liner deck clearance.

Step 6: Install Rod Bearings and Cap

Insert new bearings into the connecting rod and cap.
Lube with clean engine oil or assembly lube.
Install cap and torque to spec (typically 90–110 lb-ft).
Check that the rod moves freely on the crank.

Final Checks (Both Piston Types)

Crankshaft Rotation

After installing all pistons:

Rotate the crankshaft two full revolutions by hand.
Check for smooth movement and no binding.
Listen for any unusual contact or resistance.

Ring Gap Verification (Optional)

Before piston installation, you may check ring end gap by:

Inserting the ring into the liner.
Squaring with the piston.
Measuring the gap with a feeler gauge.

Typical end gaps:

Top compression: 0.012–0.025 in.
Intermediate: 0.010–0.020 in.
Oil ring rails: 0.015–0.030 in.

Common Mistakes to Avoid

Installing pistons in the wrong bank (left/right-specific designs)
Aligning ring gaps over wrist pins
Forgetting to install wrist pin retainers
Forcing pistons during insertion
Incorrect torque on rod caps
Reusing old rings or bearings

Summary of Key Differences

Feature	Trunk-Type Piston	Crosshead-Type Piston
Configuration	One-piece	Two-piece (crown + skirt)
Application	Trucks, light marine	Heavy marine, generator
Wrist Pin Location	Rod to piston	Rod to crown
Skirt Function	Integrated	Guided separately
Side Thrust Management	Minimal	Superior

FAQ – Detroit Diesel 8V71 Piston Assembly

Q: How can I tell if I have crosshead or trunk-type pistons?
A: If the piston separates into a crown and a skirt with a wrist pin between, it is crosshead-type. One-piece units are trunk-type.

Q: Do both piston types use the same rings?
A: No. Ring grooves and heights can differ. Always use the ring set matched to your piston part number.

Q: Can I convert a trunk-type setup to crosshead-type or vice versa?
A: No. The liners, rods, and engine configuration are different. Stick with the original design.

Q: Are ring gaps really that important?
A: Absolutely. Misaligned ring gaps lead to blow-by, oil burning, and reduced compression.

Q: Should I pre-lube the rings and skirt?
A: Yes, use clean engine oil or a proper assembly lubricant before installation.