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Understanding the Plastic Strip Test for Detroit Diesel Engines: A High-Level Guide for Owners, Operators, and Fleet Managers
Detroit Diesel engines—especially the classic 53, 71, and 92 series—are known for their durability, long life, and ability to perform under the most demanding marine, industrial, and commercial conditions. But even the toughest engines rely on precise internal measurements to operate correctly. One of the most important of these measurements is the bearing-to-crankshaft clearance, which determines whether the engine will run smoothly, maintain proper oil pressure, and live up to Detroit Diesel’s reputation for reliability.
One of the simplest and most trusted methods for checking this clearance is the plastic strip test, commonly known by the brand name Plastigage. It is used worldwide by mechanics, rebuilders, fleet maintenance departments, and marine engineers because it delivers accurate information without the need for complicated tools.
For owners, operators, and fleet managers who rely on Detroit Diesel engines daily, understanding this test—and why it matters—is essential for protecting the engine and extending its lifespan.
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1. What Is the Plastic Strip Test in the Context of Detroit Diesel Engines?
The plastic strip test involves placing a thin piece of soft plastic material across a crankshaft journal before the bearing cap is torqued down. When the cap is tightened, the plastic crushes and spreads out. The wider the strip spreads, the tighter the bearing clearance. The narrower it spreads, the looser the clearance.
On Detroit Diesel engines, this test is commonly used during:
- Major overhauls
- Bearing replacement
- Crankshaft inspections
- Block alignments
- Rebuild verification
- Troubleshooting low oil pressure
Because Detroit Diesel two-strokes rely on high RPM operation and consistent oil flow through the main and rod bearings, this measurement is absolutely critical.
2. Why Bearing Clearance Is So Important on Detroit Diesel Two-Stroke Engines
Detroit Diesel engines—particularly 71 and 92 series—are designed to operate at higher RPM ranges than many four-stroke heavy-duty engines. This means:
- Oil must flow freely and consistently
- Bearings must maintain proper oil film thickness
- Oil pressure must remain stable under load
If clearances are wrong, the engine cannot survive long-term operation.
If clearance is too tight:
- The oil film becomes too thin
- Metal can contact metal
- Bearings overheat
- The crankshaft may score or seize
If clearance is too loose:
- Oil pressure drops
- The engine knocks
- Bearings wear rapidly
- The crankshaft loses support
A Detroit Diesel can’t tolerate incorrect clearances—especially under marine load, where engines run for long hours at steady speeds. The plastic strip test ensures those clearances are correct before the engine is put back into service.
3. When the Plastic Strip Test Should Be Used on Detroit Diesel Engines
For two-stroke Detroit Diesels, the test is commonly performed when:
- Performing an in-frame rebuild
- Checking the condition of a used engine
- Replacing main or rod bearings
- Verifying crankshaft condition after machining
- Diagnosing low oil pressure
- Checking for uneven bearing wear across cylinders
Because these engines often remain in service for decades, rebuild accuracy is crucial. A simple, inexpensive plastic strip test helps ensure the bottom end is assembled correctly.
4. How the Test Works (Simplified for Non-Mechanics)
Here’s a high-level overview, without technical jargon:
- The mechanic cuts a small piece of the plastic strip.
- It is placed across the crankshaft surface—never lengthwise.
- The bearing cap is installed and torqued to Detroit Diesel specifications.
- The cap is removed, and the strip—now flattened—is measured using the printed scale.
- The width of the flattened strip tells the mechanic the exact bearing clearance.
The entire process is quick, clean, and highly reliable.
5. Why This Test Is Especially Valuable for Detroit Diesel Engines
Detroit Diesel engines have unique characteristics that make this test particularly important:
A. They rely on precision oiling systems
Detroit 71- and 92-series engines use gear-driven oil pumps and depend heavily on proper oil delivery. Incorrect bearing clearance disrupts oil pressure instantly.
B. They operate at high RPM for long periods
Many marine applications run Detroit Diesels at 1,800–2,100 RPM for hours. Clearance issues show up much faster in these conditions.
C. They use two-stroke lubrication patterns
Because every cylinder fires every revolution, bearings face more frequent load cycles than in four-stroke engines.
D. They often run in harsh environments
Saltwater, humidity, industrial dust, and heavy cycles mean errors must be avoided.
The plastic strip test gives mechanics a clear, reliable measurement that helps preserve the engine’s life and performance.
6. What the Test Reveals About Engine Health
Beyond measuring clearance, the test gives insights into overall engine condition:
• Uneven patterns across cylinders
May indicate misaligned main saddles—a problem common in older blocks.
• Tight readings
Could signal incorrect bearings, crankshaft grinding issues, or debris under the bearing shell.
• Loose readings
Could point to journal wear, bearing fatigue, or a crankshaft that needs machining.
• Consistent readings across all cylinders
Indicate a well-machined crankshaft and a healthy block.
For fleet managers and operators, this information is crucial when evaluating whether to proceed with a rebuild, replace a crankshaft, or replace the engine entirely.
7. Why Owners and Fleet Managers Should Care About This Test
The plastic strip test directly affects:
• Engine lifespan
Correct clearances reduce wear and prevent catastrophic failure.
• Oil pressure stability
Detroit Diesels depend heavily on consistent oil pressure for blower, cam, and bearing lubrication.
• Downtime prevention
Engines built with improper clearances fail quickly, costing fleets trips and revenue.
• Rebuild confidence
You know the engine was assembled correctly before it is started.
• Budget planning
Catching clearance issues early is far cheaper than losing a crankshaft or connecting rod.
In simple terms:
bearing clearance is foundational to Detroit Diesel reliability—and the plastic strip test is the easiest way to verify it.
8. A Simple Tool With Massive Value
What makes the plastic strip test so widely used in Detroit Diesel rebuilds?
- It’s inexpensive
- It’s accurate
- It works under real torque conditions
- It requires no special machinery
- It reduces risk
- It gives immediate answers
Even the most advanced machine shops and Detroit Diesel specialists use it. Its simplicity is what makes it powerful.
Conclusion
Detroit Diesel 53-, 71-, and 92-series engines are legendary for their longevity, but only when built and maintained with precision. The plastic strip test is one of the simplest and most effective tools for ensuring proper bearing-to-crankshaft clearance during rebuilds or inspections.
For owners, captains, operators, and fleet managers, understanding this test provides reassurance that the engine’s bottom end—the foundation of its reliability—has been assembled correctly. Whether your Detroit Diesel powers a commercial fishing vessel, tugboat, generator, or industrial machine, proper bearing clearance is essential for safe and dependable operation.
The plastic strip test offers a low-cost, highly reliable method for confirming those clearances and preventing costly failures. It is a small tool with a big impact—helping Detroit Diesel engines continue to perform the way they were designed to: strong, steady, and built to last.
Shop Crankshaft, Crankshaft Bearings, & Related Components For Detroit Diesel Engines
