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Introduction to the Detroit Diesel 4-71 Old Style Blower

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The Detroit Diesel 4-71 engine is part of the iconic 71 Series inline two-stroke engine family. Introduced in the 1930s and used extensively in trucks, marine vessels, military equipment, and industrial applications, the 71 Series gained its legendary reputation through simplicity, ruggedness, and modular design. One critical component of the 4-71 engine’s performance—and its very ability to function as a two-stroke—is the Roots-style blower mounted on the side of the engine.

Among these blowers, there is a vital distinction that mechanics and rebuilders must understand: the “Old Style” blower versus the “New Style” blower. This article provides an in-depth look at the Old Style blower found on Detroit Diesel 4-71 engines, how it works, how to identify it, and how to maintain or upgrade it effectively.

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What Is the Purpose of the Blower on a 4-71 Engine?

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Two-stroke diesel engines like the 4-71 require forced air induction to operate. Unlike four-stroke engines, they do not rely on pistons to create a vacuum for intake air. Instead, a mechanically driven Roots blower is required to perform two vital functions:

• Scavenging: It pushes fresh air into the cylinder to sweep out the exhaust gases from the previous combustion cycle.

• Charging: It fills the cylinder with a fresh air charge to support the next combustion cycle.

The blower is gear-driven, ensuring it spins in perfect synchronization with the engine’s crankshaft. This ensures precise timing of airflow to coincide with the opening and closing of exhaust and intake ports.

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Characteristics of the Detroit Diesel 4-71 Old Style Blower

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1. External Design and Features

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The Old Style blower is typically a larger, bulkier unit with less refined housing than the New Style versions. Key visual identifiers include:

• Square rear cover plates (later versions use a rounded or contoured rear end plate).

• Heavier cast-iron construction.

• Exposed drive coupling design.

• No provision for aftercooler ports.

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2. Drive and Gear Configuration

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The blower on the 4-71 Old Style engine is driven by the engine camshaft or balance shaft, depending on engine configuration (e.g., LA, LB, LC, etc.). The gear ratios are typically:

• Standard blower ratio: 2.00:1 (blower spins twice for every engine revolution).

• Former reduction blowers (used in early 71E, 71N, 71T engines): Ratios as low as 1.55:1.

The Old Style blower uses additional gears mounted on rotor shafts for these reductions. A flexible drive hub transmits torque from the blower drive shaft to the upper rotor timing gear.

3. Rotor Design

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Old Style blowers use two-lobe rotors, which are synchronized via timing gears. These rotors are:

• Mounted on roller bearings at the front.

• Supported by preloaded radial and thrust ball bearings at the rear.

• Spun in opposite directions to trap air and force it into the intake manifold.

Rotor clearances are tightly controlled to prevent backflow and maximize pressure.

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How to Identify an Old Style 4-71 Blower

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Here are some definitive characteristics to distinguish an Old Style blower from its newer counterparts:

You can often identify an Old Style unit visually even without disassembly. If rebuilding or replacing the unit, it is critical to confirm compatibility using engine serial numbers and accessory arrangements.

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Detailed Internal Specifications

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Gear and Rotor Clearances

• Backlash between blower drive gears: 0.003″–0.008″

• Hub-to-support bushing clearance: 0.0010″–0.0025″

• Hub-to-camshaft clearance: 0.0020″–0.0070″

• End thrust (flanged bearing type): 0.005″–0.008″

Excessive backlash or thrust clearance can lead to rattling noise, reduced boost pressure, and premature gear or rotor wear.

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Rotor Timing and Wear

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The timing gears are critical in keeping rotor lobes aligned for proper airflow. Wear on these gears reduces the clearance between the leading edge of the upper rotor and the trailing edge of the lower rotor, causing performance loss.

Shimming behind the gears can adjust rotor lobe clearance. However, excessive gear backlash (>0.004″) requires gear replacement, not adjustment71 

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Rebuilding an Old Style 4-71 Blower

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Rebuilding an Old Style blower can extend the life of your engine significantly. Key steps include:

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Step 1: Disassembly

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• Remove the blower from the engine (Section 3.4).

• Remove the drive shaft and drive gear.

• Remove the front and rear end plates.

• Extract rotor shafts and inspect for wear.

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Step 2: Inspection

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• Check rotor lobes for scoring, pitting, or damage.

• Inspect end plates for grooves or sealing surface wear.

• Spin each rotor by hand and feel for roughness or tight spots.

• Inspect bearings for smooth operation.

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Step 3: Replacement of Wear Parts

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Recommended replacements include:

• Bearings (roller and thrust).

• Lip seals at both ends.

• Rotor timing gears if backlash exceeds 0.004″.

• End plate gaskets and housing seals.

Use premium aftermarket parts—such as those offered by Diesel Pro Power—which meet or exceed OEM specifications.

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Step 4: Reassembly and Rotor Timing

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Rotor timing is critical. Follow these guidelines:

• Align gear marks on rotor gears.

• Use shims to ensure rotor-to-rotor and rotor-to-housing clearances.

• Torque all fasteners per spec:

  • End plate bolts: 13–17 lb-ft.

  • Rotor drive hub bolts: 25–30 lb-ft

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Lubrication System in the Old Style Blower

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The blower is lubricated by engine oil via passages from the cylinder head:

• Oil enters at the top rear of the blower housing.

• A slinger wheel on the lower rotor distributes oil forward.

• Dams in the end plates hold oil in the gear cavity.

• Excess oil drains back into the crankcase through return holes

Common failures in Old Style blowers include leaky seals (oil on the rotor faces) and poor oil circulation due to blocked passages or worn slingers.

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Troubleshooting Old Style Blower Issues

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Upgrading to a New Style Blower: Should You?

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While the Old Style blower is reliable and rebuildable, modern upgrades provide benefits in efficiency, noise reduction, and serviceability. You might consider upgrading if:

• Your Old Style blower housing is cracked or corroded.

• You’re seeking easier access to parts (New Style parts are more common).

• You require a new blower for a marine or high-performance application.

However, an upgrade to the New Style unit requires matching the blower drive and end plates, as they are not directly interchangeable with the Old Style.

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Practical Tip: How to Check Your 4-71 Blower by Hand

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When inspecting a 4-71 Old Style blower on an engine or workbench:

1. Remove air silencer or air shutoff cover.

2. Rotate the top rotor by hand:

   • Should move 3/8″ to 5/8″ at the lobe tip, spring back 1/4″ minimum.

   • No binding or metallic scraping sounds should be present.

3. Shine a flashlight on the rotor tips:

   • Look for oil film, scoring, or misalignment.

4. If inspecting a disassembled blower, spin the rotors by hand to confirm smooth motion without side-play.

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The Role of Diesel Pro Technicians and Video Demonstrations

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At Diesel Pro Power, our in-house technicians regularly inspect, rebuild, and test 4-71 blowers. In one of our videos, our technician rotates the rotors by hand to demonstrate proper rotor backlash and alignment in a freshly rebuilt blower. This simple action shows that:

• There is sufficient clearance between rotor lobes.

• No metal-to-metal contact occurs.

• The rotors are balanced and smooth-turning, showing proper bearing installation.

This demonstration is part of our commitment to quality control and helps you feel confident that your blower is ready for years of service.

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Final Thoughts on the Detroit Diesel 4-71 Old Style Blower

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The Old Style blower is a foundational part of early Detroit Diesel 4-71 engines. Though no longer manufactured in new form, it remains highly serviceable and reliable when properly rebuilt. By understanding how to identify, inspect, and maintain these units, you can continue to run older 4-71 engines in trucks, boats, generators, and heavy equipment.

For those in need of parts, rebuild kits, or entire rebuilt blowers, Diesel Pro Power offers high-quality aftermarket solutions that match or exceed original performance. Whether you’re restoring a classic truck, servicing a marine workhorse, or maintaining military-grade equipment, we’re here to support your 4-71 engine needs.

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FAQ – Detroit Diesel 4-71 Old Style Blower

Q1: What is the main difference between Old Style and New Style blowers?
A1: The Old Style has heavier, square end plates and sometimes reduction gearing, while the New Style is more compact and standardized with improved housing designs.

Q2: Can I use a New Style blower on an engine originally equipped with an Old Style?
A2: Not directly. You will need to update the blower drive support, possibly change the end plates, and confirm rotor clearance compatibility.

Q3: How do I identify a 4-71 Old Style blower?
A3: Look for square end plates, heavier casting, exposed flexible couplings, and lack of aftercooler ports.

Q4: What causes a blower to leak oil?
A4: Worn lip seals, improper rotor timing, or clogged oil return passages can lead to oil leaks at the end plates.

Q5: Can I spin the rotors by hand to check them?
A5: Yes, spinning the rotors should be smooth with some resistance. A slight spring-back shows the drive coupling is intact.

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Additional Resources For The Detroit Diesel 4-71 Blower

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Disassembly Of The Blower For Detroit Diesel 71 Series Inline Engine (271, 371, 471, 671)

Disassembly Of Blower Drive Gear and Support Assembly For Detroit Diesel 71 Series Inline Engines (271, 371, 471, 671)

AIR SCAVENGING AND BLOWER SYSTEM FOR DETROIT DIESEL 71 SERIES INLINE ENGINES (271, 371, 471, 671)