Yanmar Diesel Refresh With Transmission Rebuild

Keeping a Yanmar 3JH4E running strong in the Santa Barbara, Ventura, and Channel Islands region means staying ahead of critical maintenance—especially when dealing with aging marine transmissions and rear main seals. In this project, we restored a well-used Yanmar 3JH4E paired with a Borg Warner Velvet Drive by rebuilding the transmission, replacing a leaking rear main seal, correcting mounting surfaces, and giving the entire assembly a fresh protective paint job. Below is a complete walkthrough of the process, based on real work performed aboard a coastal cruising sailboat in Ventura.

 

Overview: Yanmar 3JH4E + Borg Warner Velvet Drive Restoration

The Yanmar 3JH4E remains one of the most respected small-to-midrange marine diesels used throughout the Santa Barbara and Channel Islands boating community. Known for reliability and excellent fuel efficiency, the 3JH4E is widely used in sailboats from 28–40 feet.

But like all hardworking engines, time eventually brings:

• Transmission fluid leaks
• Loss of forward or reverse engagement
• Rear main seal seepage toward the bell housing
• Paint breakdown and corrosion

In this case, the Borg Warner Velvet Drive showed clear signs of external leakage and aging seals. The rear main seal behind the flywheel had also begun to leak—an issue commonly caught only after pulling the transmission. With the engine already lifted, we took the opportunity to clean, prep, repaint, and restore the entire assembly.

 

Step 1: Removing the Borg Warner Velvet Drive for Rebuild

Once the boat was safely secured and systems disconnected, we removed the Borg Warner Velvet Drive transmission. These units are tough—especially the 70C and 71C models commonly paired with Yanmar diesels—but decades of service can fatigue internal clutches and old gaskets.

The original transmission showed:

• Darkened fluid with particulate
• Worn seals at several cover plates
• Minor output shaft play
• Surface corrosion due to bilge moisture

We performed a full teardown, inspection, and rebuild. All wearable components were replaced, including:

• Forward and reverse clutch plates
• Pump seal
• Input shaft seal
• Output shaft seal
• Gaskets and O-rings throughout the case

Rebuilding a Velvet Drive instead of replacing it is often the best value for boaters in Ventura and Oxnard—these units are known to last for decades when properly serviced.

Step 2: Lifting the Yanmar 3JH4E to Access the Rear Main Seal

With the transmission off, access to the rear main seal becomes straightforward. The seal sits behind the flywheel and prevents engine oil from entering the bell housing.

Most rear main seal leaks occur from:

• Age-hardened seal material
• Improper crankcase ventilation
• Excess crankshaft end play
• Worn or grooved crank sealing surfaces

After lifting the 3JH4E, we removed the bell housing, flywheel, and pilot bearing. The rear main seal area showed clear oil tracking, verifying the need for replacement. We cleaned the mating surfaces, inspected the crankshaft, and installed a new Yanmar OEM rear main seal.

Step 3: Cleaning, Prepping & Painting the Engine and Transmission

With the transmission rebuilt and the rear main seal replaced, we turned our attention to corrosion control and cosmetic restoration. Marine diesels in Ventura and the Channel Islands face constant humidity and salt intrusion, so a protective coating dramatically extends component life.

Our paint process included:

1. Degreasing: Multi-stage cleaning to remove oil residue.
2. Corrosion removal: Wire brushing and polishing of exposed surfaces.
3. Etching primer: Marine-rated primer to seal bare metal.
4. High-temp engine enamel: Color-matched coating for durability.

This not only protects the engine but also makes future leaks easier to spot—an important maintenance advantage for all Yanmar owners.

 

Step 4: Reinstalling the Transmission & Aligning the Drivetrain

After refinishing the engine, we reinstalled the Velvet Drive using new alignment dowels, hardware, and torque-spec fasteners. Proper drivetrain alignment is essential, especially in older sailboats where engine beds may shift over time.

We ensured:

• Proper coupler engagement
• Accurate shaft alignment within acceptable tolerance
• No side-load on the transmission output bearing
• New transmission mounts torqued to spec

Once assembled, the engine was ready for a clean fluid fill, throttle linkage adjusted, and initial startup testing.

Results: A Clean, Leak-Free, Strong-Running Yanmar 3JH4E

The final result is a fully restored propulsion package ready for years of dependable service in Santa Barbara and surrounding waters. With a rebuilt Velvet Drive, fresh seals, and renewed protective coating, this Yanmar engine is prepared for coastal cruising, island hopping, and long-range reliability.

Benefits of the completed work include:

• Zero oil leakage at the rear main seal
• Smooth transmission shifting and engagement
• Improved cooling and cleanliness
• Corrosion-resistant paint finish
• Extended service life for both engine and gearbox

This type of deep maintenance is ideal for owners preparing for extended cruising, recent pre-purchase inspections, or major upgrades such as a new shaft, propeller, or exhaust system.

Whether you operate out of Ventura Harbor, Channel Islands Marina, Oxnard, or Santa Barbara, a smooth-running diesel is essential. We service Yanmar, Perkins, and many other marine diesels used across the region.

Additional resources:

• BoatUS – Engine Care & Safety
• Yachting Magazine – Maintenance Insights
• Perkins Engines Technical Library
• Yanmar Marine Official

If your transmission is slipping, leaking, or making noise, or if your Yanmar is due for gasket and seal replacement, we can help diagnose and service your entire drivetrain.

 

 

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