EP55 – Sailors Maintaining Our Own Yanmar 4JH4-TE Diesel | Step-by-Step Reassembly

Episode Overview

Out here, there’s no AAA — and there’s definitely no “we’ll deal with it later” when it comes to engines. Reliability is safety, comfort, and freedom all rolled into one.

In this extended episode, we walk through the step-by-step reassembly of one of our Yanmar 4JH4-TE (75hp) marine diesels. This isn’t an engine rebuild — it’s major maintenance and restoration: cleaning years of grime and corrosion, replacing seals, servicing cooling components, checking valve lash, and carefully rebuilding the engine from the inside out so it’s ready to go back in the boat.

Along the way we find the kind of issues that only show up when you actually tear things down: old leaks, salt exposure, plugged coolant passages, and an oil cooler that wasn’t doing much cooling at all. It’s dirty, methodical work — but it’s the kind that pays you back later when things are calm… and especially when they aren’t.

📺 Watch Episode 55 👉

Why Engine Maintenance Matters Offshore

Marine diesels will run forever — if you take care of them. But one of the fastest ways to lose visibility into engine health is letting the engine room stay dirty.

A clean engine:

• makes leaks obvious (before they become failures)
• prevents corrosion from quietly spreading
• helps you spot problems early, when they’re still solvable

This episode is basically our philosophy in practice: clean it, inspect it, repair what matters, and put it back together like you plan to trust it in the middle of nowhere.

Cleaning, Inspection, and the “What Happened Here?” Stage

Before reassembly starts, the work is almost entirely cleanup and inspection:

• degreasing and scrubbing the block and external surfaces
• removing belt dust and oil residue from old leaks
• dealing with rust and corrosion from past salt exposure
• confirming internal condition is still strong despite high hours

This engine has around 4,700 hours, but the internals still look solid — a good reminder that hours matter less than maintenance.

Fixing Leaks the Right Way

A big focus in this video is seals — because oil leaks don’t just make a mess, they hide the next problem.

Front main seal

We confirm the front main seal was leaking (exactly where the mess was telling the story) and replace it properly with clean mating surfaces and careful installation.

Rear main seal

With the engine off the boat, it’s the perfect time to replace the rear main seal too — a job that’s much more painful in an engine room.

Cooling System Reality Check

Cooling issues don’t always show up as overheating — sometimes they show up as “this component hasn’t been doing anything for a long time.”

Oil cooler

We pull the oil cooler and find a passage plugged solid. Once cleaned and restored, it can actually do its job again.

Automatic Cold Start Device (ACSD)

Corroded, plugged coolant passages make the ACSD questionable for this engine, so we address it and plan for the reality of warm-water cruising.

Raw water pump design rant (earned)

We install a new raw water pump and call out a frustrating design detail: the pump’s placement makes service a nightmare in situ. Doing this on a stand is a gift.

Valve Adjustment and Bottom-Up Reassembly

Once the engine is clean enough to trust what we’re seeing, reassembly begins:

• valve lash checked and adjusted to spec
• wiring harness routed early (before components bury access)
• oil cooler, dipstick, brackets, and hoses installed
• bonnet/valve cover resealed carefully to prevent future leaks
• thermostat and circulating pump installed with proper sealant approach
• heat exchanger and turbo mounted (with a quick note on a camera fail)

A lot of this stage is the “500 little things” part — chasing threads, fitting tiny hoses, replacing hardware that was too far gone, and trying not to make it harder on ourselves than it needs to be.

Clamps, Hoses, and Why We Like ClampTite

We use ClampTite in place of many traditional hose clamps: it’s clean, strong, doesn’t require carrying dozens of clamp sizes, and doesn’t leave sharp edges waiting to shred your hands mid-repair.

🔗 ClampTite (affiliate link): https://clamptitetools.com/?sid=syroam
Discount code: Gerischaffer10

Exhaust and Final Assembly Details

We cover key finishing pieces that matter for reliability:

• mixing elbow install (and why running engines hot matters)
• air cleaner details and crankcase breather routing
• final mounts, accessories, and the “almost there” phase

Upgraded Engine Mounts

We install new engine mounts from Oceanic Innovations — not sponsored, paid full price. We chose them because stainless + urethane has potential advantages over mild steel + rubber in a marine environment.

🔗 Engine mounts: https://oceanicinnovations.store/

In This Episode

• Deep cleaning and inspection before reassembly
• Valve lash check and adjustment
• Front and rear main seal replacement
• Oil cooler teardown and restoration
• Wiring harness routing and component installation
• Raw water pump install and serviceability notes
• Heat exchanger and turbo installation
• ClampTite hose clamping approach
• Mixing elbow install and engine operating tips
• Engine mounts upgrade and final reassembly steps

What Comes Next

With the engines reassembled and ready to move, the focus shifts from bench work back to the boat: getting these power plants back into their homes, aligned, connected, and running where they belong.

That brings us into EP56, where we tackle a major milestone in restoring the boat — refinishing our aluminum spars with a Nyalic clear finish… and dealing with a boatyard accident that changes the week in an instant.

Transparency Note

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