Cummins 6BT and 6BTA marine diesels: what to check before you buy

The Cummins B-series, the 5.9-liter six often called the 6BT (turbocharged) or 6BTA (turbocharged and aftercooled), is one of the most common diesels in cruising boats from the late 1980s onward. Cummins sells engines in about 190 countries through more than 600 distributors and thousands of dealers,¹ and the B-series shares its basic engine family with Cummins' truck and industrial engines, which helps with parts. Marine-specific parts, such as the cooling system, exhaust and aftercooler, are their own.

These are good engines that punish neglect of the cooling system. Ask for the aftercooler records before anything else.

Cummins B-series at a glance

Is a Cummins-powered boat right for you?

A good fit if you want a widely supported diesel with parts nearly everywhere, service records show the aftercooler and heat exchangers have been looked after, and the installation is clean and accessible.

Think twice if there's no record of aftercooler service on a 6BTA, the engine room is cramped and the cooling system neglected, or the engine has been worked hard at a high-output rating.

What to check

1. The aftercooler (6BTA)

Owners and specialists stress servicing the aftercooler regularly. One specialist forum suggests flushing it at around 300 hours or a year, with a full service every couple of years in salt water and longer in fresh. If an engine with a couple of thousand hours has no record of aftercooler service, the specialist's advice is to do it first thing, and warns the core may be stuck.² A neglected aftercooler can be damaged from the seawater side and let water into the engine.²

If the records are missing, put a full cooling-system service into your offer.

2. Heat exchanger and gear cooler

Ask for service records for the heat exchanger and the transmission (gear) cooler.³ Look for coolant crust and green corrosion around the end caps.

3. Raw-water pump and impeller

When was the impeller last changed? A failed impeller means overheating.

4. Smoke, blow-by and oil analysis

Check for a clean cold start and no persistent blue, black or white smoke. Look at blow-by from the crankcase breather. Send an oil sample to a lab: one popular lab has charged around $35 a sample.⁴

5. The rating and how it was run

High-output ratings in fast boats work an engine much harder than low ratings in trawlers. Volvo Penta's rating scheme, as an example of how makers classify duty, runs from continuous duty down to pleasure duty with limited annual hours and limited time at full power.⁵ Find the exact rating of the engine you're looking at, and ask how the boat was used. See boat engine hours.

Questions to ask the seller

• When was the aftercooler last serviced? Do you have the invoice?
• When were the heat exchanger and gear cooler last cleaned?
• What's the engine's rating, and how fast do you usually cruise?
• Has it ever overheated or had water in the oil?
• Can I take an oil sample and see a cold start?

On the sea trial

The engine should reach its rated wide-open rpm briefly with a normal load, and temperatures should stay stable. On twins, run each engine alone. Open the engine room right after the run while everything is hot. See the sea trial checklist.

Common questions

Is the Cummins 6BTA a good marine engine?

It's strong, well understood and supported almost everywhere, and a well-maintained one can run for many thousands of hours. The cooling system, especially the aftercooler, is what needs watching.

How often should a 6BTA aftercooler be serviced?

One specialist forum suggests flushing it at around 300 hours or a year, with a full service every couple of years in salt water and longer in fresh water.² Follow Cummins' current guidance and your mechanic's advice.

What's the difference between a 6BT and a 6BTA?

The "A" means aftercooled: the 6BTA has an aftercooler that cools the turbocharged intake air, which allows more power but adds a component that must be serviced.