Thermex Gearbox Oil Coolers vs. Brass Tube Coolers

Gearbox Oil Cooler Comparison

Thermex Gearbox Oil Coolers have provided marine transmission manufacturers and boat owners with a high performance, robust and low cost alternative to traditional brass tube coolers for over 5 years. Third-party test houses have confirmed a significant performance benefit and a recent re-design has made them even more robust than ever before, but what makes the Thermex cooler different from the traditional versions?

Marine gearboxes are usually cooled by a small auxiliary oil cooler positioned on the top of the box. In many cases the cooler is a brass tube construction, with rubber caps on the end to connect the pipe work. These low cost oil coolers can perform well for a short period of time, but many boat owners and operators complain of perishing rubber ends and the hassle of using an extra pipe to connect the rubber ends to their rubber hoses. In 2011 Thermex engineers began work on a new alternative range of gearbox oil coolers, using components that already exist on established designs to offer a higher quality, UK made alternative, at a competitive price.

Brass Tube Cooler

The Brass Tube Design

The traditional cooler consists of a series of brass tubes joined with solder to a brass outer sleeve.

This has two connections soldered along it's length for the oil inlet and outlet, the water connections are made from rubber end caps.

The solder can be weakened over time leading to leaks, as can the rubber end caps which have been known perish within a couple of years.

Thermex Gearbox Oil Cooler

The Thermex Design

The engineers at Thermex wanted to address these issues directly, so the soft rubber end caps have been replaced with solid GFN (Glass Filled Nylon) headers, on to which the hoses can be connected directly.

The tube stack inside the cooler is fully floating, allowing the components to expand and contract freely with the changes in temperature. The soft soldered areas have been moved to the inner-facing sides of the tube stack which means that sea water only ever comes into contact with a corrosion resistant naval brass face.

The efficient design has also enabled Thermex to fit more tubes into the same amount of space providing more cooling surface area than the brass tube versions.

Optional mounting feet, as shown on the picture above, were also designed to make installation easier for the user.



GB Oil Cooler Header

Unlike the brass tube version, the Thermex cooler features a rigid pipe allowing a hose of either 28mm or 32mm to be connected directly to it. In 2016 the design was improved further to provide more strength by upgrading the material specification and thickening the flanges around the bolts. All of the coolers that leave the Thermex factory are pressure tested to ensure reliability.


Oil Cooler Tube Sheet Comparison

The Thermex cooler is designed to be serviceable and features a floating tube stack to improve reliability.

The Tests 

To prove the merits of the design, Thermex arranged for these coolers to be tested by a third party test house along side the traditional version to see how they compare. The results for our GB60, the second smallest model in the range, are shown below.

Test Results

Test Conditions;

  • Oil: Transmission Oil
  • Oil Flow: 10, 20 L/min
  • Oil on Temperature: 85degC
  • Coolant: Texaco XLC/Water
  • Coolant Flow: 50, 100, 150 L/min
  • Coolant on Temperature: 25degC

The results showed that, depending on the conditions, the performance is at least 5% more than with the brass tube versions. This figure increases up to 20% with the larger coolers in the range.
The tests also showed that the pressure drop on the oil circuit was comparable, and lower on the coolant circuit for the Thermex cooler.

 Bladt Diesel 90HK

Engine Shown: Bladt Diesel 90HK with Thermex GB50 Oil Cooler.


For more information about our Gearbox Oil Coolers please see our Gearbox Oil Coolers page or contact us.


Gearbox Oil Coolers


Gearbox Oil Coolers Webpage

Transmission Oil Cooler Catalogue

Tel: +44 (0)1527 62210



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