Big Rig Basics: Change that 'stat

Change that ‘stat

Timely replacement of your thermostat can prevent overheating and engine damage

How do you know your thermostat needs to be replaced? John Rohde, engineering manager of Caltherm Corp. in Columbus, Ind., says the valve is held closed by a spring, and gradually opened by a wax pellet inside a sealed metal cylinder. As the ‘stat begins to fail, it loses trace amounts of wax, and you’ll see the temperature indication on your gauge start to rise gradually. If this happens, and you don’t have deposits in your radiator or problems with the fan, replace the thermostat.

1. Allow the engine to cool to get pressure out of the system. Visit your parts source while the engine is cooling and get a new ‘stat and the related seals. Note that there will be a seal around the lip of the thermostat and also a “wiper” seal that goes between the body of the thermostat and the inner diameter of the housing. The wiper seal prevents pitting of the metal on the thermostat body and motion of the thermostat inside the housing. Rohde says these can cause internal damage and early failure. While you can replace the thermostat successfully without replacing the wiper seal, and it may be OK if the seal is fairly new, it’s best to replace it.

Ask the counter person to give you all the seals, including any that seal between the housing and the block itself. Make sure you get the right thermostat for your year and model of engine, too.

2. While there, make sure you have a way to replace the wiper seal. It can be pried out with a small prying instrument called a heel bar but needs to be replaced with special tools. You’ll need a mandrel, which is a tool that fits inside the seal all around and helps you force it in and seat it evenly on a circular ridge inside the housing. You’ll also need an arbor press to provide even force to insert and seat it. You may be able to take the housing and wiper seal to an engine distributor and pay a small labor charge to have it replaced instead of buying the tools.

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Once the engine is cool, remove the radiator cap. Then, place a clean drain pan under the radiator drain cock.The amount of coolant you’ll need to drain depends on how high the thermostat sits on the engine block. If it sits on top, you’ll only need to remove about a quarter of the system’s capacity, while if it sits down near the middle, drain a little more than half of what is needed to fill it up. Drain the system till fluid level is likely to be below the thermostat.

3.  Remove any assemblies that are in the way, such as water filters, by disconnecting hoses and lines and unbolting them or screwing them off. Then loosen the clamps and disconnect the main discharge hose or metal connector and, if necessary, the smaller bypass hose from the ’stat’s housing.

4. Remove the housing attaching bolts (typically there are four), and remove the housing from the engine block.

5. Note the installation direction of the thermostat and remove it together with its lip seal.

6. Scrape all sealing surfaces to remove stuck-on gasket material or sealing compound.

7. Gently pry the wiper seal out with a heel bar. Install the new wiper seal with the special tools till it seats with the taper so the larger diameter faces the thermostat.

8. Install the new ’stat and lip seal into the recess in the block. Make sure the copper-colored wax pellet faces inside the block so the new thermostat is installed in the same direction as the old one.

9. Install the flat seal that goes between the housing and block, install the housing, and then install its attaching bolts. Tighten the bolts in several stages, alternately, until just snug.

Reconnect hoses and install and torque the hose clamps. Fill the cooling system all the way. After getting the engine hot, refill the system as trapped air will have been eliminated.

Our thanks to Craig Fread, sales manager, and John Rohde, engineering manager, of Caltherm Corp., and to West Trucking, also of Columbus, for their help with this article. Caltherm direct-markets quality replacement thermostats for Caterpillar and Cummins diesels at You should be able to find other such sources via Internet searches. It might be a good idea to visit your local distributor or engine dealer the first time you replace as they can help ensure you have all the right seals and tools.