Coolant tubes don’t get much attention until one starts weeping on the shop floor. Then the question is always the same: is this the upper or the lower tube, and which part do I order? Here’s how to sort it out, what to measure if the tag is gone, and what to check while the system is open.
What the Upper and Lower Coolant Tubes Do
- Upper coolant tube: carries hot coolant from the engine’s thermostat housing to the top tank of the radiator. This is the hottest coolant in the system.
- Lower coolant tube: carries cooled coolant from the bottom tank of the radiator back to the water pump inlet, where it heads back through the engine.
On heavy-duty trucks these are usually rigid metal tubes with short hoses on each end. That holds the routing around the engine and frame, but the tube takes years of heat cycles and vibration.
How to Tell Upper From Lower
If the tube is still on the truck, follow it. Thermostat housing to top of radiator is the upper. Bottom of radiator to water pump is the lower. If it’s already on the bench, check these clues:
- Where it connected: hose and clamp marks show which radiator neck it came off, top or bottom.
- Ports and fittings: many tubes have threaded bungs or small hose ports for heater lines, surge tank lines or sensors. The number, size and position of those fittings are often what separates one part number from another.
- Shape: upper and lower tubes for the same truck rarely share the same bends. Compare yours to the product photo before ordering.
Don’t go by diameter alone. Two tubes that look nearly identical can differ by a single fitting.
Finding the OEM Part Number
The fastest way to order the right tube is the OEM number. Check for a stamped number or tag on the tube, then your parts invoices or the dealer’s records for that VIN. Freightliner numbers look like A05-12855-000; Kenworth and Peterbilt numbers look like K181-5766, F66-6193 or 07-06503.
Once you have the number, search it on our site or browse the stainless steel coolant tubes category by make. Each product page lists the OEM number it replaces plus base diameter, inlet, outlet and fitting sizes, so you can check it against the tube in your hand.
What to Measure If the Tag Is Gone
No number? No problem. Get these measurements off the old tube:
- Base tube diameter: the outside diameter of the main tube body. Many of the tubes we carry are 2-1/2″ or 3″.
- Inlet and outlet sizes: measure each end separately. They aren’t always the same.
- End style: beaded hose ends or a flanged end that bolts to the radiator.
- Fittings: how many threaded bungs or hose ports there are, the NPT size of each, and where they sit along the tube.
- Overall shape: lay the tube on cardboard and trace it, or take photos from the top and side with a tape measure in the shot.
Common Failure Signs
Watch for these:
- Rust at the bung: the weld where a fitting port joins the tube is a weak spot. Rust there usually means a leak isn’t far behind.
- Leaks and weeping: dried coolant crust on the tube, a damp spot under the hose ends, or a coolant level that keeps dropping with no obvious puddle.
- Cracks: look near bends, brackets and welds where vibration concentrates.
- Pitting at the hose ends: if the bead is corroded, a new hose and clamp won’t seal no matter how tight you go.
Spot any of these? Plan the replacement now, before it lets go on the road.
Why Stainless Steel Beats the Factory Tube
In our experience, factory tubes last an average of 2 to 5 years. Some dealer replacements are now stainless, but often a lower-grade stainless with high carbon content that still allows internal corrosion. The auxiliary ports on those replacements are typically mild steel wire-welded to the tube, and those are the ones that rust at the bung.
Our tubes are manufactured in the USA from 304 stainless steel, built as identical replacements for the factory tube. They’re built to last, and every stainless steel coolant tube carries a lifetime warranty, excluding customer damage and modifications. For more on the material itself, see our post on heavy-duty stainless steel aftermarket coolant tubes.
A Few Tubes We Stock
- Freightliner Upper Coolant Tube, replaces A05-12855-000 (for steel bolt-together tanks)
- Freightliner Lower Coolant Tube, replaces A05-15785-000
- Kenworth Upper Coolant Tube, replaces K181-5766
- Kenworth Lower Coolant Tube, replaces K181-5350
- Peterbilt Upper Coolant Tube, replaces F66-6193
- Peterbilt Lower Coolant Tube, replaces F66-6683
- Kenworth T680 / Peterbilt 579 Lower Coolant Tube, replaces F66-2769-200
What to Check While It’s Apart
The coolant is drained and the tube is out, so take a few extra minutes. It beats doing the job twice.
- Hoses: replace any hose that’s soft, swollen, hard, cracked or chewed up inside. Old hoses often take a set and won’t seal well on a new tube. A little anti-seize on the tube end makes the new hose easier to slide on.
- Clamps: don’t reuse rusted or stretched clamps. Fresh clamps are cheap insurance.
- Radiator: check the inlet and outlet necks for cracks and corrosion, and look over the tanks and core for leaks. If the radiator is on its way out, this is the time to know.
We carry hoses, hose clamps, plugs and water valves too, so you can get it all in one order.
FAQ
Can I swap an upper tube for a lower one?
No. They have different shapes, connection points and often different fittings. Order by OEM number or by measurements for the exact position.
What warranty comes with your stainless coolant tubes?
Our stainless steel coolant tubes carry a lifetime warranty, excluding customer damage and modifications.
I don’t see my truck listed. Can you still help?
Yes. Call 734-941-1460 with your OEM number or measurements and we’ll make the arrangements to have your tube made.
How does shipping work?
Shipping rates are generated by UPS based on destination and size. We ship to the continental 48 states. For orders over 50 pounds or oversized items, we’ll call you to find the most cost-effective method. Local pickup is available; call ahead to arrange it.
I wrecked my truck and damaged the tube. Do I need a new one?
Not always. Call for an R.A.R. number, ship it back to us, and we can repair it.
Get the Right Tube the First Time
SP Truck Parts is a family-owned business with more than 40 years of experience in the heavy-duty truck parts business. Not sure if it’s an upper or a lower, or all you have is the old part and a tape measure? Steve and the team can help. Call 734-941-1460 or order your stainless steel coolant tube online.