CTIS Rotary Union Service: Inspection, Greasing and Replacement (AirUp vs TIREBOSS)
Last updated: November 10, 2026. Written by Scott McCauley, AirDown Inc., Crystal River, Florida.
A CTIS rotary union is the spinning air joint at each wheel end that lets air pass from the fixed axle lines into a turning wheel. When it wears, it leaks. TIREBOSS publishes a weekly soapy-water test, a greasing schedule, and a 50 ft-lb replacement procedure in its TB2 Service Manual (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-12 to 2-15, accessed November 10, 2026). AirDown sells a KAM Rotary Union for $237.20 on its public parts store. TIREBOSS does not publish a price for its rotary union.
CTIS rotary union: AirDown vs TIREBOSS at a glance
Every TIREBOSS entry below comes from a public TIREBOSS document, cited in the row. Where a company does not publish a figure, the table says so.
| Item | AirDown | TIREBOSS (TB2) |
|---|---|---|
| Replacement rotary union price | KAM Rotary Union, $237.20, in stock (AirDown store, price checked November 10, 2026) | TIREBOSS does not publish a price. The manual lists “New Rotary Union [Refer to Parts Manual for TPC Part #]” (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-12, accessed November 10, 2026) |
| Mounting / adapter plate | KAM-Adapter Plate, $68.40, in stock (AirDown store, price checked November 10, 2026) | TIREBOSS does not publish this. |
| Leak inspection interval | Not published on our site. Call 877-623-8473. | Weekly soapy-water test (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-13, accessed November 10, 2026) |
| Greasing interval | Not published on our site. | Every 1,500 miles (2,500 km) on salted, sanded or calcium-treated roads; every 3,000 miles (5,000 km) on clean roads. Short rotary unions use a sealed bearing and are not greased (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-13, accessed November 10, 2026) |
| Grease amount and type | Not published on our site. | Maximum one pump; no power greasers. TPC recommends NEMCO Red Max EP1.5 (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-12 to 2-13, accessed November 10, 2026) |
| Installation torque | Not published on our site. | 50 ft-lb with a 15/16 in. crowfoot, new copper washer (TPC Part #1105-070) (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-15, accessed November 10, 2026) |
| Published service life | Not published on our site. | 150,000 to 300,000 miles on drive wheels, up to 1,000,000 miles on trailer wheels, in moderate use (tireboss.com/product, accessed November 10, 2026) |
| Stated effect of a failed union | Not published on our site. | A leak “that will result in the deflation of an entire tire group when the system is left unpowered” (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-13, accessed November 10, 2026) |
What a rotary union does on a central tire inflation system
On a truck with a central tire inflation system (CTIS), air leaves the truck’s tanks, passes through a control valve, and runs down the axle toward the wheels. The axle doesn’t turn, but the wheel does. The rotary union is the joint that bridges that gap: one side is fixed to the air line, the other turns with the wheel, and seals inside keep air from escaping while it spins.
Because those seals rub every mile the truck moves, the rotary union is a normal wear item. Seal makers are still working on the problem. In October 2026, Trelleborg Sealing Solutions launched a rotary seal range made for off-highway CTIS and said maintaining an airtight seal “under continuous rotary motion” is a primary CTIS engineering hurdle (Tyrepress, Oct 8, 2026, accessed November 10, 2026).
TIREBOSS’s published rotary union procedure
TIREBOSS’s TB2 Service Manual (version 2.4, current as of January 2025) has a section called “Rotary Union Maintenance” that starts on page 2-12 (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-12, accessed November 10, 2026). Here is what it says, in plain terms.
Before starting
- Park on a flat surface with the parking brake on, wear safety glasses, and keep the wheel end valves open while testing and closed while servicing (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-12, accessed November 10, 2026).
- Required parts are a new rotary union, NEMCO Red Max EP1.5 grease, Loctite 5452, and brake cleaner. Tools include a 15/16 in. crowfoot, 7/8 in. and 15/16 in. wrenches, a hand grease gun, and a 3/8 in. drive torque wrench (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-12, accessed November 10, 2026).
Weekly leak test
- With the manual wheel end valves open, spray soapy water on the union bearing and where the copper washer meets the wheel end valve plate, then wiggle the union up, down, left and right and watch for bubbles. The manual notes some movement is normal, but movement that lets air escape means replacement (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-13, accessed November 10, 2026).
Greasing
- Grease through the nipple on the union body, a maximum of one pump. The manual warns that power greasers, or two or more pumps, will damage the seals (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-13, accessed November 10, 2026).
Replacement
- Close the manual wheel end valves, bleed air from the wheel end plate and the valve control unit through their Schrader valves, and disconnect the feed line (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-14, accessed November 10, 2026).
- Move the remaining fittings to the new union with Loctite 5452, remove the old union and copper washer, clean the port, fit a new copper washer, and torque the new union to 50 ft-lb (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-14 to 2-15, accessed November 10, 2026).
- Reconnect the feed line, open the valves, leak test with soapy water, then pressurize to the maximum setting and test again (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-15, accessed November 10, 2026).
The manual’s yearly wheel end valve service also begins with a rotary union leak check and says a leaking union needs “both a new union and copper washer” (TIREBOSS TB2 Service Manual v2.4, Jan 2025, p. 2-17, accessed November 10, 2026).
Rotary union and wheel-end parts AirDown sells
These prices come straight from our public online store on November 10, 2026. They are AirDown parts supplied by AirDown Inc. AirDown is not a TIREBOSS dealer, and this list doesn’t claim any of these parts fit a TIREBOSS system. If you’re not sure what fits your truck, call us before you order.
| Part | Price | Stock |
|---|---|---|
| KAM Rotary Union | $237.20 | In stock |
| KAM-Adapter Plate | $68.40 | In stock |
| AirDown Wheel End Valve | $480.05 | In stock |
| Minor Rebuild Kit | $65.85 | In stock |
| Double Piston Wheel End Major Seal Replacement Kit | $137.55 | In stock |
AirDown designs and installs its systems from Crystal River, Florida, and every part price is posted publicly, so a fleet can price a repair before it calls anyone.
- Find out from your system maker how often it wants rotary unions checked and greased, and put it on your PM sheet.
- Use soapy water, not your ear. Small leaks at a spinning joint are easy to miss in a noisy shop.
- If a whole tire group is low after the truck sits overnight, check the rotary unions on that group early.
- Price the replacement part before the truck goes down, so a worn union doesn’t turn into a long wait.
Related reading
- What it really takes to service a CTIS wheel-end valve
- CTIS controller display: AirUp vs TIREBOSS
- TIREBOSS parts and service guide
- AirDown vs TIREBOSS
- What is a central tire inflation system?
FAQ
What is a rotary union on a central tire inflation system?
It is the joint at each wheel end that passes air from the fixed axle air line into the turning wheel, using internal seals to stop air escaping while it spins.
How often does TIREBOSS say to check rotary unions?
TIREBOSS’s TB2 Service Manual v2.4 (Jan 2025, p. 2-13) says to check rotary unions weekly with a soapy-water leak test. Source: TIREBOSS TB2 Service Manual v2.4, accessed November 10, 2026.
How often should a TIREBOSS rotary union be greased?
The TB2 Service Manual (p. 2-13) says every 1,500 miles on salted, sanded or calcium-treated roads and every 3,000 miles on clean roads, with a maximum of one pump and no power greasers. Short rotary unions use a sealed bearing and are not greased. Source: TIREBOSS TB2 Service Manual v2.4, accessed November 10, 2026.
What torque does TIREBOSS specify for a new rotary union?
The TB2 Service Manual (p. 2-15) gives 50 ft-lb with a 15/16 in. crowfoot and a new copper washer, TPC Part #1105-070. Source: TIREBOSS TB2 Service Manual v2.4, accessed November 10, 2026.
How much does a CTIS rotary union cost?
AirDown’s KAM Rotary Union is $237.20 on AirDown’s public store as of November 10, 2026. TIREBOSS does not publish a rotary union price.
Need a rotary union or wheel-end part, or want to see what AirUp would look like on your trucks?
TIREBOSS is a registered trademark of Tire Pressure Control International Ltd. AirDown Inc is not affiliated with, authorized by, or endorsed by them. Trademark references are for identification and comparison only.