What This Air Spring Does
This is a rolling-lobe air spring with a composite piston, mounted at the trailer axle. The bellows rolls down over the piston as the suspension compresses, so it carries load without the harsh spring rate of a convoluted bag. That rolling action is what protects freight and keeps axle loading even across the group.
Physical numbers on the PartStop unit: roughly 17.5 in extended height, 7.9 in compressed height, about 10.16 in diameter, and around 14.5 lb. Those figures matter more than the box label. If your old bag measures well outside that range, you are looking at a different spring and the cross-reference stops there.
The part is sold as a direct replacement, aftermarket build. It is not a Hendrickson or Firestone product and PartStop is not affiliated with either company. The brand numbers below are listed so you can identify the part you already have.
Why These Numbers Cross
One physical air spring gets a different part number from every company that sells it. Firestone catalogues it as W01-358-9978. Hendrickson buys the same bag for INTRAAX and HT suspensions and stamps its own service numbers, S-28929 and C-28929, on the packaging. Same bellows, same piston, same mounting pattern, different sticker.
Goodyear lists 1R12-668 and 1R12-1179 for the same envelope. ContiTech carries it as 67588, also printed as 9 10-14 P 959 on some bags. Triangle uses AS-8959 and Torque uses TR9978. All of these describe a rolling-lobe spring with the same extended and compressed heights, the same diameter, and the same top and bottom fittings.
The cross works because the mounting interface and the ride-height window are shared, not because the numbers look similar. Two air springs with the same diameter but a different stud pattern or a different piston height will not interchange. That is why you confirm against the number moulded into the old bag rather than against a catalogue alone.
Checks Before You Order
Get the trailer up, chock it, and dump the air. Wipe the old bag clean and read the moulded number on the bellows or the tag on the piston. That number, plus your suspension model, is what confirms the part. A cross-reference is a starting point; fitment gets verified by the number on the part in your hand or by VIN.
Check the mounting while you are under there. Note whether the top plate is stud-mount or bead-ring, count the studs, and check the air port location and thread. Measure ride height at the axle before you pull the old spring so you can set it the same on reassembly. Wrong ride height chews bags and throws off brake geometry.
W01-358-9978 Cross-Reference & Interchange Numbers
These manufacturer and aftermarket numbers commonly cross-reference to W01-358-9978. Use them to identify the part — then confirm the exact fit by VIN or the number stamped on your old unit before you order. An interchange is a starting point, not a fitment guarantee.
| Brand / Line | Cross-Reference Numbers |
|---|---|
| ContiTech | 67588, 9 10-14 P 959 |
| Goodyear | 1R12-668, 1R12-1179 |
| Hendrickson | S-28929, C-28929 |
| Torque | TR9978 |
| Triangle | AS-8959 |
Frequently asked questions
Is S-28929 the same air spring as W01-358-9978?
They describe the same rolling-lobe air spring used on Hendrickson INTRAAX and HT air-ride trailer suspensions. Hendrickson lists it as S-28929 and C-28929; Firestone lists it as W01-358-9978. Read the number moulded into your old bellows before ordering, since suspension models change over production runs.
How do I know if the bag is bad or just leaking at the fitting?
Air the trailer up and soap the bag, the air port, and the line fitting. Bubbles at the threads mean a fitting or line problem. Bubbles from the bellows itself, or visible cracks, chafe marks, or a folded-over sleeve, mean the spring is done. A bag that sits low on one side with good air pressure is also failed.
Do I have to replace air springs in pairs?
Not required, but it is common shop practice on an axle. Bags age together and a new spring next to a worn one can load unevenly until the new one breaks in. If the trailer has high mileage and one bag has failed, inspect the rest of the group before you close it up.
Not sure it fits? We check before you pay.
Run your VIN and we’ll match parts to your exact truck, or call the counter — a person who knows trucks verifies fitment by OEM number before the order ships. No wrong-box returns.
More cross-reference guides
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Every part below is a Premium Quality aftermarket Direct Replacement with published OEM cross-reference numbers, a 6+ month warranty and same-business-day shipping from Tacoma, WA. Not sure it fits? Run your VIN — or call and a person who knows trucks will verify fitment before you pay.