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U-Bolt Torque and Replacement for Trailer Axles
U-bolts clamp the leaf springs to the trailer axle tube. They look simple, but they are sized to the tube, the thread changes from one axle size to the next, and Dexter has updated the nuts and torque values in recent product bulletins. This guide shows which U-bolt kit fits which tube, the torque Dexter now gives for each size, and when to replace the U-bolts instead of reusing them.
On a Dexter axle, always follow Dexter's instructions. On another brand of axle, follow that maker's own manual. Where a Dexter bulletin and the catalog or manual disagree, the bulletin is the newer instruction.
Key takeaway
- Match the tube first: U-bolts are sized to the axle tube diameter, not only to the axle capacity. Dexter light duty axles use 1-3/4", 2-3/8", 3" and 3-1/2" tubes.
- Torque by thread and nut: Dexter Product Bulletin 505 changed the 7/16"-20 and 1/2"-20 U-bolt nuts to serrated flange nuts and gives new torque values for each tube size.
- Use new U-bolts: Dexter Product Bulletin 488 says to always use new U-bolts and hardware when servicing or replacing a leaf spring on an existing axle.
- Check them regularly: The Dexter service manual says to inspect the suspension at least every 6,000 miles and to torque the fasteners whenever they are loose or replaced.
U-bolt kits by axle tube size
The Dexter Light Duty Catalog lists a standard tube diameter for each sprung axle model, and an optional tube on some models. A different tube takes a different U-bolt, so a 3,500 lb. axle on the optional 3" tube does not use the same U-bolts as one on the standard 2-3/8" tube.
| Axle model | Standard tube | Optional tube |
|---|---|---|
| D20 (2,000 lb.) | 1.75 | 2.38 |
| D35 (3,500 lb.) | 2.38 | 3.00 |
| D52 | 3.00 | None listed |
| D60 | 3.00 | 3.50 (per Bulletin 488) |
| D70 and D72 | 3.00 | 3.50 |
| D80 | 3.50 | None listed |
1-3/4" tube
The standard tube on the D20, 2,000 lb. straight axle. These U-bolts use a 3/8"-24 thread.
- U-Bolt Kit for 2,000 lb. 1.75" Trailer Axle - APUBR134BX
- Dexter 1-3/4" Round U-Bolt Kit for 2,000 lb. Trailer Axles - K71-730-00
- Dexter Replacement for 1.75" Axle Tube, 3/8"-24 UNF - K71-391-00
2-3/8" tube
The standard tube on the D35, 3,500 lb. axle, and the tube on the D20 drop axle. Dexter Product Bulletin 505 lists the D28H, D30, D35 and D20 Drop as the models with 7/16"-20 U-bolts on a 2-3/8" tube.
- TruRyde U-Bolt Kit for 2-3/8" Axle Tube, 3,500 lb. and 2,000 lb. Drop Axles, 5.5" Length - APUBR238BX
- U-Bolt Kit for 3,500 lb. 2-3/8" Trailer Axle, 6.25" Length - APUBR238LBX
- Dexter U-Bolt Kit for 2-3/8" Round Axle Tube, 3,500 lb. and 2,000 lb. Drop Axles - K71-731-00
- Dexter Replacement for 2.38" Axle Tube, 7/16"-20 UNF - K71-293-00
3" tube
The standard tube on the D52, D60, D70 and D72, and the optional tube on the D35. Bulletin 505 lists the D40, D44, D52, D60, D70 and D75 as the models with 1/2"-20 U-bolts on a 3" tube.
- U-Bolt Kit for 5,200 to 7,000 lb. 3" Trailer Axle - APUBR3BX
- Dexter 3" U-Bolt Kit for 5,200 to 7,000 lb. Trailer Axles - K71-732-00
- Dexter Replacement for 3.00" Axle Tube, 1/2"-20 UNF - K71-294-00
3-1/2" tube
The standard tube on the D80 and D80N, 8,000 lb. axles. Per Dexter Product Bulletin 488, it is also an upgrade option on the D60, D70, D72 and D75. These U-bolts use a 9/16"-18 thread.
Not sure which tube your axle has? Read the label first. See How to Identify Your Old Trailer Axle and Order a Dexter Replacement. Ordering a new beam on a heavier tube? See Dexter Axle Options: Tube, E-Z Lube, Pre-Wire and Brake Flange. Verify fitment before ordering or contact Southwest Wheel for assistance.
U-bolt torque by tube size
Dexter Product Bulletin 505 replaced the 7/16"-20 flange nut and the 1/2"-20 hex nut with serrated flange nuts on light duty axles. It gives these updated torque values:
| Axle model | U-bolt and nut | Tube | Min | Max |
|---|---|---|---|---|
| D20 | 3/8"-24, hex nut | 1-3/4" | 30 | 50 |
| D35 | 7/16"-20, serrated flange nut | 2-3/8" | 70 | 90 |
| D40, D52, D60, D70 | 1/2"-20, serrated flange nut | 3" | 90 | 110 |
| D75, D80N, D80 | 9/16"-18, serrated flange nut | 3-1/2" | 100 | 120 |
| D90, D100G | 5/8"-11, hex nut | 4" and 5" | 110 | 150 |
These values apply to Dexter axles and the Dexter K71 U-bolt kits: K71-730-00 (1-3/4" tube), K71-731-00 (2-3/8" tube) and K71-732-00 (3" tube). They replace the older U-bolt rows in the Dexter service manual for axles with the serrated flange nuts.
K71-731-00 note: the product page lists hex nut 006-017-01 in the kit. Per Dexter Product Bulletin 505 (November 15, 2024), Dexter now uses the 7/16" serrated flange nut 006-303-01 on 7/16"-20 U-bolts, torqued to 70 to 90 ft. lbs.
Bulletin 488 gives 100 to 120 ft. lbs. for the 3-1/2" tube U-bolts on an unloaded axle.
When to check and retorque
- Every 6,000 miles: The Dexter service manual says to inspect the suspension at least every 6,000 miles.
- When loose or replaced: Torque the suspension fasteners to the listed values whenever they are loose or replaced.
- Before a trip: The manual's trip preparation checklist includes checking the U-bolt nuts against the manual's torque values.
- After storage: The manual's prolonged storage inspection also includes checking U-bolt nut tightness.
For the full suspension and wheel nut torque tables, see Build a Complete Dexter Trailer Axle.
Always use new U-bolts and hardware
Dexter Product Bulletin 488 says to always use new U-bolts and hardware when servicing or replacing a leaf spring on an existing axle. Plan on a new U-bolt kit any time you replace springs or take the spring attachment apart.
When replacing springs, the Dexter service manual says to support the trailer and block the axle before you remove the U-bolt nuts, tie plates and U-bolts, then torque the new hardware to the listed values.
The same bulletin changed the U-bolts on 3-1/2" tubing to a fine pitch thread (UNF) on both double eye and slipper springs. The new U-bolt is 9/16"-18 UNF, 7.8" long, with a 9/16"-18 UNF serrated flange nut. Installation torque stays at 100 to 120 ft. lbs. for an unloaded axle.
Dexter Product Bulletin 515 also moved U-bolts for 1-3/4", 2-3/8", 3" and 3-1/2" tubing on the D20 through D80 to a zinc plated finish for corrosion resistance. The bulletin says the coating does not change the torque specifications.
How to tell which tube you have
The tube diameter sets the U-bolt, so confirm it before you order.
- Check the axle label: The Dexter axle label carries the axle model and description number. The model tells you the standard tube from the table above.
- Check for an optional tube: A D20 drop axle uses the 2-3/8" tube, and a D35 or D70 may have been built on the optional heavier tube. If the label is gone or the axle may not be original, contact Southwest Wheel before ordering.
- Match the old parts: The old U-bolt thread and the tie plate are sized to the tube, so they are a useful cross-check.
If you are unsure, verify fitment before ordering or contact Southwest Wheel for assistance.
Frequently asked questions
Can I reuse my old U-bolts?
Dexter Product Bulletin 488 says to always use new U-bolts and hardware when servicing or replacing a leaf spring on an existing axle.
Do 2,000 lb. and 3,500 lb. axles use the same U-bolts?
Not always. A 2,000 lb. straight axle uses the 1-3/4" tube. A 2,000 lb. drop axle and a standard 3,500 lb. axle use the 2-3/8" tube, so they take the same 2-3/8" U-bolt kits.
What torque do I use on a 2-3/8" U-bolt?
Per Dexter Product Bulletin 505, 70 to 90 ft. lbs. for a 7/16"-20 U-bolt with the serrated flange nut on a D35.
What torque do I use on a 3" U-bolt?
Per Bulletin 505, 90 to 110 ft. lbs. for a 1/2"-20 U-bolt with the serrated flange nut on the D40, D52, D60 and D70.
Does the zinc coating change the torque?
No. Dexter Product Bulletin 515 says the U-bolt torque specifications remain unchanged with the new coating.
How often should I check U-bolt torque?
Inspect the suspension at least every 6,000 miles, check the U-bolt nuts before a trip and after storage, and torque them whenever they are loose or replaced.
Does the spring seat style change the U-bolt?
Overslung and underslung springs both clamp to the tube with U-bolts and tie plates. For the differences, see Overslung vs Underslung and Camber: Spring Seat Options for a Custom Dexter Axle.
New to these terms? See Trailer Axle Terms: A Plain-English Glossary.
Find the right U-bolt kit
Need tie plates? See U-Bolt Plates or call Southwest Wheel at 1-800-866-3336.
Southwest Wheel - Your Trailers Best Friend Since 1919.
References
- Dexter Light Duty Catalog LIT-122-00, page 112 (Double Eye Suspension Components) and page 113 (Slipper Springs Suspension Components: Sprung Axle Tube Diameter and Spring Pad Height), page 114 (U-Bolts and Tie Plates).
- Dexter Parts and Kits, page 124 and page 125 (Miscellaneous suspension hardware).
- Dexter Trailer Axle Service Manual, 2,000 to 8,000 lb., page 64 (Suspension Systems), page 66 (Inspection and Replacement; Suspension Fastener Torque Values), page 67 (spring replacement), page 81 (After Prolonged Storage), page 82 (Trip Preparation Checklist).
- Dexter Product Bulletin 488, U-Bolt Thread Change on 3.50" Diameter Axle Tubing, December 22, 2023, page 1.
- Dexter U-Bolt Kit for 2-3/8" Round Axle Tube, 3,500 lb. and 2,000 lb. Drop Axles - K71-731-00, product page (Description: kit parts list).
- Dexter Product Bulletin 505, Serrated U-Bolt Flange Nuts, November 15, 2024, page 1 (Updated U-Bolt Fastener Torque Values).
- Dexter Product Bulletin 515, Light Duty Zinc U-Bolts, July 3, 2025, page 1.
