The right trailer axle U-bolts must match the axle’s shape and diameter, spring width, and required bolt length. For a 2-3/8-inch round axle and 3,500-pound application, the SPARKWHIZ 5-1/2-inch kits offer a direct size match; its 6-1/4-inch kit provides a longer option. For a 3-inch axle rated for 5,200–7,000 pounds, choose a 7-inch kit. A separate Racewill kit is made for a square 2-by-2-inch axle, not a round one.
| Product | Listed axle fit | Best for |
|---|---|---|
| SPARKWHIZ 5-1/2-Inch Kit | 2-3/8-inch round axle, 3,500 pounds | One set of 1-3/4-inch springs |
| SPARKWHIZ 5-1/2-Inch 2-Pack | 2-3/8-inch round axle, 3,500 pounds | Two sets of spring attachment hardware |
| SPARKWHIZ 7-Inch 2-Pack | 3-inch round axle, 5,200–7,000 pounds | Higher-capacity round axles |
| SPARKWHIZ 6-1/4-Inch Kit | 2-3/8-inch round axle, 3,500 pounds | Applications requiring longer U-bolts |
| Racewill Square Axle Kit | 2-by-2-inch square axle | Square boat-trailer axles |
SPARKWHIZ 5-1/2-Inch 3,500-Pound Kit
This kit is best for a 3,500-pound trailer with a 2-3/8-inch round axle and 1-3/4-inch leaf springs. It includes four 5-1/2-inch U-bolts, eight 1/2-inch-20 hex nuts, and two tie plates. The listed steel construction is described as resistant to rust and corrosion.
Choose this option when one axle’s listed spring attachment hardware is needed. Its stated fit is specific: verify the axle diameter and spring width before ordering. The product information does not list a torque specification, so follow the applicable trailer or suspension manufacturer’s instructions during installation.
SPARKWHIZ 5-1/2-Inch 2-Pack
Better for buyers who need two sets of hardware for 3,500-pound, 2-3/8-inch round axles, this kit lists eight 5-1/2-inch U-bolts, 16 hex nuts, and four tie plates for 1-3/4-inch leaf springs. The U-bolts are 1/2 inch in diameter, and the steel is described as resistant to rust and corrosion.
Its main distinction from the single kit is the doubled component quantity. Confirm that the included tie plates and bolt length suit the suspension arrangement; this listing specifies 1-3/4-inch springs and does not provide an installation torque figure.
SPARKWHIZ 7-Inch Kit for 5,200–7,000 Pounds
This two-pack is best for listed 5,200–7,000-pound applications with a 3-inch round axle. It includes eight 7-inch-long, 1/2-inch-diameter U-bolts, 16 1/2-inch-20 hex nuts, and four tie plates for 1-3/4-inch leaf springs. The steel construction is described as resistant to rust and corrosion.
Choose this kit instead of a 2-3/8-inch axle kit when the trailer has the specified larger axle diameter and needs the longer bolt size. Do not select it by load range alone: the listing specifies a 3-inch round axle, and the fit must be confirmed against the actual axle and spring setup.
SPARKWHIZ 6-1/4-Inch 3,500-Pound Kit
This kit is best for a 3,500-pound trailer with a 2-3/8-inch round axle when the specified U-bolt length is 6-1/4 inches. It includes four 1/2-inch U-bolts, eight 1/2-inch-20 hex nuts, and two tie plates for 1-3/4-inch springs. The listed installation torque range is 45–55 foot-pounds.
Choose it over the 5-1/2-inch option only if the longer size matches the suspension requirements. Its stated torque range is useful installation guidance, but the listing does not replace instructions for the trailer or suspension components.
Racewill U-Bolt Kit for Square Axles
This is best for a 2-by-2-inch square boat-trailer axle, not a round axle. The listed U-bolt dimensions are 1/2-inch diameter, 2-1/8-inch inside width, and 4-3/4-inch inside length. The kit is described as heavy-duty steel with a zinc-coated, corrosion-resistant surface.
Choose it when the axle is square and the measurements match the mounting setup. Avoid it for a round axle: its stated shape and dimensions differ from the round-axle kits above.
Buying Guide: How to Choose Trailer Axle U-Bolts
Does axle shape and diameter match?
Start by identifying whether the axle is round or square, then measure its diameter or dimensions. A 2-3/8-inch round axle and a 3-inch round axle require different listed kits. The Racewill product is specifically for a 2-by-2-inch square axle. Do not assume that similar load ratings mean the hardware is interchangeable.
What U-bolt length and spring width do you need?
Compare the required bolt length with the existing hardware and suspension configuration. The listed round-axle choices include 5-1/2-, 6-1/4-, and 7-inch U-bolts. Also check leaf-spring width: the SPARKWHIZ round-axle kits specify 1-3/4-inch springs. Verify measurements before purchase rather than choosing by length alone.
How many kits or components are required?
Check the number of axles and attachment points being serviced, then compare the package contents. A single SPARKWHIZ 3,500-pound kit lists four U-bolts and two tie plates; its two-pack lists double those quantities. Confirm that the nuts and plates included cover the intended repair.
What should installation involve?
Use the relevant trailer or suspension manufacturer’s instructions for installation and tightening. One product listing gives a 45–55 foot-pound torque range; other listings do not state a torque value. Do not apply that range to a different kit unless its instructions support it. Replace worn or damaged hardware with parts that match the axle and suspension specifications.
Frequently Asked Questions
Can I use 2-3/8-inch axle U-bolts on a 3-inch axle?
No. These listed kits specify different axle diameters. Select hardware that matches the measured axle and the suspension requirements.
Are 5-1/2-inch and 6-1/4-inch U-bolts interchangeable?
Not automatically. The length must match the trailer’s axle, spring, and tie-plate arrangement. Check the required dimensions before choosing a replacement.
Does a 3,500-pound rating guarantee the kit fits?
No. The listed load application is only one fit factor. Axle shape and diameter, U-bolt length, and spring width also matter.
Which kit is for a square trailer axle?
The Racewill kit is listed for a 2-by-2-inch square axle. The SPARKWHIZ kits described here are for round axles.
What torque should I use?
The SPARKWHIZ 6-1/4-inch kit lists 45–55 foot-pounds. For other hardware, use the applicable manufacturer’s specified torque rather than assuming the same range.