The best trailer U bolt kit is one that matches the axle shape and diameter, spring width, and required U bolt length. For a 3,500-pound trailer with a 2-3/8-inch round axle, the SPARKWHIZ 2-pack includes hardware for two spring positions, while its smaller kit covers one. MCCKLE is another option for that axle size. Racewill is designed for a square 2-by-2-inch axle, and SPARKWHIZ’s 7-inch kit is specified for larger 5,200- to 7,000-pound round axles.
| Kit | Specified axle setup | Best suited for |
|---|---|---|
| SPARKWHIZ 2-Pack, 5-1/2-Inch | 3,500 lb; 2-3/8-inch round | Replacing hardware at two spring positions |
| SPARKWHIZ Single Set, 5-1/2-Inch | 3,500 lb; 2-3/8-inch round | Replacing hardware at one position |
| MCCKLE 5-1/2-Inch Kit | 3,500 lb; 2-3/8-inch round | A kit listing nuts and washers |
| Racewill Square Axle Kit | 2-by-2-inch square axle | Square-axle and boat-trailer applications |
| SPARKWHIZ 7-Inch 2-Pack | 5,200–7,000 lb; 3-inch round | Specified larger round axles |
SPARKWHIZ 5-1/2-Inch Kit, 2-Pack
This kit is specified for a 3,500-pound trailer with a 2-3/8-inch round axle and 1-3/4-inch leaf springs. It includes eight 5-1/2-inch-long, 1/2-inch-diameter U bolts, 16 half-inch-20 hex nuts, and four tie plates. The listing describes the steel construction as resistant to rust and corrosion.
Best for: Owners replacing U bolts and tie plates at two spring positions, based on the included quantities. It was selected for its clearly stated axle and spring specifications. Confirm the measurements of the existing setup before ordering; it is not listed for square axles or 3-inch round axles.
SPARKWHIZ 5-1/2-Inch Single Set
This set targets the same stated setup: a 3,500-pound trailer, 2-3/8-inch round axle, and 1-3/4-inch leaf springs. It contains four 5-1/2-inch U bolts with 1/2-inch diameter, eight 1/2-inch-20 hex nuts, and two tie plates. The listing notes durable steel construction and resistance to rust and corrosion.
Best for: A single spring position or a buyer who needs fewer replacement parts than the 2-pack provides. Choose this if the listed axle and spring dimensions match. Its smaller quantity may not cover both sides of a typical axle replacement.
MCCKLE 5-1/2-Inch Round Axle Kit
MCCKLE specifies this kit for a 3,500-pound trailer axle measuring 2-3/8 inches. The listing describes durable steel with a corrosion-resistant coating and says the 5-1/2-inch U bolts are supplied with nuts and washers for installation. It does not state the quantity of every component or the spring width in the supplied details.
Best for: Buyers seeking a 5-1/2-inch kit for the specified round axle who want nuts and washers included. It was selected as another option for the common 3,500-pound axle size. Check the full listing and compare bolt diameter, quantity, and spring width with your existing hardware before purchase.
Racewill Square Axle U Bolt Kit
This kit is designed for a 2-by-2-inch square axle, unlike the round-axle kits above. Its stated dimensions are 1/2-inch bolt diameter, 2-1/8-inch inside width, and 4-3/4-inch inside length. The listing describes heavy-duty steel with a zinc-coated surface and identifies utility trailers, boat trailers, and trucks as intended applications.
Best for: Buyers whose axle is square and matches the stated dimensions, especially those considering corrosion exposure around freshwater or saltwater. It was selected to cover a distinct axle shape. Avoid it if your axle is round or if the inside dimensions do not fit your spring-and-axle stack.
SPARKWHIZ 7-Inch Kit For Larger Axles
This 2-pack is listed for 5,200- to 7,000-pound trailers with 3-inch round axle beams and 1-3/4-inch leaf springs. It includes eight 7-inch-long, 1/2-inch-diameter U bolts, 16 1/2-inch-20 hex nuts, and four tie plates. The steel is described as durable and resistant to rust and corrosion.
Best for: A buyer whose larger round axle and spring dimensions match the listing. It was selected because its fit specifications differ from the 3,500-pound kits. Its 7-inch length and 3-inch axle fit make it unsuitable for the smaller 2-3/8-inch axle setup listed for the other round-axle options.
Buying Guide: How To Choose A Trailer U Bolt Kit
Does the kit fit your axle shape and size?
Identify whether the axle is round or square, then measure its diameter or outside dimensions. The 3,500-pound SPARKWHIZ and MCCKLE kits are listed for 2-3/8-inch round axles. The larger SPARKWHIZ kit is for a 3-inch round axle. Racewill is specified for a 2-by-2-inch square axle. Capacity alone is not enough to confirm fit; compare the actual measurements.
What U bolt length and spring width do you need?
Compare the listed U bolt length and inside dimensions with the existing hardware and the complete axle-and-spring stack. Two SPARKWHIZ round-axle kits specify 1-3/4-inch leaf springs. The Racewill kit gives inside width and length measurements, which are particularly important for square-axle fit. Do not assume a bolt fits simply because its axle rating is similar.
How many positions are you replacing?
Check the quantity of U bolts, nuts, and tie plates against the work planned. The SPARKWHIZ 2-packs include eight U bolts and four plates; the smaller SPARKWHIZ set includes four U bolts and two plates. MCCKLE’s provided details mention nuts and washers but do not specify all quantities, so verify its package contents before ordering.
Does corrosion protection matter for your trailer?
Consider the trailer’s storage and use environment when comparing listed materials and coatings. SPARKWHIZ describes its steel as rust- and corrosion-resistant; Racewill specifies a zinc-coated surface. These descriptions do not replace checking the product details for your intended conditions.
Frequently Asked Questions
Are all trailer U bolt kits interchangeable?
No. Axle shape, axle size, bolt length, bolt diameter, and spring width can differ. Match the kit dimensions to your specific suspension setup.
Which kit is listed for a 2-3/8-inch round axle?
The SPARKWHIZ 5-1/2-inch kits and MCCKLE 5-1/2-inch kit are listed for 2-3/8-inch round axles. Confirm spring width and other dimensions before choosing.
Can I use the 7-inch kit on a 2-3/8-inch axle?
The SPARKWHIZ 7-inch kit is specified for a 3-inch round axle, not a 2-3/8-inch axle. Select a kit that matches the measured axle.
What does a trailer U bolt kit typically include?
Contents vary. These listings include combinations of U bolts, nuts, tie plates, or washers. Review the stated quantities and compare them with the parts required for the repair.