50mm Square x M16 U-Bolt Kit – 180mm Long

Part No: CM603-5/8 Category: Tag:


6 in stock

The Couplemate™️ 50mm Square U-Bolt Kit (M16) provides a strong and secure connection. Perfect for trailers and caravans up to 4500kg ATM and is suitable for salt water application.

Strong steel u-bolts and plates.
Electro-plated for corrosion resistance.
Includes galvanised nyloc nuts and zinc washers.

N.B. Customers will receive 5/8″ UNF U-bolts instead of M16 due to supply chain delays.

6 in stock

Couplemate accepts PayPal, Visa and Mastercard.

50mm Square Heavy-Duty U-Bolt Kit | Medium & Heavy-Duty Trailers

The Couplemate™ CM603 is a Spring U-bolt kit for 50mm square axles. Designed to secure leaf springs to the axle safely, Couplemate™️ calls these the “heavy-duty” kits because they have thicker U-bolts and fish plates. 

Product Specifications

  • Internal U-Bolt Width: 50mm
  • Internal U-Bolt Leg Length: 180mm
  • Thread Type: M16x2 (or 5/8″ UNF)
  • Coating: Electro-Plated

Couplemate™️ often supplies Heavy-Duty u-bolt kits with trailer axle rebuilds over 1600kg (and up to 4500kg ATM).

This kit is supplied with:

  • 4 x U-Bolts
  • 8 x Washers
  • 8 x Nyloc nuts
  • 2 x Fish Plates
  • 2 x Axle Pads/Spacers

Installation (User Guide)

Couplemate™️ 50mm Square U-Bolt Kits are easy for trailer spring maintenance.

  • Place U-shaped curve over axle beam.
  • Align the centre spring bolt and U-bolt legs with the fish plate holes.
  • Secure fish plate to leaf spring through supplied washers and nylock nuts. Tighten nuts to 100Nm with a socket.

Important: Check U-bolts and re-tighten nuts after the first trip and every 250 km thereafter.

Safety Precautions

Do not re-use nyloc nuts. Nyloc nuts are often less effective after the first use. This is because the nylon material (the key component of a nyloc nut) has already once conformed to the thread profile. 

Do not use a rattle gun to tighten. Rattle guns often tighten u-bolts too fast, too tight, and uneven. This can cause nylon overheat and risks undue wear. 

Ensure the axle has welded axle pads. If not, use the axle pads supplied with the spring U-bolt kit.

test heavy duty 16mm u-bolt kit

Are Heavy-Duty U-Bolts worth it?

Within the industry, Trailer U-Bolts lack a standardised testing procedure.

However, Couplemate™ Engineers has developed its rigorous testing method. In this process, we subject u-bolts, nuts, and fish plates to a static load test until destruction occurs.

All of our 16mm Heavy Duty U-bolt Galvanised Kit undergo a 200-tonne test. During this testing, the u-bolts in the right-hand image ultimately broke at the 23t threshold.

The u-bolts in the right-hand image broke at 23t.

Not convinced? Read why U-Bolts could be the primary cause of highway accidents.

Related Towing Products

16mm Individual U-Bolts

Additional information

Weight5 kg
Dimensions20 × 10 × 5 cm


Internal Length


Internal Width



5/8" UNF, M16x2

Our Service Lifetime Warranty

Our goods come with guarantees that cannot be excluded under the Australian Consumer Law. You are entitled to a replacement or refund for a major failure and compensation for any other reasonably foreseeable loss or damage. You are also entitled to have the goods repaired or replaced if the goods fail to be of acceptable quality and the failure does not amount to a major failure.

Couplemate™ is a manufacturer and vendor of high-quality towing, trailer, and caravan components. Couplemate™ Core Goods and Custom Work are guaranteed to be free of defects for the service lifetime of the product when used as intended.

Warranties are non-transferrable and Couplemate™ requires satisfactory proof of purchase prior to issuing a remedy. The full Couplemate™ Warranty Policy can be found in the Terms & Conditions: Section 8 – Consumer Guarantees & Warranty.

Warranties are in addition to your rights under the Australian Consumer Law. If something is wrong with your order, please contact our team.

60-Day Returns

After 60 years of experience, we know that life has its unpredictable moments.

All Couplemate™ orders (online, over the phone, or in-store) have a 60 day period for change-of-mind returns and exchanges. There are some items that can’t be returned and some returns may incur a 15% restocking fee.

You can view our full policy for change-of-mind returns here.


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