English
Français
Español
Árabe
Deutsch
简体中文
Português
Русский
日本語
한국어
हिंदी
Prancis
Türkçe
Tiếng Việt
Italiano
ไทย
A 2-inch kingpin and a 3.5-inch kingpin are different nominal coupling sizes. They need the corresponding fifth wheel specification. The larger diameter is not a universal upgrade for every tractor, and diameter alone does not define the trailer connection’s permitted application.
The Size Comparison
The nominal sizes correspond to 50.8 mm and 88.9 mm respectively. These figures describe the coupling size, not every dimension of the pin or its mounting assembly. JOST’s catalogue identifies 2-inch kingpins for corresponding 2-inch fifth wheels and 3.5-inch kingpins for corresponding 3.5-inch fifth wheels. For a fleet using several tractors, record the coupling equipment on each tractor that will handle the trailer.
Why the Mounting System Matters
A kingpin connects through a supporting plate and a specific retention arrangement. Its nominal diameter does not identify those parts. Two pins of the same size can belong to different mounting designs. JOST also explains that the permitted D-value depends on the kingpin type, skid plate thickness and bracing. The connection drawing and component designation therefore matter alongside the size shown in a quotation.
Replacement and Conversion Are Different Enquiries
A like-for-like replacement starts with the existing pin and retention-system identification. A conversion from one nominal size to another also requires review of the mounting design, fifth wheel and permitted application. Some manufacturer designs allow specified conversions; that is not permission to convert every trailer. Installation, conversion and wear assessment must follow the actual manufacturer’s instructions and applicable requirements, with qualified personnel responsible for the work.
Build the Parts Record Before Ordering
Keep photographs of the kingpin marking, the associated retention arrangement and the tractor coupling identification with the trailer record. State whether the quotation is for the pin, the associated assembly or included hardware. A worn surface is a poor basis for identifying a nominal size: the workshop should use the appropriate equipment documentation and wear-checking method. Once the part is selected, keep the approved designation for later replacements.
2-Inch and 3.5-Inch Kingpins at a Glance
| Check | 2-inch size | 3.5-inch size |
|---|---|---|
| Nominal coupling diameter | 50.8 mm | 88.9 mm |
| Fifth wheel match | Corresponding 2-inch coupling specification. | Corresponding 3.5-inch coupling specification. |
| Mounting identity | Confirm pin type, retention system and supporting plate. | Confirm pin type, retention system and supporting plate. |
| Permitted application | Determine from the approved connection system, not diameter alone. | Determine from the approved connection system, not diameter alone. |
Information for a Kingpin Replacement Request
Trailer identification and existing component markings.
Fifth wheel manufacturer and model for the tractor or tractors used.
Connection drawing or available mounting-system information.
The required assembly scope and manufacturer documentation.
Compatibility Questions
Can a 3.5-inch pin be used in a 2-inch fifth wheel?
Do not treat those nominal sizes as interchangeable. Match the pin and coupling to their documented specifications.
Does a larger pin automatically permit a heavier cargo?
No. The approved connection system, trailer and tractor ratings, load distribution and applicable limits still govern the application.
Related Products and Reading
Sources and Application Notes
Size matching and mounting-system information are referenced to JOST’s catalogue. They do not establish the rating of an unidentified GENRON-supplied part.
Ask GENRON about kingpin and fifth wheel compatibility. Include the existing component markings and tractor coupling details, and identify whether you need a replacement pin or a complete assembly.
