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Improved Turning Clearance with 2015 Ram 2500 and Keystone 5th Wheel with Fabex 663 Pin Box


Would the Lippert LC 328330-1621 be a compatible replacement for the Fabex 663. The bolt pattern looks like its the same. If not , why?


Expert Reply:

We have replacement pin boxes that will work for you, but the Lippert Components Trailair Rota-Flex 5th Wheel Pin Box # LC328330 is not a compatible replacement for a Fabex 663 pin box. The # LC328330 is designed as a replacement for the Lippert 1621 and 1621HD pin box.

The measurements between the Lippert 1621 and Fabex 663 are similar in some aspects, but very different in others. I have attached a help article on measuring to determine the correct replacement pin box for you. It includes a chart with the measurements of different pin boxes.

Based on your previous questions, it looks like you are trying to improve your turning clearance with your Keystone 5th wheel. For your setup, a Sidewinder will be the best way to go. An extended pin box will not give you the additional turning clearance your truck and trailer needs.

I would like to clarify some of the calculations that we provided to you earlier.

Currently, your king pin sits 15 inches behind the nose of your trailer, but you are using a medium length pin box. Once you replace your Fabex 663 with a Sidewinder like part # RP34FR, the king pin will actually move forward about 9 inches toward the nose of your trailer because the Sidewinder is 9 inches longer than the Fabex 663 pin box. The overhang of your trailer's nose will not be a factor when calculating clearance with the Sidewinder because it doesn't affect the pivot point between the truck and trailer.

expert reply by:
John H

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