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251601 - Castle Nut – 5/16 BSF

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OEM Part
OEM: Made by an Original Equipment Manufacturer. This part is made by an original equipment manufacturer or under their license, ensuring compatibility and quality.
Castle Nut - 5/16 BSF is a standard hexagon slotted nut in 5/16 BSF (British Standard Fine) thread, used across a range of fastening applications on Series I, II, IIA, and III Land Rovers.

The slotted crown accepts a split pin to lock the nut in position, preventing loosening under vibration - a common requirement on suspension, steering, and axle assemblies of the Series era.
Vehicle Model Land Rover Series 1
Land Rover Series 2 and 3
Fitment Notes Fitment data limited — derived from web research, verify if uncertain
SERIES I/II/IIA/III — 5/16 BSF castle nut, standard Series-era fastener
Alternative Codes
251601

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Frequently asked questions

What is the 5/16 BSF castle nut used for?
The 5/16 BSF castle nut is a slotted hexagon nut with a 5/16 inch British Standard Fine thread. Its slotted crown takes a split pin through a drilled hole in the bolt or shaft, locking the nut so it can't vibrate loose. It's used on Series suspension, steering, and axle fastenings.
Which Land Rovers use the 5/16 BSF castle nut?
The 5/16 BSF castle nut fits Series 1, 2, 2A, and 3 Land Rovers, which use British Standard Fine threads. It won't fit later metric vehicles. The castellated design is common on hub, steering, and suspension fastenings where a split pin locks the nut in place.
Do I need a new split pin with the castle nut?
Yes, always fit a fresh split pin with a castle nut. Split pins are single-use and weaken once bent and removed, so reusing one risks the nut backing off. Fit the new pin through the nut's slots and the drilled hole, then spread the legs to lock it.

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