Slotted Nut

How a castle nut locks

A castle nut (castellated or slotted nut) is a hex nut with six slots across the top. Once the nut is tightened, it is turned to line up a slot with a hole drilled through the bolt, and a split pin (cotter pin) goes through the slot and the hole and is bent over so it cannot fall out. The pin physically stops the nut from turning.

Hobson's anti-vibration paper describes the castle nut and cotter pin method as giving positive anti-loosening and as suitable for critical applications, and particularly good where the tension needed in the bolt is small. That describes the locking method, not a rating for these grade 2 and class 5 nuts: Hobson publishes no rating for them; for axles use the nut and grade the axle maker specifies. The paper also notes that in a tensioned joint it may be necessary to tighten slightly further to line a slot up with the hole. The bolt needs a cross hole: a castle nut on an undrilled bolt is only a plain nut.

Castle nuts with split pins are the arrangement found on many axles, pivots, linkages and clevis joints on trailers, farm machinery and plant. Hobson publishes no axle or load rating for these nuts, so on axle, steering and suspension parts use only the nut, thread, class and setting procedure the axle or vehicle maker specifies.

Check the thread: most metric castle nuts here are fine pitch

The zinc plated metric castle nuts are DIN 935 fine pitch: M8 x 1.00, M10 x 1.25, M12 x 1.50, M16 x 1.50, M20 x 1.50 and M24 x 2.00. They do not fit a standard coarse thread bolt of the same size. The only coarse thread metric castle nut we carry is the plain M16 x 2.00.

Hobson lists the imperial castle nuts as UNF, from 1/4 inch x 28 TPI to 3 inch x 12 TPI, plus a 1 inch x 14 TPI. Strictly, the 1 inch x 14 TPI and the 12 TPI sizes over 1-1/2 inch are not in the standard UNF series (they are UNS and 12-UN threads), so check the bolt or axle stub thread before you order.

Castle nut sizes: M8 to M24 and 1/4 to 3 inch UNF

Every castle nut we carry, metric then imperial, smallest to largest. Select a size to open it in the selector.

Strength and finish

• •Metric: DIN 935, property class 5. Imperial: ASME B18.2.2, grade 2. Both are commercial strength nuts. Hobson publishes no proof load or tightening torque for these castle nuts; where a joint calls for a higher strength nut, use what the designer or machine maker specifies.
• •Zinc plated: a 5 micron zinc coating, recommended by Hobson for indoor and non-corrosive conditions.
• •Plain (the M16 coarse): no added coating, supplied lightly oiled.
• •Outdoors these finishes give limited protection, and for parts exposed to the weather Hobson's corrosion guide calls for stainless or hot dip galvanising, which we do not carry in castle nuts. Check them at service time and replace any that have rusted.

Split pins for castle nuts

Split pins are sized by the hole they fit: an M4 split pin suits a 4mm hole. Measure the cross hole in the bolt, pick the split pin of that size, and choose a length that passes through the nut slots with enough leg to bend back. We carry split pins from M0.8 to M13 in zinc plated steel and 304 and 316 stainless on our split pin page. Fit a new split pin each time: bending the legs work-hardens the wire.

How to fit a castle nut and split pin

1. Check the bolt or stub has a cross hole and that its thread matches the nut (fine, coarse or UNF).
2. Run the nut on and tighten it to the torque or setting the machine or axle maker gives.
3. Look through the slots for the hole. Hobson's anti-vibration paper notes it may be necessary to tighten slightly further to line up a slot with the hole in a tensioned joint.
4. Push a new split pin of the hole size through the slot and the hole until its eye sits against the nut.
5. Bend one leg up over the end of the bolt and the other down against the flat of the nut, or spread them apart, and trim long legs.

Castle nuts in Adelaide

Allmat supplies castle nuts, split pins, R clips and machinery fasteners to mechanics, farmers, fitters and trade across Adelaide and South Australia. Add them to a quote with the rest of your list, and we will price the lot.

Questions