ZFK-K External-Thread Optic Retaining Rings

The ZFK family consists of black, two-slot externally threaded retaining rings. The source identifies anodized aluminum and shows the rings securing optics in a mating threaded tube. Each of the 67 online models has a dedicated specification image labeling its bore and external thread. The range spans SM, fine-pitch metric, RMS and C-mount interfaces, so a model suffix alone is not a reliable fit specification. Select the full thread designation, bore and optic-edge contact arrangement together. Lens tubes, spanners and optics pictured in the installation example are not stated to be supplied with a ring. Model-specific thickness, thread tolerance, axial load and tightening torque are absent from the source; verify these for the actual assembly. The English illustration helps identify the family and is not a dimensioned drawing of any individual SKU.

Continuous ring with two tool slots

The source describes standard externally threaded retaining rings made from anodized aluminum. Dedicated photographs show a black, continuous annular part with an open center and two notches for a spanner. It is not a split spring circlip. The lens tube, spanner and optic in the installation image illustrate use; confirm the supplied contents separately.

Four interface groups from dedicated images

The first six images label SM05 through SM4 threads. Most of the middle selections carry metric M-thread designations with 0.5 mm pitch in their own images. The last two show RMS (0.8 inch–36) and C-mount (1.0 inch–32). Preserve the photographed thread marking instead of deriving a thread class or compatibility claim from the K-series model suffix. Check the mating internal thread, engagement length and entry chamfer.

Bore and thread must be checked independently

All 67 model images separately label the bore and external thread. ZFK-K6A and ZFK-K7A both state M6.5×0.5 but have 3.8 and 5.8 mm bores respectively. ZFK-K39 and ZFK-K40 both state a 37 mm bore, while their threads are M40×0.5 and M41×0.5. The SKU notes retain these source labels, including apparent naming anomalies. An inside diameter alone does not specify an optic’s supported outside diameter or establish a verified thread fit.

Assembly and missing parameters

The two notches provide spanner engagement. Check notch geometry, tool contact, optic-edge support and axial clearance before tightening. The source does not provide per-model ring thickness, slot dimensions, thread class or tolerance, anodizing specification, recommended torque, rated load or pack quantity. Claims of a large clear aperture and support force are qualitative marketing statements without test conditions, not quantified performance guarantees.

Selection checklist

Match the full external-thread designation to the lens tube

Check the labeled bore and optic-edge contact arrangement

Confirm notch geometry, tool engagement and allowable preload

Confirm thickness, tolerance, quantity and actual delivery

To be confirmed

Thread class, tolerance, engagement length and model-specific thickness

Notch dimensions, spanner model and assembly torque

Aluminum grade, anodizing specification and rated load

Supported optic range, included items, quantity and actual delivery