With all components cleaned and prepared, the optical system of the endoscope enters the assembly stage. The objective lens, rod lenses, prism, spacer tubes and sealing components - initially separate parts - are integrated back into a complete optical path. Assembly sequence, positional accuracy and sealing reliability together determine the imaging quality and clinical service life of the repaired endoscope. Starting with installation of the front objective lens, this article provides a systematic walkthrough of the entire reassembly procedure: rod lens insertion, prism mounting and angle adjustment, instrument-channel sealing, and structural repair of the rear interface.
Objective Lens and Rod Lens Assembly
After the objective lens has been cleaned, grip it by the rear end with clean forceps and guide it steadily into the objective lens tube. Avoid touching the optical surfaces with the forceps, so as not to introduce new contamination.
The rod lenses and spacer tubes must be inserted one at a time, with each part cleaned prior to insertion. Before being fed into the tube, each rod lens and spacer tube is first brushed to remove surface dust, then wiped with a cotton swab moistened with alcohol to clean the lens surfaces and tube walls, and finally dried with an air blower to remove residual alcohol, ensuring the internal channel remains dry and clean. All parts are inserted in the designed sequence; skipping or mixing the order is not permitted.
Installation of the Rear Interface Components
The sleeve that houses the rod lens is positioned against the corresponding mounting holes; this sleeve fixes the axial position of the rod lens assembly within the tube. The assembly includes a spring spacer tube: once installed, pressing it should produce visible vertical spring movement. The washer must be seated according to the alignment between its opening and the internal structure to ensure correct positioning.
Prism Assembly
In the optical path of the endoscope, the prism reflects the image from the objective side into the relay tube, and its alignment accuracy directly affects the orientation and completeness of the image. A central screw on the prism is provided for angle adjustment. Before assembly, clean the reflective surface of the prism to ensure it is free of dust, oil or other contamination. With the reflective surface facing inward, place the prism at the washer position, install the rear end cap, then insert the screws into the two remaining holes and tighten them preliminarily.
Precise prism adjustment can be performed only after the relay system and the eyepiece system have been installed. The relay system consists of a long spacer tube, a short spacer tube and a cemented doublet lens sandwiched between them, with the following specifications: short spacer tube 24.97 mm, long spacer tube 59.54 mm, doublet lens 5.08 mm; all three are 11 mm in diameter. The long spacer tube, doublet lens and short spacer tube are inserted into the tube in that order to complete the relay optical path.
A right-angle adapter is then fitted for temporary assembly, and once the lateral objective system is installed, the scope is essentially complete. The image is then observed and the rotational angle of the prism is finely adjusted to ensure correct image orientation.
Prism Fixation
Once the prism angle has been set, a 200 °C epoxy resin adhesive is used for fixation. The adhesive is mixed at a 1:1 ratio and applied to secure the prism's elevation screw and the two lateral screws. During adjustment, the fine slotted screw on the prism controls the elevation angle of the image, while the two lateral screws both secure the prism and fine-tune the horizontal orientation of the image. The angle should be verified repeatedly until the image is confirmed to be transmitted to the eyepiece end intact and upright. The amount of adhesive must be strictly controlled: too much will make later disassembly and re-repair difficult, while too little cannot guarantee long-term stability in service.
Sealing of the Instrument Tube
Apply the 8008 Japanese special quick-curing adhesive around the edge of the instrument tube. This adhesive cures by absorbing moisture from the air and withstands high temperatures. It helps keep the internal cavity relatively dry and, when used as a temporary seal, provides moderate strength and is easy to disassemble.
Press the white Teflon washer into the instrument tube and seat it firmly to form an isolation ring. If the washer is oversized, trim the excess edge with a scalpel along the indentation; any gaps can be filled with the 8008 quick-curing adhesive. After filling, use a cotton swab moistened with alcohol to clean up the adhesive that has squeezed out, while leaving an appropriate amount of residue so that the upper epoxy layer does not seep downward.
Rear Interface Repair
In this case, the rear copper cover plate was missing and the instrument tube was cracked. The repair approach was to shorten the damaged instrument tube and fit a connector sleeve to replace the original design and join the instrument tube interface. A high-hardness adhesive rated for 600 °C was mixed at a 1:2 ratio, with a total amount of approximately 3 g. The adhesive was applied evenly to the outside of the instrument tube and at the interface, with a layer thickness of about 1 mm, filling the interface area as completely as possible. The connector sleeve was then placed and pressed into position to form a sealing layer, preventing moisture or impurities from entering the scope during high-temperature, high-pressure sterilization. Since the instrument channel comes into direct contact with water, sealing at the interface is a priority.
To further strengthen the rear interface, adhesive is applied to the tube wall at the outer edge of the interface. The scope body must be kept level when the rear bridge interface is installed, to prevent the adhesive from running. After the adhesive has fully cured, a final assembly check is performed; if further tightening is required, a suitable wrench can be selected according to the dimensions of the connection.
Channel Patency Test and Valve Treatment
After assembly, a 5 mm drill bit or the inner sheath channel is used to test the patency of the entire instrument channel and confirm that there is no blockage.
If the valve nut is loose, apply adhesive to the inside of the threads and re-thread the nut to secure it.
Closing Remarks
Endoscope repair is not merely about replacing parts; it is about restoring damaged equipment to clinical standards through careful manual work and sound process design. In the following articles, we will continue to share hands-on experience and process highlights from repair cases involving endoscopes of different brands and models. Stay tuned.
