I have a cylinder with a thread pattern in the middle, and I am trying to extend it. I tried using the boolean modifier on the middle section and was able to use the array modifier to extend it but it didn't merge. Even though the pattern matched the vertexes are at different positions.
Convert your tris to quads with an AltJ, simplify the mesh:
Hide the cylinder, use the Knife to cut a segment (K to activate, A to cut perpendicularly, C to cut through), and press V to rip right away:
Remove the other parts:
Unhide the cylinder and align it with the segment with the Snap (to Vertex) option:
Right click > Shade Smooth, in the Object Data panel click on Clear Custom Clear Split Normals Data, and Auto Smooth :
Apply the Rotation and Scale of your object, give it an Array modifier on Y, enable the Merge option :
You’ll need to correct the topology to make the vertices of the instances stick to each other: