Attached Link: http://www.bokgi.net/test/001.jpg

I want to make piston. You can download my file. [www.bokgi.net/test/Runtime.zip](http://www.bokgi.net/test/Runtime.zip) There is one character. It has 4 structure. down, up, big, small I want big face to small. But, I can't. Can you help me? How can I make it?

You should be able to use "Point At" on the two halves to make them do what you want.
The caveat is that the pieces need to be rigged with the origin and axis correct positioned so that this works.

It's not obvious from the picture whether the piston geometry is separate from the arm, or if the pieces are part of the arm geometry.
If it's the first case, you should find it relatively easy to setup the parts of the piston before parenting each end to the figure.
If it's the second case, then they must be in a separate group from the part they're meant to be attached to, so that you can add appropriate bones. Adjusting the axes and origin is trickier in the case but is necessary to get "Point At" to work as intended.

"Point at" is recommended by BL Render as well; she describes a piston in her book (Secrets of Figure Creation with Poser 5).

If for some reason you don't want to do it that way, you can use ERC.  The problem is, the rotations of the piston parts are not linear equivalents of the rotation of the joint.  For a small degree of motion, close enough.  For larger degrees of motion, you need ERC math....

Point at will solve your problem.

I selected small and object->point at and selected big, then big and pointed it at small and now they point at each other. I used point at=0.5 on small to keep it centered and it look nice.

You are limited to 20 degree rotate on up because small is short but you could make it longer, but that would also be true in real world. One idea might be to simply let small extend into down. A more detailed model would have a hole in down where big could rotate, it would also enable you to use a longer small.