I haven't installed one on a 640lx, but I did install one on my 2001 Nitro 188. The kit I bought had the older cylinder Ray mentioned. I had to loosen my outboard and move it out of the splashwell to get the cylinder installed. At 24 inches it was just an inch or two too long. The new cylinder is 21 inches according to the teleflex website so that should help in the installation. Other than that it was a pretty easy install. I ran the tubing through conduit to make it easier to get from the back to the console and to add a little more protection for the tubing. Adding the fluid and bleeding the system took just a few minutes and it has working without any problems for a year and a half.
Larry