On October 8, 1997, about 2132 eastern daylight time, a Beech 18 [C-45H], N486JB, operated by Caribe Air Cargo, as a Title 14 CFR Part 135, cargo flight, made a gear-up landing near Orlando, Florida. Visual meteorological conditions prevailed and an IFR flight plan was filed. The airplane received minor damage. The commercial pilot was not injured. The flight had departed Melbourne, Florida, at 2050. The pilot said that while on base leg for runway 18R, he lowered the landing gear and "noticed that the right main gear was not down and locked." He advised ATC of the situation and was given vectors to the west of the airport to investigate the problem. The pilot said he moved the gear handle to the "UP" position, then lowered the gear, and again it indicated that the "gear was not down and locked." He then attempted "gear emergency extension procedures...and observe[d] that the gear was still [indicating an] intermediate position." He again advised ATC, and made a low pass by the control tower, in an attempt to have the tower personnel confirm the position of the landing gear. Due to the darkness the tower personnel could not confirm the position of the gear. The pilot made additional attempts to determine the problem without successes. He then elected to make a gear up landing on the runway. The airplane touched down, slid on the runway and came to a stop. Examination of the landing gear revealed that the chain from the main landing gear torque shaft to the nose gear box had come off the sprocket at the nose gear box, exposing several teeth. This caused the landing gear to be out of sequence between the main gear and the nose gear. This resulted in the nose gear retract nut bottoming out against the nose gear box, making it impossible to extend the landing gear fully, either by normal or emergency extension methods.