Joint energon

Joint energon (ジョイントエネルゴン jointo enerugon) is a variety of energon that allows Transformer bodies to develop combiner joints. It flows within these joints and can also be produced as energon cubes known as joint cubes (ジョイントキューブ jointo kyūbu).

Fiction

Unite Warriors

Moonheart amputated the Disaster Relief Team's combiner joints to get at the joint energon within, which let the 4 Guards and Megaempress combine into Megatronia. Megatronia Chapter Trickdiamond and Thrust broke into an Autobot energon factory where joint cubes were produced, stole them and fed them to Unicron. The energon granted Unicron's disembodied head a neck in the form of a giant combiner joint that he used to combine with Cybertron itself. Baldigus Chapter, Part One