Optimus' entire frame shuddered as Megatron bucked up into him, the unexpected motion coinciding with the sudden shock of old programming rearing up at him at finally being filled again. He used to function for that feeling, and he had to fight to keep from letting it overwhelm him. Optimus froze briefly, keening softly as his internal mechanisms readjusted from clenched-tight to the sudden, sizable entry.

He only shivered again, armor rattling, as every time he self-adjusted, Megatron's spike only slipped more and more firmly into place. Every ridge and nook fit snugly into the dips and curves of his own valve; calipers and filaments meshing and ensnaring the charge between them with engine-stalling ease.

It was all he could do to hold himself upright, hands shaking against Megatron's chest as the cables twitching in his Highlord's ports sent everything across - and back again.

"...Primus. You fit perfectly..."