A Bourne man and his Plymouth sidekick pleaded guilty today to a federal charge of conspiracy to damage, by means of an explosive for the way they broke into a building at Harvard Medical School and set off a "large commercial firework" in a locker up on the fourth floor. In plea deals with Logan David Patterson, 18, of Plymouth, and Dominick Frank Cardoza, 21, of Bourne, the US Attorney's office agreed to recommend a sentence of three years of probation and fines, but no prison time, based on the fact the two both admitted their guilt and had no previous sentences. US District Court Judge Angel Kelley set sentencing for Aug. 4; she is free to disregard the recommendation. The maximum possible sentence for the charge is five years in federal prison.…