The Mechanic's Best Friend is a unique variant of the pipe wrench melee weapon in Fallout 76. While the first form of the Mechanic's Best Friend was originally given to players as a reward for completing a "Destroy different kinds of robots" weekly task back in 2019, this version of the item has since become legacy content, unable to be obtained by any players who didn't complete this task while it was active. However, a second form of the Mechanic's Best Friend is still obtainable, provided that you have an end-game character that can comfortably handle itself in a fight.
The current iteration of the Mechanic's Best Friend must be manually crafted at a weapons workbench, requiring the second tier of the Makeshift Warrior perk card, as well as a shopping list of gears, oil, steel, and a few Legendary Modules. In addition, you must also have found and learned the weapon plan for the Mechanic's Best Friend, which can rarely be earned through only a few means.
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The most reliable method of finding these particular plans is by completing Daily Ops. From level 50 onward, Daily Ops completions of Initiate Tier - within 16 minutes - or higher will earn you at least one high-level reward roll. Of these high-level rewards, the Mechanic's Best Friend plans are one of 41 items found within the rare high-level reward table, which itself rolls at a 50% chance.
As only one high-level reward can roll per day, you'll need to keep logging in every day to retry the Daily Op until you get the drop you're after. Alternatively, the roaming trader Minerva can occasionally have the Mechanic's Best Friend plans as part of her rotating stock in exchange for gold bullion, depending on when and where you find her. The plans themselves are also tradeable between players, meaning that you have a nonzero chance of finding them in player-run shops.