This may be a typo or a highly specific regional/internal term. To help clarify, it might be one of the following:
If you are referring to a transmission (often shortened to "trannie" in some regions), "longmint" might be a misspelling of a specific brand, a "long-range" modification, or a "mint" (pristine condition) long-tail transmission.
It is possible "longmint" was meant to be "long-range," "long-mount," or a specific brand name like "Longacre."
