Transmission

For a used gearbox, the inspection is the point. Photos won't show wear on shift forks, selector dogs, or bearing journals — and those are the parts that determine whether you're buying a working unit or a rebuild project. If the seller is within a reasonable drive, go. Buying blind means pricing in unknown faults and only using a seller who gives you a meaningful return window.

For late Big Twin work, 1992 to 1999 FXR and FXST donor cases are the most sought-after — the five-speed was well sorted by then and late-run cases tend to have fewer casting issues. For Shovelhead-to-five-speed swaps, the change from four to five speeds came in 1980, so anything mid-1980s onwards in the Big Twin line gives you the correct architecture. Always inspect the mainshaft bore for wear before committing.

Harley specifies their own Formula+ gear lubricant for the 6-speed; AMSOIL Severe Gear 75W-110 and Spectro 6-Speed are the most common aftermarket equivalents used by independent shops. Avoid ATF or standard motor oil — the constant-mesh gear geometry needs a lubricant that resists the specific shear loading of this design. Change it at the same interval as the engine oil; it's one of the quickest service jobs on the bike.