42mm Bone Nut For Fender Stratocaster Telecaster Squier Strat Tele Style Guitars
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Please note: pre-shaped nuts are not generally drop in replacements. Some sanding or shimming may be necessary for proper fit. To shorten the height or width of the nut, you can place a piece of sand paper on a flat surface. Carefully and evenly move the nut back and forth with equal pressure to desired thickness or height. Depending on your string size and fretboard radius, some slot filing may be required as well.

Do I need a flat bottom or curved bottom nut?
Do I need 42mm or 43mm?

That could depend on your guitar or your preference. Guitar spec's usually include nut length (See Fender website for example). Sometimes spec's can differ slightly from what is actually on the guitar. The difference between 42mm and 43mm is very slight (42mm is approximately 1.65" or 1 5/8"---43mm is approximately 1.69" or 1 11/16") Many Fender, Squier, and other Fender style guitars spec 42mm nut. Some use 43mm or (42.8). Some Afinity's spec 40.6mm. For best results, it is advisable to measure your present nut or go with what is spec'd  for your guitar. Some prefer a 43mm where 42mm would generally be used, because 43mm sometimes gives slightly wider string spacing, and/or the opportunity to sand the ends of the nut to precise width of the neck...or not.