Minimum horizontal dropout length for a singlespeed?
 

How long do I need? I'm looking at an old Trek 420 for a winter SS build.

Thanks.

with a half link any gear combination can be obtained within 1/4" of dropout.

remember that you need enough space for the nut to grab on, plus the width of the axle. 1/2" should be sufficient.

if you're willing to do some math, you can bring that length down even further.

my roommate used to have a SS with vertical dropouts. he used a chain tensioner