5/16= drill F decimal .2570
1/4-20 IS A #7 , I think. 5/16-18 is what? Thanks.
5/16= drill F decimal .2570
Here's two of many sites with all the info!..
EngineersEdge! (http://www.engineersedge.com/tap_drill_chart.htm)
NewmanTools! (http://www.newmantools.com/tapdrill.htm)
Thank you.