thread size for Lyman r55?

[Castlead]

Anyone know what thread size is used for a Lyman r55 screw in aperture?

[hunterldh]

Lyman, Redfield and Williams receiver sights are threaded 7/32-40. Marbles tang sights are threaded smaller. I can't tell you about Lyman tang sights as I sold my last one several years ago. I ordered my 7/32-40 tap and required #11 drill bit on-line and didn't have any issues finding them.