the replacement front sight on my repro measures .52 from the top of the barrel to the top of the blade. don't be afraid to go ahead and make it taller. you can always file it back down if it shoots too low. gives you some wriggle room. if this is an original spencer you will decrease its value considerably by replacing or modifying sights or anything else. original spencer carbines are starting to hit $3k on the right coast. if it is a repro thensight replacement or modification will be necessary. good luck, geo.