One of 3 barrels for my Remi 31 had messed up bead sight. I soaked end of the barrel in Ballistol Lube and it came off w/o problem. I bought HiViz Spark II front sight set (thread sizes 6-48, 5-40, 3-56, 3mmx.5 and 3mmx.6). My problem is none will fit barrel bead hole. Does anyone know what diameter in mm I should get?

I don't think a Remington model 31 would have a metric threaded bead.

I don't want to spend $ buying them $10 "light bug" sights, a caliper or micrometer to spec ole' bead, so I'm going to stop at local Lowe's or Home Dept. and look at them lil' bolts with hex screw heads. I already have can of floursecent paint.

Try shooting some clays without any bead at all. Focus on the target instead of the barrels - you might be surprised at how unnecessary "sights" can be on a scattergun.

hopefully you kept the original bead. go to a gunsmith shop and have them try it in one of their thread gauges. Most gunsmiths make their own from time to time, at least I did. then buy something that you need from them like ammo, I would have helped a guy hoping he would come back for a project gun. home depot type store don't carry most of the gunsmithing threads.

Is the hole too big for the 6-48. If so you might try an 8-40. I would definately agree with Jim, assuming originality it won't be Metric. Machine screw sizes start with 0 at .060" nominal dia & go up in .013" steps. Thus a #3 =.099", #5 =.125", #6 =.138" & #8 =.164". You can mic the old shank & it will likely be just a couple thousdanths below one of these sizes. Most all gunsight threads are in standard dia's but finer threads than "Hardware" standards.