I would like to change the front sight bead on my bss 20ga but have no idea how to determine the correct thread type for the new one, any suggestions?

Call the folks at MGW, they should know what you need. I get all my browning parts from them. Great folks to do business with.

Browning Gun Parts: MGW

Yes, i have dealt with them in the past they are good guys, i'll try them. They used to work for Browning and bought out all there old and obsolete parts.
thanks

I checked out their site-didn't see anything for a BSS but they had a ton of parts for the A-5.

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Right, they don't list the bss but i figure they might know or have access to the info.

It's made in Japan so I'd guess it's a metric,,maybe one of the 2mm sizes (2, 2.2, 2.5mm).
Our #3-56 is one of the most common sizes and very close to those 2+mm sizes.
Most metrics are simply re-tapped to a common US size and a standard replacement installed in US shops.

#3-56 is one of the most common F/Site bead thread sizes, though I've seen them as small as #2-56 and up to #6-48.

#6-48 usually reserved for the larger glow-bead type sites. I think the old Ithaca Ray-Bar is 6-48

There's also the distant possibility that the front site bead is the smooth tapered pin type (unthreaded) that simply force fits into the likewise tapered hole in the rib. Unlikely I think,,but I thought I'd mention it.