I'm attempting to convert an older freewheel hub (see bottom of page here: https://www.parktool.com/blog/repair-...ewheel-removal ) to a single speed (single cog spaced out with multiple spacers), but have never worked with a non-lockring style freewheel. Are the threads on these older style freewheels the same dimensions as modern cogs/lockrings?

Any help would be appreciated. Thanks in advance.

??? You mean a cassette style cog? Freewheels and cassette/freehubs are completely different. If you mean to make a singlespeed fixed-gear, while the cog will thread on to the freewheel side, there won't be the reverse threads for a lockring that'll keep it secured.

You can buy a singlespeed freewheel that'll thread on as a replacement to your existing freewheel. White Industries makes swank singlespeed freewheels (~$90-100), though there are much cheaper alternatives made by ACS, Odyssey, and Shimano (~$20).

Older cassettes used the small cog to hold the rest on
Teardown? It just needs 2 chain-whips,
One holds a cog on the splined center portion,
pulling in the opposite direction, [CW] from the one unscrewing [CCW]
the smallest cog.

In practical terms , Building a single speed wheel is better ,
you won't gain the improved wheel strength ,
leaving it dished, imbalanced spoke tensions,
to make room for 6 speeds.

I may just have incompatibility issues in general (or not know what I'm doing), but was hoping to use a surly converter kit:

https://surlybikes.com/parts/spacer_kit/

Incompatibility; that will only work on a "modern" hub that uses a cassette, rather than an older style freewheel hub (well, they still make freewheel hubs, though they're typically meant for singlespeed use these days).

What *will* work is a singlespeed freewheel such as:
https://www.whiteind.com/singlespeedg...reewheels.html

There are much cheaper ones available though, typically used for kids singlespeed/BMX bikes.

Thanks for the help so far--after managing to remove the cassette from my wheel, and googling for a while, I realize that my terminology was all wrong (as you pointed out anotherbrain)! Sorry for the confusion.

So I definitely have a freehub with a cassette (held on via splines), but as on the park tools website it's an older model with no lockring. I had to use the two chain whip method to remove the outer most cog, and then the rest of the cassette slid off the freehub. So I think my best bet for a SS conversion is to use a BMX style cassette cog and a series of spacers to get the chainline right. Then cap it with a lockring, as in the surly SS conversion kit. Does that sound right? I'm wondering though, if a lockring will fit on the treads on that freehub, or will I have to use the original outermost cog (or a different kind of lockring) to hold the spacers and cassette cog on. Thoughts here?

Thanks for bearing with me!

I don't think the lockring from the Surly kit will work- it's designed for a Hyperglide type cassette, with internal threads. Your hub has only the external threads- but you're on the right track. Just use the original outer cog to hold the new cog and spacers together. It'll look a little funky, but it'll hold together. If you want a neater solution, an old-style bottom bracket lockring is the same thread.