Should probably post this question over here instead of the vintage forum.

2x6 downtube friction shifter setup with a Shimano 600 rd over a Regina freewheel in the rear. If I was to replace the freewheel with a 14-28T is there anything I need to consider when choosing one?

Here is one for example.

Helps to know exactly what RD that is. The 600 or other names they put on them don't necessarily mean that any with that logo/name have the same specifications. Shimano usually has the part number stamped on the back of the parallelogram. But for that old, it might be elsewhere. It'll start out as RD- along with some numbers and maybe alphabets too.

But that looks like a rd-6200. So you should be good for any freewheel that has 28 or less teeth as the max low sprocket. As long as you aren't changing other stuff, you should be good with a 14-28. Though you probably will need a chain also.

That old Regina FW might have a non-standard hub attachment that you won't find on todays FW's. So you might also need a new wheel or at least a new hub. Depending on what you wish to do.

Thank you. When you say hub attachment you mean how the FW attaches to the hub correct? What would I be looking for? A thread direction?

Appreciate the tip because Im trying to use the original wheels which are already iffy to me and if Im going down the road of replacing the freewheel and then need new hub ... I may be better off finding some takeoffs on Marketplace or something.

Prescient admission that.

Were I on your saddle I'd start looking for new built wheels & appropriate FW.

Not thread direction. But different thread standards. Even if the threading is the same diameter and pitch, whether it was French, Italian or English might be different. And the bike the country was made in might not be a good determinant of whether it is or isn't. Things were changing between the late 60's to early 80's for bikes then. As the increasing international trade made manufacturers realize that they needed to agree on something so that a part made in one place will fit any bicycle made elsewhere.

I only know that some old Regina FW's aren't standard threading. Maybe another member here knows for certain.

This is a good point. If it's Italian threading, though, you can thread an English threaded freewheel on the hub. That said, it wouldn't surprise me if these were OEM wheels to the OP's Trek in which case they were likely English threaded to begin with.

yes.. consider first that while shimano arabesque derailleurs look really sweet, they kinda SUCK in regard to basic derailleur functions.
among other issues, they have no b-screw, which limits the size of freewheel they can be made to semi-function with.

the one pictured is a short cage version, making it even more sucky.

you'll want a different derailleur made to work with a larger cassette or freewheel, and a longer chain.

another consideration.. old 5 and 6 sp. road bike hubs are typically 126 O.L.D. axle length... that axle length may not allow room for a 7 sp. freewheel, and there may very well be frame clearance issues too.. like, when the chain is on 7, it may rub the frame... i've also seen the 7 cog Hit the frame....

been there, seen that.
my '82 trek remains a 6 speed... for a reason.
no, i won't be bending the stays to fit the old suntour 7 sp. i bought ten years ago.... the trek will remain a 6 sp.
the Cyclone derailleur is also still on that bike.. it works WORLDS better than the Arabesque i attempted to use once.... Once.
i tried an arabesque on another build last year... that bike now wears a Shimano STX der set, and the cassette is now a 13-30.....

Appreciate all the responses. Have ordered a freewheel removal fitting from Park to try and service the Regina. Will get some pictures and measurements of the threading when I'm in there. I think plan A is service it and ride it til I figure out if it's worth upgrading FW or spring for a new wheelset with modern standards.

Would you believe the bike in question is also an 82 Trek (412) ...

the 412 MIGHT have some extra room in the area... mine is a 930 race geometry framed bike (bought new by me, in 1983)... and cold setting a steel frame is a doable thing... BUT., it's not just a "grab, Yank, and done" thing.. the chain stays usually don't move an equal amount, and then you also need to to reset the DROPOUTS, or the axle gets flexed, causing fast bearing surface wear......

check out some youtubes on cold setting bicycle frames.
the '84 or so Trek Elance 300 in the sale rack is running a 7 speed, no problem.. i bought it set up as a 7 speed rear, triple front.
i don't know if it's still a 126 OLD, or ever was.
the trek '85 1500 alu. is a 7sp....130 OLD, IIRC.

You might find this Sheldon Brown article helpful or at least interesting...

https://www.sheldonbrown.com/freewheels.html

And here are a couple threads here on BF about them....

Freewheel threading

Regina Freewheel

In the last thread above, post number 6 gives how Regina marked the type threading that was on their FW's.

Regina Freewheel

Since it's a Trek, I'd tend to give better odds that it's English threaded. And that is what became the standard for most FW's today. But I'm just guessing.

Brought the wheel to two local bikeshops this morning. I was going to let them take it off and put a new freewheel on it and true the wheel to make it worth their time. Neither one had a tool to remove the Regina. They advised me to seek the council of an elder bike mechanical wizard Bob who may possess such an arcane device of antiquity. Such a man does not work Fridays though. Their guess was English threaded as well.

Your rear derailleur should be able to handle a 28T freewheel, unless you have a particularly large jump in the front chainrings, in which case you might need a longer-cage rear derailleur to wrap enough chain. And you most likely will need a longer chain to accommodate the bigger freewheel sprockets. Personally, I have little fondness for Regina freewheels. Modern profiled freewheel sprocket teeth such as found on Shimano and SunRace freewheels will shift phenomenally better than that Regina unit, even though they're cheap freewheels. Run 'em till they die, then replace.

The proper Park tool is the FR-4. Bicycle Research made one as well, the CT-2, although you probably will have to remove the axle locknut and spacer to seat the tool properly. You'll also have to do that with the Park tool if the freewheel is mounted on a Campagnolo or Zeus hub. If it's a Phil Wood hub, you need Phil's tool to remove the freewheel, as Phil Wood axles don't have a removable spacer. Phil's tool is very thin-wall, to slip over the axle despite this. The Phil tool (and Zeus tool) will also allow removal from Campagnolo and Zeus hubs without disassembly to remove the spacer.

Appreciate all the help in this thread. The elder had the Regina off in about 2 minutes today. I purchased a new 7 speed Sunlite 14-28t all chrome freewheel from them for the trouble. Spun right on after I cleaned up the hubs. Put it in the dropout and doesn't appear to be clearance issues but we'll see when the new chain gets here if there's any rubbing. Plenty room for a spacer looks like if it comes to it.

I never liked Regina, used Suntour New Winner until I switched to Shimano HG cassettes. I recently built a friction-shifting bike with modern SRAM cassette and chain, it's weird how smoothly it shifts, just dropping quietly into gear with no hesitation or chain rattle.