As stocks of freewheels dwindle, it's harder and harder to get quality parts for our vintage bikes. Has anyone ever heard of a screw-on freewheel body that accepts SRAM/ Shimano hyperglide - splined cassette cogs? Is there such a beast?

No, not available or realistically to be done for HG. The inner measurement of HG splines is smaller than the threads of a freewheel hub. This would mean that the innermost HG sprocket would have to sit quite far out from the hub flange. See below, red is about the ID of HG sprockets (this doesn't really take into account much of a FW body thickness).

Most freewheels uses large spline ID for the inner sprockets so allow the sprockets & body to sit over the threaded portion of the hub.

there was and might still be a freehub engineered for 120 mm spacing, 126 also.

conversely, a Dura-Ace EX freehub.
I have a set that was designed for 120 mm spacing. purchased NOS, NIP , before the potential value was known.

Shimano makes several freewheels with splined Hyperglide cogs. It just happens that the bore and spline dimensions are different than Hyperglide cassette cogs. As noted in the second post, the diameter of the bore of the larger freewheel cogs has to be larger than cassette cogs.

Shimano's HG37 and HG22 freewheels are one of the great bargains in cycling. Because the cogs are Hyperglide, they shift a lot better than any of the vintage stuff. Going back 50 years, I have tried all of the old stuff, and I can confirm definitely that these new freewheels are better. Yes, they are are of a basic construction, and the cogs are made of softer metal than say the ultra hard steel of an old Dura-Ace freewheel, but they only cost around $20. The old freewheels will shift poorly for a lot longer.

I haven't confirmed that the HG37 and HG22 is still made, but at our Co-op we have a bin of them heavy enough to deliver a strong youngster a hernia. We sell good condition used ones for $5.

EV-MF-HG37 (ax)-1947

EV-MF-HG22 (Y)-1948

Another thing: we have buckets of Shimano and Shimano-type rear cassette hubs designed for 6 and 7-speed drivetrains. So 126mm rear spacing. Most of these do take Hyperglide cassette cogs. Some of these old hubs were designed for the older clunky Uniglide cogs, but a few minutes of grinding on each cog will render a Hyperglide cog to be Uniglide freehub compatible.

Freewheels with modern profiled sprocket teeth are still being manufactured by e.g. SunRace, Shimano, Falcon, etc. Mechanically, they're not as nice as the classic high-end freewheels of yore, but the profiled teeth mean that they shift very well, and when they wear out, they are affordable to replace.

I've stocked up more Suntour New Winner and Perfect freewheel bodies and cogs than I can reasonably wear out during the rest of my life, as long as I keep my drivetrains clean. I don't think we've hit a dire point in freewheel availability, so it's not really an issue, in my opinion.
Phil

One idea I've kicked around is to cut out the ID of HG sprockets to fit on Suntour bodies. The best bodies for this would be the AccuShift and PowerFlo models, with their multiple splines mounting notches and the single 13T threaded sprocket to secure the stack. There are also Shimano bodies which only have a single threaded sprocket as well, which could also be adapted.

Not being a machinist, engineer, nor metal fabricator, I'm not certain how this could be done in an efficient manner. Stamping would probably warp and deform the HG sprocket. CNC cutting would probably be too time consuming and thus costly. Then there is the cost of acquiring and servicing the freewheel bodies, collecting the needed spacers, and reassembly. So as can be seen, this would be an expensive endeavor.

I do have a good supply of serviced and ready to ship freewheels and might have what you are seeking.

Plenty of freewheels out there, and I already have more than I’ll ever need.
Tim

Yeah, just a half-dozen swipes using a 1/4" round file reduces the long spline so it slides onto a UG freehub. The metal is easy to cut with a reasonably sharp file, as long as it's held in a vise.

I myself only prefer HG cogs for use with indexed shifters.
For friction shifting, I prefer the reliable audible feedback of UG and Suntour cogs which prevents me from experiencing random ghost-shifting from a misaligned gear selection.
Whenever the noise of wind, traffic or a surrounding peloton prevents me from hearing an HG shift event using friction levers, the non-HG cogs seem like a necessity,

If a greater gear-count is what the OP is after, there are now (I believe thanks to the E-bike industry) a large selection of different speed freewheels (up to 11s) readily available at low cost.

These seem to all feature HG-style cogs, and are extra-strong for mtb use with today's small chainrings.

I was able to narrow an 11-36t, 10s freewheel to a 13-36t, 9s freewheel for use on a 6s/7s hub using as small as 124mm axle spacing.

To give you an idea, BB spacers and HG spacers and freewheel spacers are the same. The HG spline is the same diameter as freewheel threads. There's no space in between to have a freewheel mechanism.

You could make some freewheel cogs out of HG cogs by cutting a new interior in them, I guess. With a splined body you could get the shift gates to line up right.

Tab A / Slot B. Couldn't be easier. And it's cheap. Doing something else is simply self abuse. A meaningless argument of course.

Wait, how?? I need this!

Thanks for all the replies! The comment on HG inner diameter pretty much explains why this isn't a thing. The comment about SunRace, etc. being in production is relevant; I'm replacing a SunRace that lasted less than 500 Km and I was hoping to find something with better build quality and similar shifting. The comment about putting HG cogs onto Suntour freewheel bodies has me thinking: I wonder if the SunRace cogs might drop onto an older Suntour body I have laying around.

Thanks for the ideas!

I'm nearly 100% certain Sunrace won't fit on old Suntour bodies.

This has hit on an idea of mine.

A lot of the nicer cassettes have the larger sprockets riveted to a spider, and there's no reason someone couldn't make a spider that interfaces with one of the freewheel bodies.

Well, there is a reason: there's no profit in it. And there's so many different freewheel interfaces, it would be hard to choose which one.

But it could be done, as long as your big cogs are large enough (no corncobs).

There are a few possibilities that might be explored, including the fitment of a one-piece PowerDome cassette onto a freewheel body.
The one-piece steel PowerDome uses a removable alloy large cog to interface with and transmit torque to the freehub body, but this one (thicker) cog could be machined out to the larger spline pattern of any freewheel body. The small end of the cassette would then need to just locate (no spline needed) to the smaller end of the freewheel body, with perhaps the freewheel's original smallest threaded cog holding things together.
But I gave up on this idea because the PowerDome cassettes are very expensive and were offered in a limited range of sizes.

[QUOTE: I was able to narrow an 11-36t, 10s freewheel to a 13-36t, 9s freewheel for use on a 6s/7s hub using as small as 124mm axle spacing....

Modern custom Ultra-9 freewheel build.

I think I remember see a thread here where someone discovered that freewheels for ebikes (yes, that is a thing) had the same splines as Shimano HG, obviously for just the smallest cogs. So I guess that freewheel interface makes the most sense.

My solution to this problem is to equip my bikes with rear wheels with cassette hubs. This way I can have many gears with close-enough spacing and no more broken axles.