29" wheel set

You most likely havent got a cassette, you've probably got a freewheel.
A good enough design for 5/6-speeds but very prone to failure when used on 135 mm wide 7-speeds.
7-speed freewheel hubs with disc mounts are rare, and an evolutionary dead end to meet a certain low pricepoint.
Your best option, since you're already invested in throwing good money after bad is to buy a freehub wheelset for 8/9-speed, a 7-speed cassette, and a 4,5 mm spacer.
Maybe an 1 mm spacer too.
These will let you mount a 7-speed cassette, which will index just like the freewheel.

It is a freewheel, I have referred to freewheels both as a freewheel and a threaded cassette for years and years back when they were a common thing.
You are not lying when you say the 7 speed freewheels are prone to failure, I only had 1500 miles on this one before it failed.

Luckily the freewheel that I purchased was dirt cheap on amazon.
Looks like I will be looking for a nice wheel set for this and getting a 7 speed cassette with the spacer. After making this thread seems like 4mm spacer seems to be the most common size when putting a 7 speed on a 8/9 speed cassette.

I appreciate the response and figured this is what it would come down to. Probably for the better anyways since these wheels have a slight wobble in both front and rear since I got the bike and tried to fix it with no luck (and I have fixed some real messed up wheels)

At least the frame on this bike was good when I got it (since that will be the only thing left on it besides the fork.

Thanks again!

dabac said:

... 7-speed freewheel hubs with disc mounts are rare, and an evolutionary dead end to meet a certain low pricepoint.
Your best option, since you're already invested in throwing good money after bad is to buy a freehub wheelset for 8/9-speed, a 7-speed cassette, and a 4,5 mm spacer.
Maybe an 1 mm spacer too.
These will let you mount a 7-speed cassette, which will index just like the freewheel.

FWIW ...

[INDENT]I was thinking about this ...[/INDENT]

And, while a specific '7-speed Freewheel hubs with a disc mount are rare', one can easily be cobbled together using ANY Flip-Flop "track" hub (vs. Single Speed hubs which may have cartridge bearings) which has the traditional threading on both sides ...

[INDENT]BECAUSE, I believe Shimano's Centerlock rotors share the same threading ...

[INDENT]So, it's just a matter of ponying up 10x as much for a Centerlock rotor (or, for a Centerlock-to-ISO-rotor-adapter) and then adding the appropriate spacers to the Freewheel side of the hub.[/INDENT][/INDENT]

[INDENT]DONE!!![/INDENT]

BTW. The Track hub's solid axle will limit the likelihood of a bent axle being a problem ...

[INDENT]SunRace Freewheels are reasonably good ...

[INDENT]Freewheels & Freehubs do need occasional maintenance.
[/INDENT][/INDENT]
Regardless, if the frame's rear dropout spacing is 135mm, then my inclination would be to lace up the rim on a Shimano MTB hub which has ISO disc mounts

OR, simply buy a ready-made wheelset as dabac has suggested.

alfeng said:

FWIW ...

[INDENT]I was thinking about this ...[/INDENT]

And, while a specific '7-speed Freewheel hubs with a disc mount are rare', one can easily be cobbled together using ANY Flip-Flop "track" hub (vs. Single Speed hubs which may have cartridge bearings) which has the traditional threading on both sides ...

[INDENT]BECAUSE, I believe Shimano's Centerlock rotors share the same threading ...

[INDENT]So, it's just a matter of ponying up 10x as much for a Centerlock rotor (or, for a Centerlock-to-ISO-rotor-adapter) and then adding the appropriate spacers to the Freewheel side of the hub.[/INDENT][/INDENT]

[INDENT]DONE!!![/INDENT]

BTW. The Track hub's solid axle will limit the likelihood of a bent axle being a problem ...

[INDENT]SunRace Freewheels are reasonably good ...

[INDENT]Freewheels & Freehubs do need occasional maintenance.
[/INDENT][/INDENT]
Regardless, if the frame's rear dropout spacing is 135mm, then my inclination would be to lace up the rim on a Shimano MTB hub which has ISO disc mounts

OR, simply buy a ready-made wheelset as dabac has suggested.

Well, no.
Centerlocks mount with the same (kind) of lockring as used for cassettes. Meaning the lockring needs an internal thread in the hub to engage. Freewheels and track sprockets engage an external thread.

However, there are (were?) thread-on rotor mounts with the same thread as for freewheels and track sprockets. Different items than the 6-bolt-to-splines centerlock adapters.
Buying one of those - and a flip-flop hub - would kinda-sorta get the job done - assuming you're able to get both the chainline and the rotor-to-dropout spacing right simultaneously.

dabac said:

Well, no.
Centerlocks mount with the same (kind) of lockring as used for cassettes. Meaning the lockring needs an internal thread in the hub to engage. Freewheels and track sprockets engage an external thread.

However, there are (were?) thread-on rotor mounts with the same thread as for freewheels and track sprockets. Different items than the 6-bolt-to-splines centerlock adapters.
Buying one of those - and a flip-flop hub - would kinda-sorta get the job done - assuming you're able to get both the chainline and the rotor-to-dropout spacing right simultaneously.

Ack!

[INDENT]I believe that YOU are 100% correct ...

[INDENT]For some reason I was only thinking about the lockring (which could be used on the non-Track cog side) ...

BUT, I wasn't picturing the hub's Centerlock interface's splined pattern and cannot imagine what I was thinking of (while I do remember the threaded rotors you mentioned from 40+ years ago, I was apparently hallucinating about something else) ...[/INDENT][/INDENT]

[INDENT]My apologies.[/INDENT]

alfeng said:

Ack!

[INDENT]I believe that YOU are 100% correct ...

[INDENT]For some reason I was only thinking about the lockring (which could be used on the non-Track cog side) ...

BUT, I wasn't picturing the hub's Centerlock interface's splined pattern and cannot imagine what I was thinking of (while I do remember the threaded rotors you mentioned from 40+ years ago, I was apparently hallucinating about something else) ...[/INDENT][/INDENT]

[INDENT]My apologies.[/INDENT]

No worries.
With the scope of "standards" available today, I rather see it as a sign of sanity not to be letter-perfect on all potential combinations.

One thing I think is a pity though is the almost-but-not-quite interchangeability of Shimano roller brakes and the centerlock standard.
They both use a splined fit to engage the hub.
Granted, not many would probably use the option to turn a disc brake hub to a roller brake hub (or the other way around). But having the parts so close yet so far away is simply annoying.