How do I open this cassette?
 

Can you please let me know the name of this cassette so I can look up the tool and how to open it?

https://cimg1.ibsrv.net/gimg/bikefor...6e9625245.jpeg

Looks like a Suntour 6sp from the late-70's.
If it's a little balky, sometimes you can put some drops of thin oil to free it up.

Thats a SunTour freewheel which takes a two notch remover. Park tools version works well. FR-2.

…as it is a freewheel it comes off as a unit.
1. Remove QR skewer
2. place tool in the notches completely
3. put skewer back on and tighten until the springs compress against the tool
4. place wrench/adjustable wrench on flats and pray it turns counterclockwise easily. If not, Kroil from the back!

That appears to be the technological predecessor of a cassette, commonly called a freewheel. Removing it is conceptually simple, you just need a Park FR-2 freewheel remover or equivalent and a large bench vise or equivalent, plus a lot of brute strength and a little bit of technique.

Actually disassembling it and reassembling it is more challenging. This one does not appear to be in a condition where it would need to be disassembled, but if it is actually broken then it is probably a lot cheaper to replace it than fix it.

Thank you for the info, I found the two notch remover tool for this freewheel.

I'll try the oil method before I attempt to open it since this looks a bit more complicated.

Youtube is great for stuff like this. Just look up "disassemble freewheel" and you'll have a lot of hits. You can also search for a creator called "RJ the Bike Guy," who has a series of instructional videos on many basic bike maintenance topics.

Manufacturers from that era had proprietary methods, so your Suntour will come apart differently from Shimanos (and different Suntour models as well), so just be sure to research before deciding to tackle it.

Insert the freewheel tool and secure it just less than finger tight. Now, put the wheel in a vice, snug the vice up to freewheel tool and, using both hand, break the freewheel threaded fit loose. Just a wee bit loose. Remove the wheel from the vice and loosen the skewer a bit. Now, with the tool still installed, unthread it. Best of luck...
https://cimg2.ibsrv.net/gimg/bikefor...6b8b0f321d.jpg

"Unscrew >>>>>>>"

That is a SunTour Winner freewheel and there's a tool for the disassembly, in addition to the one you already know about for removal. I have one of these but this net photo is handier than me having to get off my butt and go into the cold garage to take pic for you. Good luck!
https://cdn11.bigcommerce.com/s-1rmg...691730.JPG?c=2

That's not a cassette; it's a SunTour "New Winner" freewheel. To remove it from the hub, you need the proper two-prong remover tool to fit the wide notches in the freewheel body. The other, narrower notches are to adjust the bearings, for which you need a special tool. Chances are, that will be unnecessary; if it is necessary, let a bike shop or Pastor Bob deal with it.
https://cimg4.ibsrv.net/gimg/bikefor...0b40909e48.jpg

Just out of curiosity- are you trying to take the freewheel apart or are you just trying to remove it from the wheel?

The freewheel was stuck and I was trying to open and grease it, but per roadcrankr suggestion I put some oil in it and it starts to spin again.

IMO, dripping oil is only a temporary solution. Something was causing it not to freewheel. It could be corrosion, contaminated grease, a broken bearing or pawl, or a combination of these causes. You may have caused a temporary fix only to run into trouble while out on a ride. Do yourself a favor and properly service this New Winner.

I agree; in my experience, Suntour freewheels are pretty robust. If you rebuild, take your time, be prepared for extremely small bearings to be accounted for and, as mentioned above, maybe look for YouTube tutorials. Not ready for that? Maybe hit up a co-op in your area fir a replacement.