The cog spacing is same when comparing 6 and 7 speed. Removing the largest cog should work fine. You may want to find a newer IG 7 speed cassette which had threaded pins holding the cassette together. I have seen lots of HG cassettes with riveted pins making disassembly more difficult.

 

If your hub is an old Shimano 6-speed cassette hub, those cassettes you pictured are not compatible. The older Shimano 6-speed cassettes have splines that are all the same width. The cassette in the image (and all 7, 8, 9, 10 speed cassettes) have one spline that is wider that positions the ramping on the individual cogs.

Additionally, the cassette in the image uses a lockring that threads into the inside of the cassette body. The old Shimano 6-speed cassettes used the small cog that threaded to the outside of the body to secure the cogs.

It is possible to replace your 6-speed cassette body with a newer 7 or 8 speed compatible body.

 

The cog spacing is same when comparing 6 and 7 speed. Removing the largest cog should work fine. You may want to find a newer IG 7 speed cassette which had threaded pins holding the cassette together. I have seen lots of HG cassettes with riveted pins making disassembly more difficult.

Thanks for the tip on rivets. I do have an old HG 13/30 7-speed cassette sitting around that's bolted together so I thought they were all that way.

If your hub is an old Shimano 6-speed cassette hub, those cassettes you pictured are not compatible.

It is possible to replace your 6-speed cassette body with a newer 7 or 8 speed compatible body.

The bike is a 1995 with a MJ Freehub, which according to what I can find on the web should use the CS-HG30-6 cassette above which is an 11/24

I still don't have the bike in hand, but if it's the older style 130 mm spacing, can I still cram a 7-speed freehub on there... at least w/o redishing the wheel?

The PO sent me this picture. This is the lock-ring style... right?

 

Thanks for the tip on rivets. I do have an old HG 13/30 7-speed cassette sitting around that's bolted together so I thought they were all that way.

The bike is a 1995 with a MJ Freehub, which according to what I can find on the web should use the CS-HG30-6 cassette above which is an 11/24

I still don't have the bike in hand, but if it's the older style 130 mm spacing, can I still cram a 7-speed freehub on there... at least w/o redishing the wheel?

The PO sent me this picture. This is the lock-ring style... right?

Yep, lockring type. And those riveted pins are easy to punch out with a hammer and punch.

 

Yep, lockring type. And those riveted pins are easy to punch out with a hammer and punch.

So basically, I can get any 7 or 8 speed cassette and re-stack it to whatever 6 speed combo I want -- right?

 

So basically, I can get any 7 or 8 speed cassette and re-stack it to whatever 6 speed combo I want -- right?

Right, except you will have to use the 11t or 12t cog as the last one, as the hole through the larger cogs is bigger in diameter and they don't have the ridged ring to mesh with the lockring ridges that "lock" it in place. And make sure you use that funky spacer (labelled "4" in the diagram above, used to clear the heads of the bolts) between the last cog that is/was bolted together and the smaller loose cogs, otherwise the spacing will be off between cogs 2 and 3.