Are the 7spd HG and 8spd.HG cogs interchangeable ?
Want to build up a specific 8 cog cassette.
wondering if I can use replace two of the 8spd cogs with 7spd of the tooth number I need.

older cassettes were held together with 3 long thin bolts, so could be .
newer ones there is a bigger sub assembly and only a couple small cogs are loose

I believe the "spider" mounted cog clusters started at 9-speed.

Yes, you can substitute a couple of 7-speed cogs in an 8-speed casette but be sure to use 8-speed spacers. Shimano 7-speed cogs are 1.85 mm thick and 8-speed cogs are 1.80 mm thick so the difference is minimal.

And if they happen to be "end cogs", it's even more minimal.

Back in my "poorer" days, I used a bunch of 7 speed cogs and thinned the spacers to equal 8 speed spacing.
I had index shifting, so I wasn't too concerned about it being "perfect".
Last summer I bought a "real" 8 speed bike and tried my home brew cassette (13-21). It worked much better than the 11-32 that was on it!

The only drawback to mix n match cassettes is that the shift gates will be in the wrong place. What puts the Glide in Hyperglide is that the sprockets are phased so when the chain comes through the shift gate it's rollers are positioned exactly (phased) to engage the neighboring sprocket. When the gates aren't in the right place the chain may climb onto the tips of the teeth on a shift then only drop in when you pull it forward causing a small skip.

pre-index riders are familiar with this, and with your mixed cassette you'll feel it too. It's no big deal under you like to shift under load.

Shift quality MIGHT be fine, depending on the cogs you are using.
In my case I used a-
12-28 (12-14-16-18-21-24-28) and a-
13-26 (13-15-17-19-21-23-26 and made a-
13-14-15-16-17-18-19-21.
All but the largest 2 were from "alternating" cassettes.
I think the 1 tooth increments made the shifting "spiffy".

Any shifting problems you see would probably be where you were using wider increments.

Great (on the compatability) The clusters are held togeather by a long rivit setup.Grind the rivit head off and...
This (interchangeability )will afford me the cogs I needed without needing to purchase more complete cassettes.
BTW apparently the newer cassettes DO come completely apart.
I upgraded my 08 Ultegra equipped bike (went with the 6750 derailleur so I could use a 11-28 10 cog cassette)
Had the LBS do the changeover and when I got the old cassette back it was held together with a zip tie, cogs completely free of the body. Apparently Bikes Direct had the origional 6600 cassette upgraded and that required disassembly of the origional cassette.
Anyhow, Thanks HillRider!

Yep on the cassette buildup.
Used Sheldons gain chart to come up with something narrow spaced at either end of the spectrum as I'm using Velo Oranges Grand Cru 55.4bcd 46/30
going with the 12-14-16-18 up to 21-23-25-28
Having gotten used to the one step increments with the CFs 10 cog setup I guess I got spoiled.
Those giant leaps when you're climbing truly suck.
The lower end will work fine with the 46t
The upper end with the 30t chain ring should make life (on the climbs) easier.
If I need to I can swap out the 25 for a 26 and the 28 for the 30 (not as crisp but...)

I'm getting ready to shovel out my workshop again. Every time that I do this I throw away a lot of junk that I didn't think I could live without the last time I cleaned. This year some old cassettes and various sized spokes amont other things are hitting the dumpster because I need the shelf space.

+1. Last month I finally discarded a used 7-speed Shimano freewheel I'd kept around for years thinking maybe someday I'd find a use for it. Not going to happen and I haven't had a freewheel wheel in over 15 years. Now I'm wondering about the pair of plastic toe clips.......

RG, if you have any 7-speed cassettes with some life in them, I'd be willing to save them from the dumpster. PM me.

Nah, I once had an 8 speed XT cassette that had the spider for the larger cogs. Unfortunately it was so worn out I had to toss it.