If you can't stick a thumbnail in them and make any kind of impression, they are too hard, plastic, forget 'em.
If you CAN stick your thumbnail in 'em, and they are resilient, spring back with just a trace of your impression, likely will hold. Compressible. If it can be compressed, at water line pressures, it will be sealed. Hand tighten, you got a seal.
Charlie, you ever been to the Herminie Feed Store? Burned down about 35 years ago. You wanted ANYTHING, you went there. They damned well had it. Might have not been made for 20 years, but if you needed it, they could probably go to the shelf or bin, or in my case, up to the attic to find it.
Loved the store. Had EVERYTHING. Like a "pick it up and touch it" Sears and Roebuck catalog. The STORE was always there. When the new Sears catalog came out, the old one went to the outhouse for as+wipers.
Ah, them was the good old days.
Coulda gone to them and bought an SB lathe, run it to death, 48 to 60 pieces per day, made a little bit of a living.
Now, you gotta go for a 1/4 million automatic machine, run full out, program a little wrong, 3 seconds too much per part, they come and repo the machine, you ain't making enough to pay the financing of it.
I feel for some of you. I can't help, but I can sympathize.
Cheers,
George