On Sun, 09 Jun 2013 22:08:51 -0700, DD_BobK wrote:

One useful definition of "Quality" is "fitness for use".

Indeed. You are a wise soul.

For example, it turns out that the 25-inch diameter "cord" ring
around the Sta-Rite System 3 Model S8M150 filter tank is an
odd "Alladin" size of "Alladin O-486".

Upon first inspection, this looks like a standard o-ring trade
size; but there is no o-486 in the standard inch-size charts!
http://www.oringwarehouse.com/

A typical buyer would shell out $27 to $40 for the Sta-Rite /
Pentair part number 24850-0009, which is what the market will bear:
$27 http://www.theoringstore.com/index.p...ct s_id=18551
$33 http://www.amazon.com/Sta-Rite-Syste.../dp/B005MIZV6S
$35 http://www.poolsupplyworld.com/Penta...24850-0009.htm
$40 http://www.yourpoolhq.com/sta-rite-2...nk-o-ring.html

However, for me, bulk cord glued together is "fit for this use",
as long as the price for materials is well below those numbers above.h

It may not work, but, it seems to me that for a buna-N material,
your basic 100 viscosity cyanoacrylate adhesive should serve well
as an O-ring Glue (applied sparingly due to the sheer strength
of this particular style of glue).

So, my initial plans are to make my own 25-inch diameter o-rings from
bulk cord stock, if I can get the stock for about $1 to $2 a foot.
http://www.theoringstore.com/index.p...ndex&cPath=117

If I can't get the stock at that price, I'll buy the Buna-N 70 durometer
o-rings for about $30. The decision is all about quality & fitness for
use, against the classic cost tradeoffs.

Just like you advised.