I've been following your thread about your Kohler K121S engine.
The part number for the Ring Set - Std was Kohler part # 31-108-05, according to the pdf posted by Bob.
That ring set was not used on the K90- K91, as you were told by Hastings.
However, that ring set was also used on the Kohler model L654 - 7.5kw generator.
I found a company on the 'net who had that ring set in stock back in 2009. ....I called them this morning, and they are going to check to see if they still have them, and they are going to call me back.
The company is Wavetop Technology in New Bern, North Carolina. ....Phone is 252-636-0715 .....Website is www.wavetop.com. Update: I just heard back from Wavetop, and they do NOT have them in stock, and do not know of a source for them. ...He suggested I contact auto parts suppliers.
The guy at Wavetop also suggested trying Seafarer Marine Supply at 727-595-8813. They are in Largo, Florida, and buy (and sell) obsolete Kohler parts. ....I'll let you contact them.
If they do have the ring set, you may want to ask them to verify the ring sizes.
Incidentally, the original part number for the STD Piston was Kohler #31-074-06 which was discontinued and superceded by #31-074-11, which was superceded in March, 1982 to part # 50-074-03, which has also been discontinued by Kohler.
The latest numbers (also discontinued parts) for pistons are:
Piston - .010" part # 31-074-13 replaced by # 50-074-04
Piston - .020" part # 31-074-14 replaced by # 50-074-05
Piston - .030" part # 31-074-15 replaced by # 50-074-06
From what I can see in my books, Kohler Piston Assy's included the wrist pin & retainers, but not the rings.
There was a Kohler Service Bulletin issued July, 1979, having Service Data on the K121S engines. ....It is referenced to Microfiche card # 67, Grid # B1. .....I don't have the microfiche cards, so I can't help you there, but maybe someone else on GTT has them.
Hope this helps!
Thanks Bruce for digging into this so much, I really appreciate the effort.