Harold, I've used the Stens rod, piston, rings, and gaskets in a few K series singles. Each piston and rod measured right in line with Kohler book specs. To me the quality is there. I've never had new Kohler parts at the same time to make direct comparisons, but the std. Stens rod was better finished with less casting parting line flash than the used Kohler part. I like to smooth off the flash and sharp corners on any of them.
Gaskets, seals, and valves were fine too.
In the past, Stens didn't have -0.010 rods or governor gears and other gov. parts available so have had to go OEM with some parts. Don't know if they have them yet.
It would be nice if someone rebuilt two original, identical engines, used for identical work, one with the OEM and one with Stens parts. We might not be around long enough to find out the results though. :-(
tommyhawk