The Contax/Kiev "standard" is 52.4mm. Tolerance for the focal length of the Russian lenses was 1% during the mid-60s. Some of the Russian lenses require a little "fiddling with" and even some filing to get onto a Contax. The Helios-103 is a double-Gauss formula lens that is inexpensive and very good.
The J-8M is a simplified formula Sonnar, the triplet of the traditional Sonnar is collapsed into a doublet. I've seen a few good ones, several "okay" ones, some not-very-good ones..