Does anyone know if there is a way to tell if an o-ring is Buna-N by looking at it or analyzing it in some way (preferably without destroying it)?
Tool identifies O-ring material
Almost by definition, all O-ring seals look pretty much alike. With some basic measuring devices, a mixed bag of O-rings can be sorted by size, but how can you identify their material for chemical compatibility - and do it without destroying the O-ring? One way: use the ORID 70-C O-ring identifier, built by Bachus Instrument Co., Nashville.
Just hold this simple tool - which is about the size of a pencil - in a vertical orientation with its bottom end against an O-ring of unknown material that is lying flat on a table or desk. Slide the tool's internal weight to the top of its slot and release it. The straight edge at the bottom of the weight should drop directly on the O-ring cross-section so that the edge strikes and bounces off that cross-section's arc. Watch the height of the first bounce when the weight rebounds off the O-ring. Marks inscribed along the slot identify Viton, Kalrex, nitrile or Buna-N, and ethylene propylene. Because minor differences in operator technique may produce variations in bounce, it's best to take multiple readings.
The stainless-steel ORID 70-C will accurately read 100, 200, 300, and 400 Series (0.103 to 0.275 in.) cross-section O-rings with durometer hardness of 60, 70, or 80. Note that the surface supporting the O-ring should be hard and rigid. The ORID 70-C tool should not be used with damaged or failed O-rings, or O-rings older than their indicated shelf life, and it cannot identify material correctly that is not in an O-ring configuration.