Lycoming valve seats

[gossend]

I just finished lapping-in-place a leaking exhaust valve on a Lyc O-320. The compression went from ~40:80 to 76:80. Borescoping the valve and seat had shown the seat to be rough and pitted (1900 hours), but now it's smooth.
My question: is it possible to overdo this treatment? Specifically, are Lyc valve seats through-hardened, or can one lap through a hardened surface into a softer material - which would presumably wear quickly post-procedure?

 

[BoydBirchler]

Seats are routinely reground during cylinder overhaul. Whether the seat gets replaced is a question of how much meat is left to regrind.

That said; regrinding the seat is different than lapping: regrinding is achieved with optimal angles (usually 3) and resulting in optimum contact area for heat transfer and wear characteristics.

So in final answer to your question, the seat is the same hardness all the way through.

 

[lr172]

I believe that both the valves and the seats are stainless and most SS cannot be hardened in the way that carbon steel can. Lapping removes very little material relative to what is done during a traditional valve / seat grinding operation, so quite unlikely you have done anything of concern here. Lapping is like using 120 grit sandpaper with your hand. Your arm would fall off before you removed .002" of steel. I often take off the last .001" of steel material off manually on a lathe when going for precision. It is spinning 1000 RPM and still takes 30 seconds with A FILE. Would take many minutes with 120 grit sandpaper.

FYI, the lapping takes material off equally from the seat AND the valve.