What kind of crush washers for Superhawk head?

[GG]

Hello!

I'm reassembling my Superhawk, and would like to know about the crush washers at the very top of the head. Some of them have to seal oil (all)? The originals appear to be aluminum. I can get copper washers. I always had a bit of leakage from some of these washers. I just don't know if copper is hard enough.

Any suggestions are welcome! Thanks!

GG