[rrobson]

Copper grease wont protect the subframe and fastener from corrosion unfortunately, you need to electrically insulate the washer from the carbon steel, or use carbon steel. It’s the reason you won’t find any stainless steel in any car construction (apart from the one famous example).

Have a read up on galvanic corrosion, not a lot will happen for a while but if it’s a long term solution I would strongly suggest using carbon steel instead.

I certainly don’t mean to waz on your bonfire, just making you and others aware about the possible issues to surrounding items that may cause issues in years to come. You certainly won’t have catastrophic failure of any components but you certainly will see more corrosion in the long term.