Using a sram/avid mount with a shimano caliper? ok?

one with conical washers? Got a fork with a 160mm post mount, post mount caliper (shimano), 180mm sram/avid disc. Can i use the sram/avid adapter off the old brakes?

It depends, if it’s one of those that only has two tubes (like this: https://images.amain.com/images/large/bikes/avid/00.5318.008.005.jpg?width=475), it won’t work. The tubes have a scalloped feature in them so they replace half of the conical washer. In order to fit a non-Avid caliper you will need to remove the washers between the adapter and the caliper, but that is impossible with these.

The ones with a brace between the posts (like this: https://forums.mtbr.com/attachments/brake-time/961941d1423191894-avid-brake-caliper-adapter-avid-brake-post.jpg ) will work, provided that you only use the conical washers on the outside (between the caliper and the bolt head, like this: https://p.vitalmtb.com/photos/stories/2017/09/03/max_Shimano_XT_Update_367324.jpg).

it is same as the top one – why does removing the washer make it impossible? could i not keep the washer in there?

The washers will space a non-Avid caliper further out than necessary. I think there are two variations on the top one as well – if yours have both faces perfectly flat and use a full set of washers (8 pieces in total), then it will work.

This is how the incompatible ones look like: https://r2-bike.com/media/image/product/32435/lg/avid-adapter-pm180vr-pm160hr~2.jpg . Removing the washer will cause either the caliper or the fork to sit against a scalloped surface.

How thick should the top and bottom mounts Be? I.e both 10mm?