NMPRA

Over the years I have found that I generally have better luck running new motor mounts than running ones that have been crashed. The reason is that most times the mounts end up twisted and don't adequately hold the engine after the crash. This is sometimes evident by seeing black gunk coming from an unknown source.

Someone passed a tip on to me that if the twist is minor you can true up the mounting surface with minimal effort. The pictures show what can sometimes be done to true up the mounting surface.

1) Remove the mount from the model.
2) Identify a flat surface. This can be a piece of glass or other machined surface. I used the surface of my saw.
3) Slowly work the mount surface on a piece of wet-and-dry sandpaper. This was 400 grit paper used dry.

The other issue that is common is that you can't tighten the compression nut enough to hold the needle valve in place.

I've continued to tighten the compression nut so much that the needle valve broke off inside the mount. At this point I recognized that I waited too long to fix the issue.

The better thing to do is replace the compression nut and nylon fitting when you first identify that the assembly won't hold a needle setting.

shouldn't you also flatten the backside of the mount where it sets on the firewall, so as to not introduce stress if it's not flat?

If you flatten only the engine bearers with the mount unbolted, it seems the back may also no longer be flat.

It wouldn't hurt to also true up the base of the mount.

I apply epoxy to the firewall before the mount is pulled up tight. The epoxy levels it all out.