Tower has the manual for the Avistar Mark II ARF online. The link is [link=http://www.hobbico.com/manuals/hcaa2016-manual.pdf]here.[/link] This may be a little different than yours, but should be better than the manual for the RTF. As you can see, although they don't seem to mention it in the instructions, they have cut some of the cheek for the muffler. In their installation, it looks like the thrust washer is about 1/4" ahead of the cheeks. That sounds about right to me.

-Scott