The firewall mount tubes ar made from 0.5" x 0.120" 4130N. This leaves a 1/4" inside diameter for the AN4 bolts to pass through. The tubes are 2-1/2" long at the moment but will be reduced in length once the rest of the engine mount has been welded. I wanted to be longer than find I have gone too short and have to make them again.
An AN970-4 washers are bolted to each base and then welded before sliding the bolt out.