Engine will be a YS 110...mounted on it's side with a Pitts slimline muffler...

I thought about washers...but after tightening the mount...and given the vibration...I would think the washers would just "Sink" into the Wood of the firewall....

It would be much better to spread the load over a larger area...to insure the mount doesn't work loose over time.

also..you say use a washer...but what size washer...how thick????

What about adding down thrust as well as right thrust??