qwazzy123... My rubber stopper was also tight.. It's good that way..
Anyway I lubed the rubber then... worked it in ....:-)
Most mounts donot have holes predrilled.. This gives you a lot
of leeway for you to put what ever size engine you want....
I used a longer drill bit for mine.. Worked like a charm..
Most ARF kits are like this..
Just keep learning on this build... Once it's in the air, it makes it
all worth it!!... Then it makes you want to take on a kit... Talk
about tricks...

Have fun and enjoy!!