[pampadori]

Yes, with plugs in you no longer have any escape path for the air you are compressing by moving the piston up while the valves are closed.
This is why I think every kid should have a dirt bike growing up. You'd know what the pressure was a result of because you'd have scars on your shins from the lesson.

Good for you for tackling such a major endeavor! You'll learn way more than you thought you would when its all over. Especially if you have to redo some parts a time or two.