I don’t have much experience with small engine valves. I can say however that larger engine valves (i.e. automotive gasoline and diesel engines) will properly seal from the moment they’re installed - provided the valve guides are perpendicular with the seats, and that the valve & seats are properly ground and lapped in. Normally compression issues during the break-in period are a result of ring seating.
One thing is for sure – the air is blowing by somewhere prior to break-in, and is doing so less afterwards.
Mathematically, a perfectly round O-ring will have an undefined surface area against the cylinder wall (a pure tangential line of action). Pressure is equal to force per unit area – thus if the O-ring is round, it will have no defined area against the cylinder wall, and be incapable of holding back any pressure. Obviously there is a slight interference fit, which causes the O-ring to deflect into the cylinder wall creating a real sealing surface. Nonetheless, perhaps the O-ring even better conforms to the cylinder wall after a break-in period? Either by stress-relieving and conforming from the heat cycle(s), or maybe the O-ring develops a small flat-spot against the cylinder from friction?
How smooth is the bore? I.E. maybe the O-ring is somehow polishing the cylinder?
I’ll be following this post with interest!
Last edited by Entropy455; 10-04-2012 at 04:16 AM.