Ted,

You can go to about 75% of bore width before you notice exponential reduction in ring life (so to speak).

BUT, that assumes you dont have any sharp edges in the port. It has to be a perfectly oval shape with a good chamfer around the edges. Like this intake port:



Once you cross 70% though, symmetricity of the port becomes critical.

If you have limited porting skills or dont have the right equipment, then 65% is VERY safe. But you must still retain the oval shape.


Ananth