the 800 is a stroked 750. the biggest you can over bore std 750 cylinders is 90.5mm i believe, 2.5mm over std. bcm have pistons for that. which would give 849cc in an 800. altho you'd need pistons with 2.25mm less deck height than the 750 based ones to allow for the increased stroke.

the stud spacing is smaller than the 900 engines which limits the cylinder wall thickness to a given maximum which in turn limits the max bore size.

there's more capacity stuff here: http://moto-one.com.au/perform....html