Valve size
 

What are the larges size valves that can be placed in a 96mm bore cylinder head.

I think 46x40 would be getting towards the limit. I personally wouldnt go above 44x38 in that bore size

Valve size
 

is their a reason for not going 46x40, does it weaken the head.

I think he was saying that is wasnt wise to go above 46 x 40 if you are going to use 96mm pistons.

If you are using big bore pistons (102mm - 105mm) then I have heard of people using 50 x 40 valves in a T4 head.

A lot of people just put in bigger valves because they flow more air. Ever heard of someone "unshrouding" a valve? This means you move the wall away from the valve to allow the side nearest the wall to flow properly. If you go too big on valve sizes, you actuall put a lot of the valve along and very close to a wall and thereby actually reduce the airflow of the head simply because 30% or so of the valve is not being used porperly or at all.