300/440/550 Stock waterbox mod question

[yamarider]

I set out to do the stock waterbox modd for my 300 but gave up when i measured the cast outlet pipe and found that the internal diameter was only 34mm. The carb is a 38mm the exhaust manifold outlet is 38mm and the end of the kerker pipe is 38mm and also the inside diameter for the inlet pipe to the stock waterbox is 38mm but the crappy casting outlet is like 34.

Could there be a reason it was made like this?

 

[Jetpilot106]

I just deleted mine it gives it a lot more power with my mariner exhaust.u can make ur own pipe from pvc.lmk if u need measurements