How do you make smooth transition from collector to round pipe?
 

I have schedule 40 manifold which I remade with 316 for a 3 rotor engine. I have a quick question. How do I make a smooth transition from the 3 joined pipes into a round pipe? The shapes are little different. Schedule 40 is too thick to hammer form into any shape... I could grind out the inner diameter but before I do that I wanted to ask for what people usually do in this situation... I don't want to get a new collector because I don't want to remake the primary runners.

Any suggestion is much appreciated. Thanks!

I would tack the collector to the sch. 40 pipe and try to pound it out from the inside of the pipe. Might need a long punch or something with a round end on it.

Lots of tacks and hammering should work.