if your using 38 mm pipe just cut a slot down length wise and weld it back up with the smaller diameter. The pipe should have an internal diameter of something similar to 35-36mm, unless you were talking inside diameter.

To reduce the outside diameter of - 38.1mm pipe to 34mm outside you would cut a 13mm slot.
- 38.1mm to 36mm outside you would cut a 6mm slot.

Work out the outside diameter of pipe you need, and measure the outside diameter of the pipe you have.

then you can use this formula to work out how much you have to cut out.

(3.146 x outside diameter of current pipe) - (3.146 x outside diameter of ideal pipe)

When its hot after welding the seam back up, then you can beat the egg shape out of it. Leaving the end egg shaped to match the inlet port.

Most if not all individual inlet setups do not do this, expanding the mixture before it reaches the cylinder will most likely reduce the pressure and velocity of the mix.

hey guys im just making a inlet manifold for a set of mikunis i have. basically its just 4 pipes heading out of the side of the head. now my carbys are 34mm and my ports on my h89 are 30mm x 36mm. to go from the head to the carbs i have some 38mm pipe. is this 2 big? just wondering if its ok to go from the inlet port to a larger port (the manifold) then choked down to 34mm