Can anyone guide me in this particular model?

Offset plane from centre to side flange and draw sketch (without inner hole) and extrude to next and taper and cut the hole to next. Try or i will send you.

Draw circle on flange and on middle plane and cut it or use revolve it's better

I'll send blowcock casing as soon as possible. Currently busy till next 2 weeks. Don't worry

Here is the solution. Hope my design is correct. New experience

Can you spot the error? Even outer 72 diameter circle has a taper.

I have tried it in past few hours. It looks something like this.

Ooops...

The section bridging the two is not easily created as a solid... you need to start with sheets, sew, thicken and unite.

Keep in mind that controlled transitions require a bit more work. Ideally, you'd like the same number of sections in each profile. In this example, you have a 'rectangular' section transitioning to a circular, so you'd want 4 sections in your circle as well. See the attached image.

I projected (vector) the port shape to the inner wall, then used a curve (point to point) to drive an extrude along the vector using 'until extended'. This bridged the two shapes. The only reason to do this is so that "stop at intersection" can be used in your selection intent.

I then selected the right-side section and created a sheet. Then the left side. Then a sheet between with continuity set to 'tangent' for the top and bottom, using the previous sheets to drive the continuity.

From there, you'd sew, thicken, extend and unite.