Heat riser get stock when warm

[Nel]

Hi, couple years ago, I replace the heat riser on my manifold, since the old one was leaking. Got a new one from roseville. Today my dad noticed that there less exhaust coming out from the right side. We found out that when cold the heat riser move very easy but when the car get warmer and before the heat riser have the time to open, it get stock. Stock so hard that it's impossible to turn at all by hand. We are lost on that one.

[Brads70]

Must be something seizing when it gets hot. I'd start looking at the shaft and at the manifold, something is either not the right size or bent/holes not aligned. Butterfly Valve to big for bore in manifold?  Parts expand when hot so if clearances are not right for what ever reason, things will seize.