SpudFiles

I have read that a 1 1/4" coupler will fit inside 2" pipe very well. The main question is whether a 1 1/4" piston has a big enough diameter to seal against a 1 1/2" SDR-21 Barrel. I don't have the coupler on hand, so any help is appreciated!

It will just fit onto the pipe
It will be very close but it will fit onto it

Your piston HAS to be larger then your valve seat. (DUH)
But with larger, it means not the same size, but larger.
It needs frontal area to get pressure work out some force on.

If this area is very very small, the piston will probably open slower (less initial force)

But if it fits, I think it will work.

It will work. I've used 1.25" couplers for many 1.7" porting piston valves.

But you will need something around the coupler for a good fit in 2" pipe. They are a bit smaller than the ID of 2" pipe.