A picture may be nice. Are you talking about street 90 bends into a Posi-Temp valve?
My original tub/shower piping is 1/2" i.d. but then goes down to these narrow elbows into a Moen 1016p valve body. Is it standard to have the pipe connect in the valve body instead of o.d. of the valve body?
Many shower valves have male threads to accept a female threaded connector from the supply line. But, the inside of that valve is often bored out to accept the pipe in as if it was a coupling (i.e., it is the proper sized socket for accepting the supply pipe to be soldered in). Without seeing what you have, it probably is okay, but we'd need to see it to be sure.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
Here it is
My guess is that it is 3/8" i.d. The outside appears to be 1/2". The actual bent part, not the elbow openings.
The connection IS 1/2", but then they connect 1/2" PEX to it which DOES decrease the i.D. to about 3/8".
Licensed residential and commercial plumber