Per [this thread on MTBR](https://www.mtbr.com/threads/sr-suntour-triair-3cr-anyone-using-this-shock.1101104/page-2) the only difference between the 210x50 and 210x55 shocks is just a spacer, clipped on the main shaft, right behind the end stroke metalic washer:
In the thread the procedure to lengthen the stroke is debated, so I guess the reverse operation should be equally simple. You could machine/make a 5mm spacer, clip it behind the end stroke washer, and call the day off.
Feb. 25, 2021, 6:32 a.m. - Luix
Per [this thread on MTBR](https://www.mtbr.com/threads/sr-suntour-triair-3cr-anyone-using-this-shock.1101104/page-2) the only difference between the 210x50 and 210x55 shocks is just a spacer, clipped on the main shaft, right behind the end stroke metalic washer:  In the thread the procedure to lengthen the stroke is debated, so I guess the reverse operation should be equally simple. You could machine/make a 5mm spacer, clip it behind the end stroke washer, and call the day off.