For reference, the stock gearing for the WR250R is 13/43 while the X model runs with a 13/42 stock configuration because of the SM wheels/tires. Both use a 108 link chain with the stock gearing.
The JT rear sprockets listed are made from the finest 545C high carbon steel and are then heat treated and hand finished to meet or exceed OEM standards. Outlasts many mild steel or surface hardened sprockets as much as two and three times over.
Limited availability on Aluminum from PBI. Manufactured from aircraft 7075 T651 Plate Aluminum.
If you want the additional low-end grunt that the 46, 48 or 50 tooth rear sprocket will give you, you will need longer chain as well.
New Rear Sprockets require new Sprocket Bolts because of a difference in the holes vs. the stock bolts.