your bar is the newer oversize, 31.8 is 1.25" essentially.. BMX bars are 7/8" tube at the clamp (& all the tubing) 22.2 mm
Machined shims are made you want one with a 22.2 inside and a 31.8 outside to continue using the stem you have..
the difference, in radius , so machined shim thickness is 4.775 mm [1/2 9.55.. in combined difference of 2 diameters..]