without a print or geometry It's hard to help. from what I see the math is wrong.
try this code and see if it works for you.

O51712(PRESSURE BLOCK)
(MATERIAL -YELLOW BRONZE)
(ORIGIN-X,Y,Z ORIGIN CENTER OF STOCK RADIUS MINUS .150 CLEANUP)
(TOOLS)
(TOOL - .200 SQUARE GROOVE TOOL)
(TOOL -55 DEG)
G18 G20 G40 G80 G90 G99
G00 G28
(55 DEG-FACE AND TURN)
T101
G50 S150
G96 S60 M03
G00 X18.5 Z.250
G71 P1 Q2 D.05 U.05 W.05 F.005
N1 G0 X0.0
G1 Z0.0 F.005
G03 X16.5 Z-.81824 K-42.0 (-.8182 CORRECT VALUE FOR END OF ARC @ X16.5)
G01 X16.5 Z-1.125
G02 X16.75 Z-1.125 I.0625 ( I.0625 CORRECT FOR X16.5 TO X16.75 DIA.)
G01 X17.00 Z-1.125
G01 X17.0 Z-3.0
N2 G1 X18.5
G0 X18.5 Z.25
M00