Use genpt with ptbreak=5 for gen_stop (MCFM-8.3 default), this seems to work really well.

src/Phase/gen_stop.f

@@ -93,8 +93,8 @@ c       xjac = exp(2._dp*log(sqrts/2._dp) - 2._dp*(1-r(ijet))/r(ijet))/r(ijet)**
         !if ((kcase == kbq_tpq_jet) .and. (kpart == kreal) .and. usescet .and. (ipsgen == 1)) then
 c           if (ijet == 1) then
                 ! in 1/sqrt(pT)
-                pt = r(ijet)**2*sqrts/2._dp
-                xjac = pt*sqrts/2._dp*2._dp*r(ijet)
+                !pt = r(ijet)**2*sqrts/2._dp
+                !xjac = pt*sqrts/2._dp*2._dp*r(ijet)
 c          else
 c              ! 1/pt^2
 c              pt = 1._dp / sqrt(4._dp/sqrts**2 + (1-r(ijet))/r(ijet))
@@ -105,7 +105,7 @@ c           endif
             !error stop "todo"
         !else
             !error stop "please explicitly setup gen_stop for this case"
-            !call genpt(r(ijet),ptbreak,.true.,pt,xjac)
+            call genpt(r(ijet),5._dp,.false.,pt,xjac)
         !endif
         wt=wt*xjac