Mr C are you using a small washer YB head or a large washer YB head?..I have small washer and the only socket that will fit is a 3/8th snap on multi spline 17mm...My mate looked at a 19mm socket and said no way mate.

As PGT said head mod is best when having long stud!!!...and i bet you have got that hence why your 19mm machined is ok...on mine no way ho zay