size 19 on the crank bolt - rather tedious

what i have done to make life easier to remove valve springs as follows:
get a big open ended spanner(20 mm plus)
lay this on the valve spring retainer with the other end of the spanner towards the cam laying on the rocker stud
with the rocker removed , mark the spanner where it touches under the spanner
drill an appropriate hole through the spanner
attach your new "tool" with a rocker stud bolt and compress the spring - sorted!
you now have a spring compression tool
a modified plug with an air compressor attached will keep the valves up , alternatively , use some nylon rope , feed into the chamber , rotate the piston up