Valve Seal Tool

M7 is a close enough match to fit Ludwigs valve spring compressor tool to the rocker arm adjuster threaded hole.

Compressing the valve spring against the frame tube is not a bad idea, but the compressor in the video is rather crude.
If you want to do it my way, these are the steps:
Remove plugs, remove stud valve cover.
Piston on compression stroke, 1 cm below TDC.
Lock the valve , remove rocker spindle and replace it with 12 mm or 7/16 bolt (rod, drill bit..)
Remove adjuster screw from rocker, install compressor, compress spring, remove collets, remove compressor.
Replace 12 mm bolt with a 4-5 mm rod.
The rocker can now be pushed far enough back to remove the valve springs.
If you like more working space for the seal, loosen the valve stop and lower the valve against the piston.
Replace valve seal, replace springs and collar, remove 4 mm rod and replace 12 mm bolt.
(make sure the ball end of the rocker sits in the pushrod cup)
Compress spring, place collets, remove compressor.
Remove the 12 mm bolt and Install the rocker spindle.

It seems that this is a stumbling block for some:



A thin nut between rocker and valve tip also works:



The 12 mm bolt will create some extra space for the nut.