Valve stem oil seals

My valve stems seals are letting some oil past so going to replace them.
Has anyone done them without removing the head? If so what method did you use?
I have done them many years ago on a car with an airline adapter on down the spark plug hole.

 

Same process, cams have to be removed and cam cups , as I recall there is a yamaha tool , Xj4ever may have answer . Note you will need to recheck valve clearance after seal replacement. Should be the same but could be different.

 

I can't see how it can be done without removing the cylinder head.
The seals are on top of the guides which are below the valve keepers and inside the springs down in the tunnel. Even if the valves were held open how will the seals be removed?
What i've done before now is once the valves are out pull the old seals off with long nose pliers and push the new ones on with my fingers.

 

I have done it on cars before with pliers designed for pulling off seals. Refitted can normally be done with a snug fitting socket.
I have heard of people carefully threading string into the spark plug hole then bringing the piston up to hold valves closed.
I will experiment and let you know how I get on

 

Once you've got the cams out you've still got to get the valve keepers off to get access to the seals.

 

Might be better just removing the head. Down side you will need a new gasket.