35mm or 1-3/8" will do it. Whole front has to come off to drain properly as no drain plugs on these.

it is possible to leave the front end on.

remove axle and wheel and you will see there is a 12mm socket head bolt in the bottom of the fork. If you remove this you will drain your forks too.

i don't like that method for these reasons:

There is no way to hold the part that the socket head bolt screws into when either undoing or doing up.

you really need to pump the forks a few times to get all the oil out. Much easier to do this with the fork off bike and spring out.

oh yeah the top cap will be under quite a bit of spring pressure so be careful when you remove it. Cover your tank etc with old blankets and wear safety glasses.

undo top Triple tree pinch bolt first.

i made a jig that allows me to fit the cap or undo it easily while it us under spring pressure.

35mm..

SHOWA are Japanese