There is no tool on the market for this job that I know of...I made one using an old 1/2" drive socket and a bit of hexagon bar....I cut off the 'socket' to just leave the shank with the square hole...I then filed that to be a good fit on the square head of the follower guide...
Next I drilled the hex bar to the correct diameter to take the socket shank with a 5/16" hole all the way through for the tappet shaft to pass through...Then I welded the prepared socket in place...A bit crude but it works a treat...I made it years ago after I got fed up with stretching spanners and damaging the tappet guides...Heat the cases before unscrewing the guide...Ian

email (option): ian@wright52.plus.com

Yes, it is my video... just like Ian I was also struggling with open-end spanners :upside_down_face: and I made the tool straight after Trevor shared his clever idea here, Thanks again Trevor :+1: :+1:

Short piece of square tube, reinforced with a ring:



BR, Michiel