8mm.

If you do not have a service manual,please read some threads on the topic (almost everything has been discussed) :

https://www.google.com/search?q=valve+stem+seals++site:https://www.peachparts.com/shopforum/diesel-discussion&safe=images&hl=en-US&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjHxLKC7-T9AhUej4kEHfgRAkQQ0pQJegQIAxAC&biw=1920&bih=955&dpr=1

https://www.google.com/search?q=rocker+tower++site%3Ahttps%3A%2F%2Fwww.peachparts.com%2Fshopforum%2Fdiesel-discussion&tbm=isch&ved=2ahUKEwjhg_GG7-T9AhV3KN4AHZyQCE4Q2-cCegQIABAA&oq=rocker+tower++site%3Ahttps%3A%2F%2Fwww.peachparts.com%2Fshopforum%2Fdiesel-discussion&gs_lcp=CgNpbWcQA1CHFFjeRWCESGgBcAB4AIABeogB2QeSAQQxMi4ymAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=YVwVZKHPFPfQ-LYPnKGi8AQ&bih=955&biw=1920&safe=images&hl=en-US

https://www.google.com/search?q=valve+adjustment++site%3Ahttps%3A%2F%2Fwww.peachparts.com%2Fshopforum%2Fdiesel-discussion&tbm=isch&ved=2ahUKEwjM89nM7-T9AhXEE94AHR5cAi0Q2-cCegQIABAA&oq=valve+adjustment++site%3Ahttps%3A%2F%2Fwww.peachparts.com%2Fshopforum%2Fdiesel-discussion&gs_lcp=CgNpbWcQA1DsIFijPGDRQmgAcAB4AIABRIgBiAiSAQIxN5gBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=81wVZIyuLsSn-LYPnriJ6AI&bih=955&biw=1920&safe=images&hl=en-US

...and do not forget to remove the oil from the towers' bolt holes in the cylinder head before reassembly.