I just measured it to be 36.5mm, I'm not sure what that accounts to. If you go to preston motors and tell them your engine they will know, or someone here might have better info
There a bit of a pain in the ass to get, if you cant get them from underneath you will have to remove the manifold. Jack it up and have a look from underneath to see how accesable it is, I haven't done them myself so not sure