Once you remove the cir-clips the bearing pushes out with a light tap.
Printable View
If you have access to a lathe then a 4-jaw chuck, centralised using a DTI on the existing bore, then bored out to 80mm. Creep up on the final cut as it's quite a deep part.
If you have access to a milling machine then a flycutter, or a proper boring head if it will cut a part that deep. I don't have one myself but imagine they are expensive.
Do all this before you make the slit etc.
If you have neither, then do you have the receipt! Would the supplier swap it for an 80mm version?
I have one of these 75mm mounts sitting in the workshop at the moment which was given to me. When I measured it, I realised why it was being thrown out... When I get round to it, I'll stick it on the milling machine and run the boring head through it. There seems to be plenty of meat in the casting to do this. Only minor difficulty is that someone has already cut the slot down one side so I'll have to shim that and clamp up before boring.