Thread: how can i do this?

Cant see why it couldnt be done on a lathe,I think when they reffered to milling the inside of it,it may just be a term they used.
If they did do it on a mill it would of been a CNC and they would of used Circular Interpolation.
Can the Conrod be held at the Crank end and the Piston spun without causeing damage to the Gudgen.
If the Piston can be Rotated while the Conrod is Fixed you should be able to machine the Bore of the Piston without a Problem.
Give the size of the Piston and the amount of material to be remove your work holding methods will be Critical.
I would think that you would be better off useing soft jaws in a 3 jaw chuck and making a purpose made clamp for the Crank end with an appropriate sized Centre to locate the Conrod whilst machining the Piston bore.
Is this going to be a 1 off or will you be making more.
If its a 1 off, apart from the experience you gain and the amount of time and care involved it would probably be easier to buy them.

I'd be making quite a few. The Conrod is attached with a socket. so it can spin freely.
You could probably just tie the con rod at the crank end with string to keep it out of the way.

What Tool would i use that can fit inside though?

Thanks for help!

You would be better off using a piece of HSS,without knowing the dimensions its hard to judge what size piece of HSS.
Another allternative would be to use a small piece of Carbide silver soldered to a bar of a suitable size to allow you to get between the Conrod and Piston.
I would be cautious about using a piece of string or similar to hold the Conrod out of the way whilst machining,but the choice is yours,trial and error.