This is the manual feedpump for SL Isabella.

What is a manual feedpump?

After I received the castings I put it for three months on the shelf because I did not know how to make the bore for the piston into the casting.

Problem:

The bore should be 20mm diameter and 170 mm long.  Since I have only little experience in machining castings I did not know how to do this. There was already a bore of 12mm in the casting so you can not just put a bigger one in to without the serious risk for snapping (Gun Metal).  And even if I would manage to change the geometry of the bore so it won't snap, most of the bores of this diameter are about 140-150mm long and I did not want to buy a new one only for this single hole.

Solution

I used a single cutting toolbar I normally use for making cylinders bores on the milling machine. i made a support for it so I could use it on the lathe. The bar is 180mm long and I managed to let 140mm stick out of the support. This way I could very carefully make a  very precise hole of 20mm over a length of 140mm.

The trick

Than I turned around the casting, spend an hour or two to get it aligned and made a hole of 20,5mm over a length of 30mm from the other side. The idea behind this is, that as long as the piston fits very well into the first 140mm, it does not matter if the remaining 30mm are a bit a loose fit.

The only disadvantage of this approach is that I had to turn down the outside of the casting until I got a smooth surface that could be used as a reference when I turned the casting around in the three claw.
 

What is it and why is it in every steamlaunch?

A manual feedpump is used to pump water into the boiler by hand.

This can be necessary for a couple of occasions:
.    To run a pressure test with water to see if the boiler is still safe,
.    to pump water into the boiler when your engine driven pump is malfunctioning, 
.    To pump water into the boiler when the engine is not running

Because low water in the boiler is the worst thing that can happen every Steam launch will always have at least a manual feed pump besides the engine driven pump.