In 303, I have gone down to 1-72, dont think 0-80, thats pretty tiny!  For stainless I always use the slightly larger drill size the chart shows, 50% vs 75%, and add a drop of oil to the tap. If the tap is getting dull at all it will not cut well in it, and snap if forced.
I've also never tried a form tap in 303, just brass, mainly since I only have one form tap so far! Works well in brass, though I have to remember that it requires a different drill size than a cutting tap.

Not sure I've attempted anything smaller than #4-40 in stainless.  Glad you got it down on the 2nd try, Kim. 

That's looking great, Chris! 

I assume when assembled, the cylinders will pull up tight to the crosshead guides, so there won't be that little gap there?

Kim

Correct. For now there are temporary  screws holding the crosshead guides to the plates, the heads are keeping the cylinder out. I have tapped the screw holes at both the cylinder flange and and also the end plate end to facilitate  this. The last thing to do before final assembly is to drill half of them out to clearance  size. Some at the cylinder end, some at the guide end, depending  on which way the studs can go in. The steam chests are in the way on the upper ones, so those go in from next to the guide tubes. Lots to keep track of!

The connecting rod looks 'interesting' - I'm wondering the best way to set this up in order to machine between centres, these are the smallest lathe dogs I have and again same problem as I had before which is to set it up and drive them. I'll need to make centres at either end then taper from one end to the other as per the instructions .......any ideas welcome

The issue with the valve stem seal is annoying. If necessary, it can be remade from round stock.

Good progress 👍.