Stuart Number 9 Engine

[delalio]

I've been following this build very closely. I recently built my first steam engine, a Stuart 10V, and I've a Stuart S50 awaiting starting too.

I have the same new project nerves you do! So I've been doing almost every other job I can think of instead...

 

[jcreasey]

Part 9 - Making the Piston​

First job was to clean up the casting.


Despite what the plans specified I decided not to rely on the thread for alignment and machined a small rebate into the piston. This will be a tight fit against the piston rod.


Now that the piston is cleaned up it was time to make the piston rod. I didn't want to rely on the three jaw chuck but I also did not want the 4 jaw chuck to cause any damage to the piston rod so I made a small fixture from some brass to clamp the rod.


This allowed me to dial in the concentricity very closely before single pointing the 2BA thread and the matching boss for my piston. The boss is a very tight fit.



I used the piston rod as a mandrel to finish machining the piston to size. Hopefully this will ensure everything is as concentric as I can possibly get it.



I ground up a little stubby parting tool to cut the slot for the piston rings.


Once I had the piston done I reversed the rod to machine a thread on the other end for the cross head guide. It took quite a bit of messing around to get the piston rings fitted with just the right amount of clearance.


A bit later I drilled two blind holes in the end of the piston and made a pin wrench to suit so that I can easily remove the piston from the cylinder. This makes life much easier when fitting the cross head guide.

 

[Steamchick]

A proper job!
Recently, trying to drill a 1.25mm hole - using the 3 jaw chuck to hold the drill on my 8in lathe, I found the chuck runout was to great for the drill too not be true. So had to resort to the 70-year-old Unimat SL.
Maybe my 3-jaw chuck has some swarf where it should not be, so it needs a strip and clean? - I was OK less than a year ago...
But I like your fastidious approach to precision of concentricity.
What dimensions are the piston rings? Expanding them over the piston must have been a nervous moment!
K2