Timing the Trevithick dredger engine-3. How embarrassing….

by John

As planned I removed the engine from the boiler, to discover why it would not work with compressed air yesterday.


After yesterday’s failed attempt with compressed air, I removed the cylinder and piston and valve assembly from the boiler to check the steam passages, throttle and rotary valve. 


These components were removed so I could see what was going on.

I wish that I had checked this before going into print yesterday.

I discovered that the bronze cylindrical insert with the steam ports had never been fixed/Loctited into position.  Clearly it had rotated with the first engine movements yesterday, causing all steam passages to be blocked.  No wonder I couldn’t get the engine to do anything!

And worst was that I had mentally made a note to secure the insert, when I inserted it, a month or so ago.

So today I drilled and tapped a hole for a stainless steel grub screw to hold the insert into position.  I decided not to Loctite it at this time, but will do so when I am sure that the timing is correct.  I might need to alter the shape of the ports, and the insert would need to be removed to do that.


Tapping the thread for the grub screw using the shop built tapping head which I made to Mogen Kilde’s design a couple of years ago.  I have never broken a tap while using this tool.

After reassembling the components I ran another test with compressed air.

Somewhat to my surprise, at 20psi the engine ran for a few revolutions, before  settling down into a back and forth motion.  It still needs to be timed, but hey, it moves.  But by this time it was becoming hot in the workshop, so I left the timing for another day.  Maybe tomorrow pm.   Not Thursday because it will be HOT.  42ºc  (107.6ºf)