Timing the Trevithick Dredger Engine_ 2 making the fittings

by John

Today I made the fittings which would enable timing the dredger engine…


I needed access to the boiler for compressed air, and a temporary installation of a pressure gauge.  I decided to add a valve, so that I could shut off or adjust the compressed air.

I had made a safety valve, with a lead weight, but I had no idea of what pressure the valve would release, hence the gauge.  A spring had been specified in the plans, but I did not know whether it was actually required, so initially I left it out.

So, with the above setup completed, I turned on the compressor, and waited to see when the safety valve would vent.

It vented at about 20psi, a bit low.   So I added a spring.   That pushed the venting pressure up to 30psi.   At 30psi, the engine was still not turning over.

I was expecting that the engine would move at 30psi, but that the timing would need adjusting.   After an initial movement back and forth…. nothing.

The engine is still a bit tight, so maybe more pressure required, so I installed a heavier spring.


So I installed a heavier spring.  Pressure rose to 50psi before venting.  Still no movement in the engine.   Something is wrong.


  1. The throttle is not working properly.  Maybe the plans are back to front, or I have made the throttle back to front.  I have made both of those errors in previous builds.
  2. The steam valve is back to front.  Yes, I have done that before too.
  3. There is a blocked steam-air passage.  Yes, I have managed that one before too.

Next steps…   re-examine the plans.    Do a tear down of the throttle and steam valves.   If no obvious cause for the failure of movement…. call for expert help.   Yep.  I have done that before too.