On Mon, 25 Jul 2011 02:52:31 -0700, Andy Dingley wrote:

On Jul 25, 2:03Â*am, Clive wrote:

Priming is normal for any engine with an over full boiler,

Priming might be "inevitable" if you over-fill the boiler, but it's
never "normal".

Look at the sad tale of 532 Blue Peter, if you want to see how bad
priming can get.

Going back to the earlies, the /extremely/ large clearances at the ends
of the cylinders on the two surviving Hedley (Wylam) machines could
be a recognition (possibly by Trevithick, given that the cylinder patterns
are very like ones in his stationary engines, and the cylinders may
in fact be cast from the patters made for the 1805 Gateshead engine)
that they were going to prime.
In the Wylam application, of course, it would have been even more important
to have this clearance as the cylinders were mounted externally (rather
than within the boiler, as the cylinder design suggests they were intended
to be fitted) so condensation would be an issue - and there were no drain
cocks on those (vertical) cylinders...

--
From the Model M of Andy Breen, speaking only for himself