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  Not a vacuum, It is just a large dent in the outer heat shield.

Ah, OK.

Thank you for that. 

This is a question for the model engineers among us:

I’m planning to use O-rings as piston rings in a new steam engine design (instead of PTFE piston rings as used so far), and I’m trying to figure out the ideal amount of compression for the O-rings.

The technical reference manuals of the manufacturers recommend at least 6% compression for dynamic applications (like sealing a piston rod or a piston ring). For example, a 1.5 mm cross-section O-ring would be compressed to about 1.41 mm. In my experience, though, 6% is way too much for small model engines. When I turn the engine by hand, there’s noticeable friction, and the engine only starts moving smoothly at fairly high steam pressures. Getting a smooth, slow operation seems almost impossible.

Some people suggest that a piston O-ring might not need any compression at all, and that the seal created by the steam pressure pressing the O-ring against the surfaces is enough. Without pressure, there’s barely any sealing—but then, there’s no steam pressure anyway, so maybe that’s fine.So, is the “right” compression somewhere between 0% and 6%?

What compression do you use in your own designs, or have you measured in commercial engines during maintenance?

Donation made 

Hi All
Sadly missed Lomex however, did manage a trip up to Lincoln Tues to MMM where I got answers, took along a Cheddar Puffin plant and also a Cheddar Plover plant, both will fit so now just need to make up mind which to use.
Made very welcome at MMM by Ann and immediately offered a cup and tea and biscuit, now how's that for customer relations.


Rosegarth ordered and expect delivery in around 28 days, worth the wait I think.


Mike 

I also did it for MBM. !

I filled the stern-tubes with grease by which all the works connected with the drive trains have come to the end at last.
In the next place, building processes go to the rudders, water cooling pipes installation and radio setting before long-awaited launching about which I am anxious a bit.

While I have no experience with the kit you reference, I have two kits by Trumpeter and both were excellent.

Do you take Crypto?    No matter.$$ will work. What would the world look like without MAYHEM?

Thanks Chris........interesting images


It is easy to see why photography in restricted light can influence or enhance  the illusion of a model being seen as a full-size vessel 


Derek