Dave

I made Stew's Vertical Cross Single which uses the same cylinder and valve chest. I found that I was able to drill and ream the hole for the valve deep enough for it to operate without going all the way through the chest. There is even enough room for the chest to be made a little longer if necessary.

In this way you need not make a plug at all.

Jim

fit in the same metal will be as good as it can be and won't vary with temperature.

Well, for certain the hole and plug would change size by exactly the same if their temperature changes if they remain equal.

There is, of course, nothing preventing you making a plug with the cap being of different material to the actual plug core.

I wouldn't want a plug taking full pressure, if there were any chance of it being fired from its barrel at high velocity!

This should work just fine. Make the brass plug a thou oversize and tap it in. Bronze (a fancy grade of brass) bushings are pressed into steel connecting rod small-ends in full sized internal combustion engines millions of times per day without a problem. I daresay they come under way more stress than your exhaust steam port will provide.

The difference in expansion between different metals is going to be negligible at the size and temperature you are working, and anyway the brass expands more than steel so if anything the grip will become tighter.

When I built mine, made as per plan, tight fit in bore, used vice to squeeze it in place. Not elegent but you cannot tell just by looking. On!y run mine on air so as Neil says, not really needed. Mine only needs a few pounds of air pressure to run, probably more down to my loose machining tolerances than precision fit. If it is a loose fit you could always glue it in.

Edited By Howi on 15/11/2017 09:19:31