Plug and Hole: Should the metals match?

I'm making Stewart Hart's Potty Mill engine and am assembling the valve. One end of the valve box has a gland entry for the valve rod. The other end is blanked off with a plug specified to be a push fit into the cylinder.

The plug takes the pressure of any steam leaking past the valve piston. In one of my builds that could be quite a lot!

Normally I'd make the plug in the same material as the valve box. I assume that the strength of an interference fit in the same metal will be as good as it can be and won't vary with temperature. Is that true? Or would I get a stronger or weaker fit by using dissimilar metals? If so, are there any combinations that are particularly weak or strong?

My real-world application is sealing the steel valve box with a brass plug to improve its appearance. But not if the pretty plug is going to fall out!

Can the team advise please?

Dave

Hi Dave,

In theory brass expands more than steel with temperature and so if you use a brass plug it'll get tighter at higher temperatures **LINK**.

The only doubt I have is whether the plug will drop out when the temperature goes down again.

Not sure this helps

Jon

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

The plug is only ever exposed to exhaust pressure as the space between piston valve and plug is vented to the atmosphere by the exhaust hole, ditto between other end and the valve 'gland'.

In practice neither gland nor plug are needed if running on air.

Neil

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!

I know this isn't directly relevant, but if it were stainless, you might run into all sorts of difficulty press fitting SS plug into SS hole as they could gall and get stuck part way in. Been there, done that, and the onle solution is machining it out.

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.

How many brass welch plugs have been pressed into cast iron engine blocks and never fell out ? Millions ? More ?

Sure they only operate in an environment that is running around 15 psi and they are operating around 80 deg C but they do the job.

If you are running more pressure than that you need something to ensure it doesn't move - a thread or cotter pin etc but the temp of steam won't budge it .

Edited By XD 351 on 15/11/2017 05:20:32

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

Just a quick response as I'm already running late but Neil and Howi are right that the plug isn't needed!

As is obvious from the photo any leakage past the valve piston will vent to atmosphere through the exhaust ports before reaching the ends of the valve box. Should have engaged brain!

However, I'm still interested in the effectiveness of interference fits between different metals. I hadn't realised that brass plugs in steel are in common use and Duncan's point about stainless galling is a good one. I have a decent selection of Engineering Books and I don't think any of them cover force-fitting plugs let alone recommend which metal(s) to use.

I shall read all the answers properly when I get home this evening.

Ta,

Dave