Repairing a high quality model marine engine

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bogstandard:
To carry on from the last post. Yesterday I got the valve apart and ordered the o-rings.

Now to get back to a rather bad problem that I had notice before. I started this yesterday but didn't really feel up to it. But I did think I had found my 7BA tap. But when I really checked it out, it wasn't, so this is where having friends locally comes into it's own. A quick call to Stew and the required tap was on my workbench, ready for doing the job today. Thanks Stew.

So this is the problem, and one of the main reasons I think that this engine was finished off by an amateur who knew nothing about how this type of engine works.

If you look at this pic, it shows the problem. When the steam slide is in the back position, all is well, and the nut is prevented from turning by the bottom of the slot it is sitting in pressing against one of it's faces. The problem starts when the slide goes into it's normal operating position, pushed forwards by steam pressure and forcing it against the port face, ready to do it's job. There is enough gap between the slot bottom and the nut face to allow the nut to do full revolutions around the screw, so allowing the critical timing to lose position. You can spend hours fine tuning the engine timing, only to have it all undone in a matter of minutes when the engine is running. If you look at the drawing, you can easily see that even if the slide is very loose, there is no way that the correct shaped nut can rotate and allow the timing to move. I can only assume that a bit of Loctite was used on the nut after timing setting, but that is a long time burnt away, if it was there in the first place.

So yesterday, I modified the slot in both slides, and cut a piece of brass down until it was a smooth sliding fit in the slots.

Holes were drilled in the required offset positions along the length of the bar. Should have been 2.05mm, but 2mm will do fine, so that is what they got.

Then onto the tapping stand. Not having the correct width parallels, the way I easily got it level in the vice was pop the part onto the table, turn the vice upside down over it, and tightened up the jaws. Once the right way up, they are seen to be spot on along the whole length. All holes were tapped in a matter of seconds.

After a date with the bandsaw and mill, the nuts were to the right size, and as you can see in this pic, there is no way that the nut can rotate. This is a standard method that is used on slides on this type of engine.

The other block, ready to go in the engine, with a few spares for the customer. Another job out of the way, now onto the previous one.

I ended up cutting the stuck handle off, I couldn't risk damaging the very finely lapped spool.
Once it was off, I could see what damage had been done, and measure up the o-ring. They came today, so I can carry on with the job now. Looking at it now it is apart, I think it has been like this from the very beginning. As it looks like the o-ring seating has been cut too deep, so it wasn't sealing properly from the start, so a shim was put in there to get it to compress enough to stop the leak. If I am careful, I might be able to skim off the inside of the flange to get the seating to the correct depth, then take the same amount from under the flange face to put everything back to original. That will save me having to make a complete new flange, and everything that it involves.

So now we are on the downwards slope, just this job to do, plus a few parts to make from the drawings that are missing from the reversing linkage, and I will be ready to start the build up.

Bogs

Darren:
It's the size that gets me, these parts are so small.

I'm not sure I could see them never mind machine them :bow:

bogstandard:
Darren,

These are large compared to some bits I have to make for steam engines. It is all in the mind. As long as you can hold it rigid enough, you should be able to machine it.
Not used on these bits as they are fairly large, but I have a 1" micro sized grinding vice that holds the small bits and then that is mounted into the normal vice.

http://www.arceurotrade.co.uk/Catalogue/Workholding/Machine-Vices

The smallest one in the first lot.

John

Darren:
I nearly fell off me chair laughin....9mm opening, jaw 25mm width.... :lol:

Sorry, I'll go to the back of the class...... :hammer:

Bernd:
Bogs,

This is going to be a bit :offtopic: but I followed the link you gave. Very nice place to get tooling and the like.

But what interested me was link to the location via google map. I looked around using the satelite veiwing. Can't believe how those houses are packed in there like sardines in a can. I can now understand why you chaps that live in cities have such small shops. I can also see why it takes you so long to get somewhere. Don't seem to be any direct routes to were you want to go.

Bernd

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