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Mangled valve ports - help!

How might I repair mangled valve ports in CI cylinder castings?

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Bob Corrie02/12/2010 16:21:01
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 My apologies if this should go somewhere else or the answer is obvious but I’m a newbie to both this forum and model engineering and would appreciate some advice from those who have been round the block a few times more than I have.
I have acquired a part built 5” gauge locomotive (a Firefly 45XX GW prairie design) to complete as my first serious project. On inspection I have fund that the slide valve ports in the CI cylinder castings have been mangled (probably attempting to drill the steam passages)  such that there is not enough material left to give the required lap. Being a tad reluctant to fork out around £250 for new castings, I wonder if I could repair them.

Two approaches occur to me: 1, to silver solder/braze up the ports and machine them again (but will the solder  “take” in the ports, will the casting suffer with the heating and will silver solder provide a hard enough edge for the ports?) or 2, to mill off say 3mm from the valve face and then sandwich a 3mm plate between the cylinder block and the valve steam chest to form the valve face with new valve ports milled in plate. It occurs to me that making the  steam passages could be simplified by this if I milled shallow cavities beneath or in the underside of the plate to meet very short drilled passages into the ends of the cylinder bores.

What do you think, is this a goer? If so, what material would you use for  the plate with a GM valve? Or are there any better ideas? Any advice you can give is most welcome!
 
Bob
JasonB02/12/2010 17:17:45
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If there is sufficent material to allow the existing face to be be milled back then you can add a new face, this would be best made from a slice of CI bar and can be retained with some slow set araldite and a couple of small CSK screws.
 
Soldering CI is not easy.
 
Have a look at the pics in this guys album on a face being added to a cylinder block, they are a bit out of order but start with this pic
 
Jason
 
 

Edited By JasonB on 02/12/2010 17:18:26

Nicholas Farr02/12/2010 17:30:56
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Hi Bob, I to would go with the sandwich idea. Like Jason says solering would not be easy, you would need a fair amount of heat and it probaly wouldn't get hot enough in the right place. I was going to suggest gauge plate for your sandwich, but I've never had to try it. The pics in Jason's link look the business 
 
Regards Nick.

Edited By Nicholas Farr on 02/12/2010 17:34:07

JasonB02/12/2010 18:43:51
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If you join Traction Talk Forum (just an e-mail address req'd) you can read the words as well as having the music.
 
This thread covers the work on Kevins 4" Mac cylinder
 
J
Richard Parsons03/12/2010 09:09:13
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JasonB will Blue pack Araldite stand the 'racket'?
Bob Corrie03/12/2010 16:34:09
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Thanks very much guys, that is really helpful. I guessed that I wouldn;t be the first with this problem and your thoughts and the links to the TE thread are most encouraging.
 
Bob
JasonB03/12/2010 16:58:37
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I think the slower setting Araldite does have the blue/white tubes, reason for teh slow set is so you can squeeze the excess out with the screws before it goes off.
 
As for standing the heat when I drove the finished engine it was fine.
 
Jason
Richard Parsons03/12/2010 17:21:46
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JasonB Yes Blue pack araldite sets slowly at normal temperatures and the result is opaque.  At about 50C it can set in a few minutes and at 70-90C it is liquid but it has polimerised. 
At higher temperatures is can change into a sort of carbon fibre or 'buckyballs'.  As you know Iron (& steel) love carbon and Carbon loves iron so it can form molecular bonds which can be very strong.
You can delay the setting 'Red pack' araldite by putting in the fridge/freezer.  This delay can be from a few tens of minutes to hours.
Stub Mandrel03/12/2010 21:06:38
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I haven't had any problems getting silver solder to stick to fresh, clean cast iron surfaces.
 
If you mill the cavities out oversize you should be able to fill them with silver solder OK. Use plenty of flux as it will take a while to heat.
 
Neil

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