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Tapered Blow Down Valve

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Alan Worland06/01/2010 21:28:56
247 forum posts
21 photos
I must confess to putting this question in the steam boat section - but I am now getting desparate!
I need to 'grind in' a small brass tapered draincock, I have tried a variety of compounds, which all look to do a good job - but it still leaks!
Any suggestions/hints/guidance? - other than replacing it!
russell07/01/2010 02:15:15
142 forum posts
Are you sure everything is still circular? I am wondering if the body distorted?
 
if you were to put engineers blue on one part and rotate slightly, you could (should!) be able to tell if it is making contact all the way round.
 
regards
 
russell
Alan Worland07/01/2010 21:40:06
247 forum posts
21 photos
It probably wasn't! I didn't have marking blue but used a permanent marker which showed good contact. However I think I have done so much grinding in that the hole has only just above got a clear way through! Bordering on scrap.
mgj07/01/2010 21:59:16
1017 forum posts
14 photos
Depends on how easy the thing is to hold and how priceless a replacement - either in terms of £ or convenience.
 
Make a tapered d bit/reamer and at the same setting make the new  plug cock. Then recut the seat/taper in the body  with the reamer.
 
It works but you do need to get a decent finish on the reamer. I polish the outer before milling to 1/2 D with emery and oil. Its not a difficult process- to do a reamer and harden and temper is a 1/2 hour job and it should save the day. The risk is with a very little D bit , that tempering could soften the reamer too much - difficult to control in those sizes, so you'd get away with care with not tempering at all probably. Worth a stab anyway.
Mike Gibbons07/01/2010 23:01:00
8 forum posts
Hi Alan,
              I think you've answered the question yourself - - too much grinding.
Taper plug cocks are lapped - in; not ground. I have'nt done any small ones, but i've lapped in a few of the "big ones"; although it was 50+ years ago. From memory we used to finish up with Brasso. LBSC in his book "Shop, Shed and Road" states - - - "grind the cocks-plugs in with a scraping off your oil stone; just a few turns back and forth should be all that is needed".
 
Meyrick's advice will certainly work, it will be like making a new unit.
 
But if there is enough metal left try with Brasso, or even tooth paste. But when grinding or lapping in a valve or cock-plug, only turn the plug through 90 degrees or so, never turn a complete circle. After a few turns in one place reposition the plug and start again.
 
regards    
Mike
Alan Worland07/01/2010 23:40:54
247 forum posts
21 photos
Sounds like a good idea regarding the reamer and making a new plug (but I will put the hole through last!)
On closer inspection the through hole in the plug is so far 'into' the taper the hole is barely visible in the body - could the plug be tinned to return some metal or solder the hole up?
The brass plug has the usual tapered and bent handle which is bent really close to the taper and quite tight - any advice on bending it?
When grinding it in I did start off with fine media and because it still leaked I ended up using coarser material which was the error of my ways!
I can only think that it distorted when the silver soldering was going on.
mgj08/01/2010 00:01:18
1017 forum posts
14 photos
Solder up. Nope.
 
2 reasons. First it will become impregnated with medium and just be a permanent lap. Second its too soft and draggy. You need a good but slippery fit. Bite the bullet, put it down to experience and do it right. Sorry.
 
At which point someone will chip in and say they have done it,  and I'd still say no, not for me..(but with respect )
 
Brass plug. Anneal first, then make straight, put retaining nut on!!!! then bend in one. I assume its fairly small.
 
I've just done one in stainless running in a gunmetal body. Heated that (handle) after machining to red and that worked fine, so there is a hint.
 
 
 
 

Circlip08/01/2010 12:32:59
1723 forum posts

Just to re-iterate the last two posters Alan, NEVER a "Grinding" abrasive, "BRASSO" for coarse and Toothpaste for fine. Graphite as a lubricant. The Bricks you've been using will now be embedded into the fittings, so enjoy a turning exercise and make some new ones COMPLETE.

  PS., not ALL grades of Brass CAN be annealed.

   Regards  Ian.

mgj08/01/2010 19:29:03
1017 forum posts
14 photos
And ream etc AFTER silver soldering!
 
Anything that gets metal very hot can introduce distortion, not just from differential heating but from stresses coming out too. So IMO its best ot do anything that requires a critical fit after heating if possible.
Alan Worland08/01/2010 20:19:54
247 forum posts
21 photos
I can now see how it's all gone wrong - still at least I have learnt something, just the hard way.
Guess I will be honing up my small lathe tools. Thanks to everybody for your input(s)
Alan Worland10/01/2010 19:19:33
247 forum posts
21 photos
I established that the taper was about 3 degrees and made up the reamer as suggested using it for truing up the bore of the valve (I had really made a mess with the grinding) While setup at 3 degrees I went about making the new plug - I have a slightly different handle design no bending involved I just tapped a brass cross pin in the end which has a larger boss.
Drilled the 1mm through hole and 'lapped' it in with toothpaste, assembled with graphite, and hey presto it works a treat with no sign of leaking!
Thanks to you all for your patience and guidance - a result!
Alan Worland10/01/2010 19:20:31
247 forum posts
21 photos
I established that the taper was about 3 degrees and made up the reamer as suggested using it for truing up the bore of the valve (I had really made a mess with the grinding) While setup at 3 degrees I went about making the new plug - I have a slightly different handle design no bending involved I just tapped a brass cross pin in the end which has a larger boss.
Drilled the 1mm through hole and 'lapped' it in with toothpaste, assembled with graphite, and hey presto it works a treat with no sign of leaking!
Thanks to you all for your patience and guidance - a result!

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