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Post by runner42 on May 27, 2015 23:05:19 GMT
Hi all,
the 1/4" x 40 tpi ME threads in the backhead bushes of my boiler has suffered due to excessive heat ups and over exposure to citric acid pickle and added to the fact that I used LG gunmetal instead of PB for the bushes. I suspect the efficacy of using fine ME threads where the thread depth is so small, they are candidates for such degradation. I am aware that it would have been better to leave final thread cutting until after all the silver soldering, but I made mistakes this being only one. The workaround is to re-tap them at 9/32 x 32 tpi, the bush having sufficient metal to accommodate this. However, this will require that I re-make all the backhead fittings.
Before I embark on this course of action is there any novel thread repair methods that I could try? I thought about removing the suspect bushes, if I used oxy-acetylene, or filling the threads with silver solder and re-tapping or some gunk that hardens and can be re-tapped.
Brian
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Post by Shawki Shlemon on May 28, 2015 9:05:58 GMT
If you have meat in the bush , the oversize is the way to go , just you have to join a lot of us that have a scrap box that keep growing , welcome to the real world of this hobby .
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Post by Deleted on May 28, 2015 9:10:30 GMT
I wouldn't recommend re-tapping the damaged 1/4-40 threads 9/32-32 The very small difference in diameter and the mismatched tpi could just produce another set of bad threads .
Re-tapping 9/32-40 would be far less problematic and is the easiest solution .
Can't think of any sensible way to fill the bush threads with metal and re drill .
If it were me I would make an oversize 1/4-40 tap to clean out existing threads and accept that backhead fittings needed reworking .
As Shawki says welcome to the real world of steam .
Very few steam locomotives in full size ever had imortant fittings just screwed into bushes on the backhead . The use of bolted flange fittings was almost universal .
The pads on the backhead for these fittings were often just thick washers rivetted on .
If you ever build another engine give some thought to using bolted flanges - look nicer anyway and you don't have the problems of the water gauge fittings tightening up at 37 degrees off vertical !
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nonort
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Post by nonort on May 28, 2015 13:27:09 GMT
If there is enough meat in the bush why not produce a step down bush from say 3/8" 40TPI to 1/4" 40TPI.
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steam4ian
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Post by steam4ian on May 28, 2015 23:02:18 GMT
G'day Brian.
You could drill out the existing bush and make a flanged insert which you silver solder in and then tap to suit the fittings you have made.
I will send you a pm
Regards Ian
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Post by runner42 on May 30, 2015 6:44:12 GMT
Thanks for your replies. The thread degradation is shared between the female thread in the bush and the male thread in the plug or associated backhead fitting. It must be a feature of the 1/4" x 40 tpi tap and die, when making the male thread it was nearly impossible to engage it in the female thread and required that I use disproportionate force to do it up. So to overcome this problem I engaged the male thread in the die and put the die in the vice to close it down on the thread and then unscrew it ostensibly cutting a deeper and smaller OD male thread. This allowed me to readily fit the plug or backhead fitting. I have remade a couple of 1/4" x 40 tpi plugs using the die in its normal position, whereas the originals are like the proverbial pr*ck in a shirtsleeve these are a better fit and might make the grade in sealing the boiler. I will have to await fixing the pin hole leaks in the boiler.
At Shawki's suggestion I did nothing further in attempting to fix the pin hole leaks and waited until I showed the boiler to the BI. His view and other onlookers is that I need to use oxy-acetylene to provide high pin point heat, large area heat from propane torch will only precipitate further leaks. This is one view, others may have a different view. He was surprised that I achieved what I had with only a propane torch, especially in the staying of the firebox which is narrow and reasonably deep. Dr Allan Wallace, a highly experienced ME at SASMEE is kindly assisting me (no that's wrong I am assisting him by providing another pair of hands) in fixing the leaks. We have had two sessions and a third is scheduled this week.
Brian
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nonort
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Post by nonort on May 30, 2015 10:22:11 GMT
Thinking laterally would it be possible to make the male thread of the fitting larger by pressing a ball or plug into the fittings swaging them out. I have used this method on very expensive vacuum fittings to save the day. If you heat the fitting then quickly insert the swagging tool it will strech very easily you may have to take several goes to reach the diameter that you require, a bit like broaching a keyway.
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Post by runner42 on May 30, 2015 22:47:01 GMT
Thanks nonort, that is a bit of lateral thinking. However, I think that I am going to have to make the backhead fittings and use an O ring to seal them instead of relying on the thread to form a seal. I am told that the use of an O ring will provide sealing when the fitting is done up only finger tight. After all it is not a complete remake it is only the body part that screws into the bush. It may even be the old threaded section cut off and a new threaded section silver soldered to the existing body part.
Brian
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Post by Deleted on May 31, 2015 8:33:00 GMT
However, I think that I am going to have to make the backhead fittings and use an O ring to seal them instead of relying on the thread to form a seal. I am told that the use of an O ring will provide sealing when the fitting is done up only finger tight. After all it is not a complete remake it is only the body part that screws into the bush.
Brian Hi Brian , You are getting some strange advice and I suggest that you question competence of person advising you . I know it's only a model size engine but steam is dangerous in any size and all design and construction should recognise this . All fittings should be made in such a way that they can be strongly attached to boiler . With screwed in fittings this means having fully formed full depth threads on all mating parts and the parts done up tight . Apart from actual strength fittings which are not properly tight will inevitably move at some time when in use . With fully formed threads sealing becomes less of a problem . Traditional way of sealing is to use thin fibre washers since these both seal and allow for alignment . Sealant on threads then just provides backup . Since you are making some new fittings you may like to look afresh at the water gauge design . I don't know what type LBSC specified for Doris but if it his standard screw in and hope design you could usefully change it to a backnut type . Best , Michael . PS: Just a note - this is in general not specifically about your engine . I tried explaining this on the ME forum once but got nowhere as usual : The basic integrity of a steam fitting connection as regards strength and non leakage comes fundamentally from the well fitted dry parts . Use of small amounts of simple sealant should only be needed to stop minor leakage . Any steam fitting connection which requires a great deal of sealant or use of volume filling or high strength sealant to secure the parts or prevent leakage is unsound and should be reworked or scrapped . Use of high strength thin film sealants is sometimes acceptable where a particularly strong connection is wanted but not to repair poor work and design .
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jma1009
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Post by jma1009 on May 31, 2015 8:58:11 GMT
hi brian,
i would strongly endorse michael's advice above.
as you know i dont use gunmetal for bushes. hard drawn phos bronze produces far stronger and more durable threads for boiler bushes.
so far as your current problem is concerned i didnt quite understand your description of using dies when threading the fittings and plugs. there is no shame in opening out the dies as much as the holder allows to produce an over size male thread for boiler bushes. i would also use a well worn die as a new die will tend to cut undersize anyway on brass fittings. also turn the part to be threaded oversize to start with.
cheers, julian
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Post by runner42 on May 31, 2015 23:26:40 GMT
Hi Michaelw,
the advice is sound the reason being that the thread integrity is good to ensure a sound mechanical connection able to withstand the pressure, however the O ring is providing the sealing. Remember the thread is being re-tapped to 9/32". The use of the finger tight expression is just to indicate that all it takes for an O ring to provide sealing, it is not an indicator that the fitting is being only tightened finger tight. I assume that the backnut type of water gauge is having an extended threaded section that can accommodate a thin nut that when the water gauge is fitted in its correct orientation that the nut is then tightened against the backhead bush.
Hi Julian,
yes I was aware of your concern on the use of gunmetal for bushes, I wish I had used PB. I am concerned that assuming the boiler eventually passes the pressure testing that in use the constant removal of the safety valve for boiler filling is going to be undertaken without thread wear.
Brian
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nonort
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Post by nonort on Jun 1, 2015 8:10:07 GMT
I personally never fill the boiler via the safety valve. I use a windscreen washer pump conected to the boiler blow down valve.
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Post by ejparrott on Jun 1, 2015 8:44:53 GMT
I personally never fill the boiler via the safety valve. I use a windscreen washer pump conected to the boiler blow down valve. If there's one thing I can't stand it's safety valve's being removed for filling or blanking for pressure test. Non of my engines ever have any fittings removed unless it's for re-work, which isn't very often. Boilers are filled through built in points via the pumps, safties are clamped for tests.
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Post by runner42 on Jun 1, 2015 23:29:39 GMT
I personally never fill the boiler via the safety valve. I use a windscreen washer pump conected to the boiler blow down valve. Sounds a good idea, any chance of a picture?
Brian
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Midland
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Post by Midland on Jun 2, 2015 4:24:07 GMT
I personally never fill the boiler via the safety valve. I use a windscreen washer pump conected to the boiler blow down valve. MAgree with all before. we all have connections for the handpump. Make a new connection and use a campers little £10 pump. Works a treat. Anyway does not removing a safetly valve require a new boilert test? Cheers David
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Post by runner42 on Jun 2, 2015 7:39:51 GMT
hi brian, ............ there is no shame in opening out the dies as much as the holder allows to produce an over size male thread for boiler bushes. i would also use a well worn die as a new die will tend to cut undersize anyway on brass fittings. also turn the part to be threaded oversize to start with. cheers, julian Hi Julian,
your advice was spot on. I opened the die as far as I could without stressing it, turned the OD to 0.257" and produced an oversize male thread, the plugs all fitted as tight as a duck's.....
Brian
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