Boiler bush threads

[Speedy Builder5Participant @speedybuilder5]

When making bushes for boilers, should you thread them to full depth of thread? I believe that during the heating / silver soldering of bronze bushes, that there is surface degradation of fine threads. If the bushes were made and threaded to perhaps only 3/4 depth of thread and then threads fully tapped out once silver soldered into place, would this be better practice?

[Speedy Builder5Participant @speedybuilder5]

[David George 1Participant @davidgeorge1]

Hi do you mean just start the thread with a taper tap and then use a finnish tap after silver soldering in place. Personally i would finnish the thread with correct size drill for a full depth thread and finishing before silver soldering as bronze can be quite heavy to thread and can you be sure that the thread is square to sealing face. I would just clean up thread after soldering.

David

[JasonBModerator @jasonb]

Depending on the thread it can get burn by overheating so things like 40 and 32 tpi its usually best to start the taper tap into the hole when the bush is still in the lathe and then complete after soldering. On a bigger model with BSP threads not such an issue.

[BazyleParticipant @bazyle]

If you are worried make a loose fitting iron plug if you haven't drilled right thorugh until after pressure test or put through a small steel nut and bolt. Contaminate the plug to ensure solder doesn't stick.

[nigel jones 5Participant @nigeljones5]

You can use fully formed bronze bushes without issue – I use hundreds and have yet to have one fail. If you were to use brass then there is a high chance that the whole bush will melt and the threads dont like the heat regardless so dont go there.

[Speedy Builder5Participant @speedybuilder5]

Well, thanks, there are 3 different ideas, all of them I understand. The iron plug idea for smaller threads would allow the bush to be supported in a jig to ensure that it was "square" to the required axis but never heard it talked about and a heartache if a spot of silver solder made the plug stick in the thread.

As Jason said, the fine threads are at risk of burning but Fizzy who has made more boilers than most of us put together hasn't had a problem.

And David rightly is concerned that a unless the bush is tapped in the lathe, then there is a risk the face of the bush is not square to the thread.

Has anyone tried some sort of "fire clay" to put into the bush during soldering ??

[BazParticipant @baz89810]

I know that at least one of the professional boiler makers didn’t drill right through the bush and therefore can only put one to two turns of thread into the bush. When the boiler had been hydraulically tested the bush would be drilled through with a pistol drill and the tapping done then. Saves messing around with making blanking plugs, so I was told.

[Nigel BennettParticipant @nigelbennett69913]

I've never had any bother with fully-threaded bronze bushes (DON'T USE BRASS!!) but Tipp-Ex on the threads would be an idea if you're worried. Mind you, I only use propane, not having oxy-anything, so that may be an issue.

[nigel jones 5Participant @nigeljones5]

just to add….if im doing a decent sized loco boiler the bushes are 90% drilled, soldered in and drilled and tapped cold. The amount of heat that goes into a 5" gauge boiler is unreal and controlling it is difficult. On the small stuff which I do most (3.5inch dia to 6inch dia) I have cnc bronze bushes made. Sometimes silver solder will creep up and over the face and into the threads but chasing them out is easy enough.

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