Boiler Bush Size

[timb]

Hi All,

Help please!  I am constructing a Canterbury Lamb boiler to LBSC design.  The drawings show fittings tapped into the boiler shell which is a no no.  My question is what size bush to fit?  There are two threads in question the first 1/4 x 40 and the second 5/16 x 40.  I was going to drill 3/8 and 7/16 dia holes and make the bushes 7/16 and 1/2 inch respectively giving a 1/16 shoulder 1/16 deep to allow for soldering all round.

Bushes made from GM or Bronze.  Any advice to the contrary?

Thanks

Tim

[uuu]

Those sizes seem wrong - hole sizes and shoulders OK. But if you're drilling a 3/8 hole and have a 1/16 shoulder, you'll be adding an eighth, so your bush will be 1/2. And the larger one 9/16. You've not said how long you're making them. I'd suggest the overall length should be at least as long as the thread diameter.

Wilf

Edit: as with all things boiler-related, check with your inspector! These sizes conform to "Jessie" - so a published design.

[timb]

Sept 10, 2020 12:21:38 GMT uuu said:

Those sizes seem wrong - hole sizes and shoulders OK. But if you're drilling a 3/8 hole and have a 1/16 shoulder, you'll be adding an eighth, so your bush will be 1/2. And the larger one 9/16. You've not said how long you're making them. I'd suggest the overall length should be at least as long as the thread diameter.

Wilf

Edit: as with all things boiler-related, check with your inspector! These sizes conform to "Jessie" - so a published design.

Duh!

Quite right Wilf, (I was wondering when someone was going to spot that, Well done Pike!) thank you, I will check with 'The Man!'

Tim

[Roger]

Sept 10, 2020 11:03:22 GMT timb said:

Hi All,

Help please!  I am constructing a Canterbury Lamb boiler to LBSC design.  The drawings show fittings tapped into the boiler shell which is a no no.  My question is what size bush to fit?  There are two threads in question the first 1/4 x 40 and the second 5/16 x 40.  I was going to drill 3/8 and 7/16 dia holes and make the bushes 7/16 and 1/2 inch respectively giving a 1/16 shoulder 1/16 deep to allow for soldering all round.

Bushes made from GM or Bronze.  Any advice to the contrary?

Thanks

Tim

Hi Tim,
My preference is PB102 or PB104 for bushes because the threads are much less prone to damage. Personally I'd make sure the bushes were fairly long to give more thread length and allow a nice fillet of Silver Solder on the back.
You might consider making a pocket in the face of the bush for an 'O' ring sealing on the diameter instead of using a fibre washer or sealant. That way you can get a seal with the fitting done up finger tight because the sealing is done radially. All the thread has to do is to stop the fitting coming out.

[ettingtonliam]

One thing I do is to start the thread in the bush with a taper tap, then when its been silver soldered and acid pickled several times, and the boiler is finished, run the tap through to full size. This means you don't end up with loose threads because the acid pickle has eaten the threads a little.

[92220]

Hi.

That sounds a good idea, but one problem has just come to mind: How do you prevent tiny metal chippings from getting into the boiler and later migrating to the cylinders with the steam flow? Another possible problem could be that a tiny chip jambs the regulator open during a run, or am I just being over cautious.

Bob.

[Roger]

Sept 13, 2020 16:49:47 GMT 92220 said:

Hi.

That sounds a good idea, but one problem has just come to mind: How do you prevent tiny metal chippings from getting into the boiler and later migrating to the cylinders with the steam flow? Another possible problem could be that a tiny chip jambs the regulator open during a run, or am I just being over cautious.

Bob.

This is something I should have done. My fittings would be more snug had I tapped them that way. I don't think it's an issue. I think you can blow it down and get the swarf out easily enough.

[ettingtonliam]

A little grease in the flutes of the tap will trap most swarf, followed by a good washing out using a nozzle on the hose.

[uuu]

If you have a nice big dome hole, you can give it a good rinsing.

Wilf

[gwr14xx]

Or connect a compressor to the boiler while you finish the tapping - then the swarf will be blown up the flutes of the tap, instead of dropping into the boiler. (Make sure you wear a good pair of goggles - bronze swarf in the eye is not good!).

Eddie.

[timb]

The 'Dome Hole' as such (ie where the regulator is) is outside the main body of the boiler above the smokebox, fortunately the safety valve has a 3/4 inch bush so I will be able to use that for washing out!

Thanks all for the advice, especially the taper tap and 'O' ring shoulder!

Tim

[theflyingscotsman]

it occured to me whilst reading this that when building the boiler would it be prudent to make the shoulder thicker and put it from the inside outwards to allow depth for the thread, then silver solder them in as a belt and braces approach then the pressure will always be pushing against the shoulder effectively. when iv done it before i did it the other way which works fine but just a thought i guess

[coniston]

Mar 3, 2021 21:37:48 GMT theflyingscotsman said:

it occured to me whilst reading this that when building the boiler would it be prudent to make the shoulder thicker and put it from the inside outwards to allow depth for the thread, then silver solder them in as a belt and braces approach then the pressure will always be pushing against the shoulder effectively. when iv done it before i did it the other way which works fine but just a thought i guess

Although I have seen this done before it does limit the size of the 'flange' on the outside for the fitting to seal against and the flange inside can be a bigger hindrance to water flow possibly creating hot spots if in the water space between the inner and outer firebox. As I say it has been done before.

I see this is also an older thread so not sure if the boiler has already been made?

Chris D

[theflyingscotsman]

im not sure about the author of the thread as i only saw it today, but im in the process of making mine. like i said i have made a boiler before but it just made me think whilst reading the thread. the points you raise make sense and i think il stick to what i have done before.
kind regards lee

[timb]

Mar 3, 2021 23:16:28 GMT theflyingscotsman said:

im not sure about the author of the thread as i only saw it today, but im in the process of making mine.

Hi Lee, My boiler has been on hold as I have been unable to get to see the boiler inspector due to the Covid restrictions.  I did not want to get going in the wrong direction and make a hash of it.  Hopefully this will be picked up this summer, I expect I will have a Juliet boiler to make as well by then.

Some pictures of your boiler would go down well I think!

Thanks

Tim

[theflyingscotsman]

Unfortunately I no longer have the locomotive that I completed a boiler on, currently I have my 3 1/2" rob roy boiler but only started making the flanged plates and barrel so not at the Bush point just yet