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Boiler fittings

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Peter Simpson 330/11/2021 09:01:51
40 forum posts

I need to make fourteen 6ba screws to secure my boiler dome cover. The plans state they should be Bronze. Would be Stainless steel be OK.

noel shelley30/11/2021 09:48:25
1442 forum posts
23 photos

Could you change the thread to a metric or unified and buy in, in stainless ? If not in a possition in or under water use brass ? Just a thought ! Noel.

Peter Simpson 330/11/2021 10:08:11
40 forum posts

I have no problem making them in either material. I have some 0.250" diameter S/S in stock. I don't have any Bronze in at the moment. Also 12" of 0.250" diameter Bronze is stupid money.

Tim Stevens30/11/2021 10:23:09
1622 forum posts

I foresee no problem with stainless in place of bronze, for threaded fittings. The expansion with heat is a bit less, and the friction characteristics are not so good, but let's hope you only need to tighten them once or twice.

There will be a violent reaction from the odd purist, of course, especially if your design originally started life before about 1930. In my experience, these tend to generate a lot of heat, but no illumination.

Cheers, Tim

Peter Simpson 330/11/2021 10:29:24
40 forum posts

Cheers Tim, I will spend the next few hours on the Myford. I did a test piece this morning, using cutting fluid and a slow speed, the first one turned out fine.

John Baguley30/11/2021 10:32:21
507 forum posts
56 photos

I've always used stainless with no problems. You can buy 6BA x 1/2" cheesehead screws from EKP but not cheap.

Can I suggest that you make studs and fit nuts if possible rather than use screws. Much easier to remove the dome then and avoids the risk of shearing off a bolt when they seize up (as they often do!) Fit the studs with threadlock which will seal and protect them if the threaded holes in the dome bush go through to the boiler space.


Peter Simpson 330/11/2021 11:07:04
40 forum posts

John what a good suggestion. You always get a good response on this forum.

Nigel Graham 205/12/2021 19:10:59
2284 forum posts
33 photos

Agreeing with Tim and John -

Studs and nuts would be the the much better mechanical, and indeed prototypical, option.

The rivet-counters won't spot stainless-steel fittings hiding under the outer dome; besides the full-size inner dome was probably cast-iron or steel, not bronze, and secured with steel studs and nuts.

If you are the UK, not the USA, use metric rather than UN, as being much easier to obtain now. Using the Thread Comparison Chart supplied by Emkay Screw Supplies, I would pick M3 as nearest to 6BA, at only 0.2mm (0.008 " ) larger in diameter; but verify nut and spanner clearance from the wall of the dome.

Though tempting to use stainless-steel studding, to make a decent job of it, turn proper studs with central plain shank. This gives a self-limiting depth control in the tapped flange.

Edited By Nigel Graham 2 on 05/12/2021 19:11:23

Peter Simpson 305/12/2021 21:52:21
40 forum posts

Well I did as John suggested. Purchased some 3mm 303 Stainless. Turned down to 0.110" at 1/2"long. Machined 6 threads on one end and 9 thread on the other this gave a nice central plain shank. The nuts were made of 5mm hex 303 stainless.

Speedy Builder506/12/2021 06:51:07
2653 forum posts
219 photos

Peter, not a good idea having the same stainless for both nut and stud as they can cold weld and seize together. Perhaps not at the loads we put on the threads but once you go up in size, its a definite no-no. I worked at a lab testing high pressure steam stuff for CEGB, and there was a steam chest where the cover was held in place by about 40, 15mm bolts, the fabricated chest was the same stainless as the bolts and more than half had to be flame cut off. It took ages to clean the fan off afterwards !!


Brian H06/12/2021 07:40:52
2312 forum posts
112 photos

In the defence industry, when using stainless for studs and nuts, the studs would always be silver plated to prevent cold welding. The silver acts as a lubricant.

It would be an idea to check with your boiler inspector to confirm that stainless is acceptable. And you need to be sure as to the grade of stainless. Personally, I would use 316.


J Hancock06/12/2021 08:57:39
843 forum posts

As above , not a good long term idea to use same grade st. nuts.

noel shelley06/12/2021 09:50:34
1442 forum posts
23 photos

As with some of the others, cold welding of stainless nuts and bolts. If the theads are rough this will start it and then the stud will fail as you try to get out of the mire. Noel.

SillyOldDuffer06/12/2021 10:40:48
8891 forum posts
1998 photos

Is cold welding of stainless nuts and bolts a problem on models? Most of my fastener stock is stainless metric, and I've not had any trouble with them. Is that because they've not spent 10 years holding a boiler together, or because the manufacturers of nuts and bolts avoid alloys that cold weld? I'm wondering if the advice to avoid stainless is good if the fasteners are home-made 303, but over cautious if commercial items are used.

Seems hard to buy retail quantities of small bronze fasteners or small diameter rod in the UK. Bronze fasteners seem to start at 1/4" minimum because they're sold for corrosion resistant structural work and boats.

Bronze rod is available 12mm and up, which is pretty wasteful turning down to 6BA/3mm/⅛"! Could 3.2mm bronze brazing filler rod be used to make studding suitable for Peter's application?



Edited By SillyOldDuffer on 06/12/2021 10:41:09

duncan webster06/12/2021 11:25:07
4120 forum posts
66 photos

Stainless in boilers is only a problem in contact with liquid water. Studs holding down the dome should be dry, certainly not in contact with liquid. We happily use ss superheaters.

In my professional life we had issues with stainless cold welding, we eventually used satellite weld deposit on the male, not the hard facing version, this was machineable. For this instance I'd work out if brass nuts would be strong enough, if not make bronze.

J Hancock06/12/2021 11:25:12
843 forum posts

Of all the applications , the dome is most likely to , over time , see the affect of 'steam creep' through the joint and up into the stud apertures, then condense in the gap in the threads.

Seize guaranteed.

Baz06/12/2021 13:18:25
756 forum posts
2 photos

A finger full of copperslip would probably stop the stainless seizing.

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