By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

boiler fittings

best material

All Topics | Latest Posts

Search for:  in Thread Title in  
RRMBK28/11/2016 13:07:43
152 forum posts
18 photos

Hi All

Anyone got any recommendations for the best jointing material to use with brass parallel thread boiler fittings. I am struggling to get the boiler with all its fittings in place, to hold the 1.5 times pressure test for any significant period of time. It appears to be just very slight seepage at a couple of the connections between the boiler bush thread and the boiler fitting ( eg gauge glass frame) threads.

Not sure about PTFE tape as I have had experience before of little strands coming off and clogging various items e.g. valves, or preventing them seating properly.

Many thanks


David Jupp28/11/2016 13:23:30
806 forum posts
17 photos

Typically parallel threads are not designed to seal pressure, and it will be very difficult to get them to do so (except when you are relying on them not to seal!).

Where parallel threads are used, the seal is usually somewhere else - e.g. on a shoulder at top of thread (perhaps with a bonded washer), or an 'o' ring sat in chamfer at top of the female thread.

BTW - shouldn't boiler fittings be bronze rather than brass?

Nigel Bennett28/11/2016 13:27:43
431 forum posts
11 photos

Loctite 542. And yes, David's point about them being bronze is a good one, certainly if they're at or below water level. I've seen brass fittings dezincify and they just snap off, with the fracture site looking like bits of Aero chocolate.

Edited By Nigel Bennett on 28/11/2016 13:28:26

colin hawes28/11/2016 13:58:06
552 forum posts
18 photos

You must not use brass fittings directly into a boiler if you intend to obtain a boiler test certificate. As stated above it can lead to failure of the component in use and danger to anyone near it. Colin

RRMBK28/11/2016 14:26:54
152 forum posts
18 photos

Hi gents, rest assured they are bronze ! I used the term brass to simplify matters.

I have made all of the fittings with lands for copper washers, which I have annealed and used. However, making the fittings forces me down the road of parallel threads and also to get the correct alignment of certain fittings they have a separate hex nut on the threaded portion and this allows the fitting to be fixed in an exact orientation.

I think also, I am wary of any further tightening onto the copper washers as the threads are mainly ME 32 or 40tpi which have very limited thread depth, and I don't want to risk stripping the bushes in the boiler . I am coming to the conclusion that I should screw cut the threads instead and leave them a little oversize. This method though then takes me full circle back to using ptfe tape for its thread lubricant properties, which is what I wanted to avoid in the first place.

Is it worth screw cutting onto a 1/2 or 1 degree taper? anybody tried this approach?

Nigel Bennett28/11/2016 14:39:30
431 forum posts
11 photos

In order for tapered threads to seal, they need to be fully formed at the crests and the roots so that you get complete engagement and no helical hole left at either the top or bottom of the threads. I don't think tapering your thread will guarantee success unless the threads of the boiler bushes is impeccably formed to 100% thread depth.

I've never come across anybody using tapered threads in boilers in "our" sizes.

I have always used Loctite 542 (hydraulic thread sealant) and I do not have any leaks where I have correctly applied it. (Other makes of this product are of course available.) It is a low-strength adhesive and there is usually no problem in unscrewing fittings.

Phil H128/11/2016 17:04:22
425 forum posts
46 photos

Its interesting that everybody suggests that the boiler fittings should be bronze. However, some of the suppliers clearly state brass for their fittings. Are they using the term 'brass' in a similar way to RRMBK was using it but actually selling bronze?


fizzy28/11/2016 17:55:35
1835 forum posts
120 photos

Phil - there was a recent thread which went into depth on this subject, but most fittings are brass, they just need unscrewing and inspecting once in a blue moon. I think people are confusing boiler bushings with boiler fittings. All my bushings are bronze, all my fittings are brass.

Roderick Jenkins28/11/2016 17:58:22
2134 forum posts
586 photos

As far as I am aware, all the commercial steam fittings for models are made from brass. Yes, any parts that are a permanent feature of the boiler, such as threaded bushes, should be bronze but I can't see why any part that can be removed and inspected for de-zincification cannot be made from brass. I'm not sure where this myth that bronze fittings are mandatory has come from (the Tin Marketing Board perhaps?) but it doesn't feature in any regulation I'm know of.

I wait to disabused,


J Hancock28/11/2016 18:18:37
798 forum posts

Apologies if I have misunderstood this but it reads as though you are using an annealed copper washer (good)

squashed up by a nut against the bush in the boiler to effect a seal ?

Won't work.

Liquid threadseal is way to go.

duncan webster28/11/2016 18:48:34
3697 forum posts
69 photos

If alignment doesn't matter or one is using a check nut PTFE tape works for me, just make sure when you wind it on that there is none on the end face which can get cut off and lost in the fitting, and wind it clockwise looking at the steam/water end of the fitting so it doesn't unwind when you screw it in. Otherwise, as many others have said Loctite Hydraulic Seal

Sam Longley 128/11/2016 18:59:10
915 forum posts
34 photos

Plumbers hemp & boss white

fizzy28/11/2016 19:43:32
1835 forum posts
120 photos

If its a semi permenant join I use bearing retainer which is especially good for sight glasses, otherwise a fibre washer gets used and screwed in only hand tight these never leak - i do tighten them as well!

Phil H128/11/2016 19:47:56
425 forum posts
46 photos

Thanks for the clarification Fizz and Rod. I suspect it could be well worth using bronze for valve bodies if you are making your own fittings because the quantities are so small.

Back to the thread, I was under the impression that there is a great Loctite product as suggested by Nigel that allows you to screw the valves into position with the correct alignment. No copper washers or tapered threads necessary. I'm interested because I am about to start making some valve bodies and other fittings (finally).

Phil H

RRMBK28/11/2016 21:16:08
152 forum posts
18 photos

Thank you all for the information so far.

Re bronze/ brass. I was lucky enough in a former job to get the task of replacing quite a few old sailboat propellor shafts as I had the facility to machine the tapers , keyways and threads for the new ones. These were mostly bronze and up to 2 " diameter. The owners didn't generally want the old ones and if one asked nicely they would be glad of the offer to dispose of them! I've got a few odd lengths left of various diameter bronze shafting and I keep this stuff exclusively for boiler bushes and fittings . I do agree however that if the part can be removed for examination regularly then brass can be used. De -zincification too the extent where the item would fail, seems to suggest a lack of regular examination.

Nigel - thanks I will try that .

John Hancock I have machined a land on the face of the check nut and I would like to understand better why this method doesn't work as this particular fitting that does have to be in an exact location is the one thats giving me the most trouble. .

Sam I agree about Hemp & boss ! used it for so many years but it just looks so out of scale on a 1/4 x 40 tpi fitting !

Looks like I am going to have to add to the " Lucktite" shareholders dividends !!

Thanks again all and don't let that stop the thread. Others may still benefit from your wealth of knowledge and experience.

julian atkins28/11/2016 22:37:45
1246 forum posts
353 photos

Hi BK,

It is a difficult to get these bits to fit. The major problem is lack of undercut at the end of the threaded fitting, or lack of counterbore on the bushes. Also remove all burrs. I always aim for a metal to metal mating, with, on screwed in fittings a smear of red hermatite type sealant (no longer obtainable, but there are other substitutes).

I would never use Loctite on boiler fittings. Think what you may have to do to remove them for maintenance.



fizzy28/11/2016 23:21:48
1835 forum posts
120 photos

Julian - loctite bearing seal is very low strength and seals brilliant, very good for this application

Brian Baker 129/11/2016 07:43:20
186 forum posts
36 photos


loctite 572 is ideal for this application, and since it is made for low pressure (under 15 bar) pipe sealing, it would seem to have been made for us.

i have used it for 30+ years, never had a problem breaking open a joint, and never had PTFE tape clogging the working parts.

It is best described as "liquid PTFE".

try some!

Brian B

Edited By Brian Baker 1 on 29/11/2016 07:43:33

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
Eccentric July 5 2018
walker midge
JD Metals
rapid Direct
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest