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Pros and cons of 32 TPI vs 26 TPI

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Greg H05/12/2016 09:19:58
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46 forum posts

Hi,

I need to make some boiler fittings for my loco and was wondering about the preferred thread type. The plans state 1/2" x 26 TPI, but I have noticed that some suppliers sell safety valves with a 1/2" x 32 TPI thread.

So before I make the fittings I was wondering what the pros and cons of these two threads was?

I have taps and dies in both 32 and 26 TPI, so could go either way.

Regards,

Greg

Jon05/12/2016 15:06:37
650 forum posts
32 photos

Both will take the pressure if that's your concern assuming done to size.

Have tested the 26TPI cycle thread/Brass threads to 10000psi near on metric 1mm pitch.
32 TPI tad larger pitch than metric 0.75mm but only taken to 4500psi.

General rule if tapping, finer the pitch the easier to turn due to less metal removal.

Finer pitch can leave more material thickness over the thread so safer and usually over looked.

Speedy Builder505/12/2016 15:44:53
1262 forum posts
91 photos

Sometimes the finer pitch will allow you to position the fitting more accurately i.e.: backhead sight glass fittings etc
BobH

julian atkins06/12/2016 00:09:25
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1131 forum posts
351 photos

Hi Greg,

I use 1/2" x 26 tpi ME thread for boiler bushes and fittings of this size. I go down to 32 tpi ME thread for 3/8" and below till I get to 1/4" when I use 40 tpi and for smaller sizes.

I have 32 and 40 tpi dies and taps for 1/2", but are not used on boiler work.

Cheers,

Julian

Neil Wyatt06/12/2016 07:23:29
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Moderator
11310 forum posts
527 photos
62 articles

As a fully paid-up lover of imperial sizes it pains me to suggest this, but perhaps in the future 1.0mm (25.4 tpi) and 0.75mm (33.8 tpi) will become the preferred sizes? 0.6mm pitch is ~42tpi but only easily available get as M3.5.and 0.8mm (31.75 tpi) is just M5.

0.75 has the advantage of being available for M4.5, M6,M7,M8, M10 and M12 giving a relatively fine constant pitch series in the most useful sizes.

The problem will obviously be people trying to screw metric fittings into imperial holes and vice-versa.

Just a thought....

Neil

Greg H06/12/2016 09:55:59
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46 forum posts

Thanks guys,

So 26 tpi is a little stronger as expected but not really needed. A finer thread as mentioned could make alignment easier on my turret, but then I would probably tighten the turret base in the bush, then work out the alignment before soldering the top on.

I might just stick with the 26 tpi.

Neil,

I am a metric man, but just as good working in imperial. I dismissed your thought very quickly because my loco is a scale model of British locomotive, and a build of a 1958 Martin Evans design (264T) so I couldn't possibly have any metric threads on it, but my mix of BA, ME, BSW and UNF threads sits just fine with me smiley

Nigel Bennett06/12/2016 10:03:04
218 forum posts
4 photos

If you're in the habit of removing the safety valve at every steam-up in order to fill the boiler, go for 26TPI.

Greg H06/12/2016 10:18:35
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46 forum posts

Good advice. Due to one of our more experienced members telling me in my earlier days that it was not a good practice I don't do it and so I pump away on my hand pump each time for some time. A a bit of effort, but at least I know my hand pump is working.

richardandtracy06/12/2016 11:25:28
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639 forum posts
7 photos

When having fun as a stress engineer at work we had one customer who needed proof that the thread would not strip on every nut & bolt as well as proof the bolt would not snap under load. So, hundreds of times over I had to go through the same calculations with different loads. It hammered into my brain that:

  1. Both coarse and fine threads break in tension or shear before the threads strip, provided the thread engagement length is similar to that of a nut and the nut/(bolt/screw) are materials of similar strength.
  2. The thread strips more easily on a fine thread than a coarse thread, but at a load greater than the tensile strength of the bolt.
  3. A fine thread has a greater tensile capacity than a coarse thread.
  4. Bolts & nuts are actually very well designed.

That was fun. Summarised 3 years of calculations in 4 points. 4 points the customer would not accept as generally applicable cases.

Regards,

Richard.

Worksengineer25/09/2017 15:51:43
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1 forum posts

Jon you mentioned "26TPI cycle thread/Brass threads" do you know which is used as they are differnt thead forms

Regards.

Paul

duncan webster25/09/2017 23:01:53
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1180 forum posts
21 photos

I am a metric man, but just as good working in imperial. I dismissed your thought very quickly because my loco is a scale model of British locomotive, and a build of a 1958 Martin Evans design (264T) so I couldn't possibly have any metric threads on it, but my mix of BA, ME, BSW and UNF threads sits just fine with me smiley

BA is about the most metric thread you can get! Intensely logical in that the threads are in a geometric progression (I think that's the right term). The pitch in millimeters is 0.9^k where k is the BA number, and the OD is 6*pitch^1.2, again in mm. This is why they are so ridiculously difficult to screwcut. I very much doubt you'll find a BA thread on a full size Stanier 2-6-4

Brian G26/09/2017 00:08:38
135 forum posts
2 photos
Posted by Worksengineer on 25/09/2017 15:51:43:

Jon you mentioned "26TPI cycle thread/Brass threads" do you know which is used as they are differnt thead forms

Regards.

Paul

Is there a reliable way of telling which is used for an internal thread? I have bought individual BSB taps and dies where needed, but on second hand models there is always a fear that they will have used cycle thread - after all, there are plenty of sets of "BSB 26 TPI Cycle Thread" tap and die sets on eBay.

Brian

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