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How Can I Determine Threads

Posted: Thu Jan 24, 2019 8:13 pm
by rhbroehl
Hi,

I recently purchased an LE 0-4-0 that was never completed. One issue that I am having is how can I determine the threads on the boiler fittings? Is there a way to determine this?

Thanks

Rob

Re: How Can I Determine Threads

Posted: Thu Jan 24, 2019 9:44 pm
by Bill Shields
a thread pitch gauge and a micrometer are a good place to start.

next - eyeball if the threads appear to be tapered...as there are variants of many model threads that are identical except for taper (or lack thereof).

Re: How Can I Determine Threads

Posted: Thu Jan 24, 2019 10:07 pm
by rhbroehl
Makes sense. Thanks for the suggestion

Re: How Can I Determine Threads

Posted: Thu Jan 24, 2019 10:37 pm
by Greg_Lewis
Here's how I once determined a small internal thread. It looked to be a #4 or #5. I tried inserting screws with all the standard #4 and #5 threads including the closest metric without success. I then took a piece of plastic like that from a weed whacker and twisted it down into the hole. Pulling it out the plastic carried an impression of the thread. Checking with a thread gauge revealed a 50 TPI thread. I think the manufacturer did this on purpose to try to force folks into buying their parts.

Re: How Can I Determine Threads

Posted: Fri Jan 25, 2019 12:14 am
by Dick_Morris
If they are tapered, they are probably model taper pipe. 5/16-40 is a relatively new variation on MTP threads.

http://www.americanmodeleng.com/id13.html

If they are straight, it is likely that they are UK fittings using 40 or 32 tpi model engineer threads. Watch out for the thread angle, I believe they use 55 degrees in the UK compared to 60 degrees for taps typically available in the U.S.

https://www.pmmodelengines.com/model-pipe-threads/

Re: How Can I Determine Threads

Posted: Fri Jan 25, 2019 5:30 pm
by Bill Shields
Whitworth threads are 55 degrees...not very likely to be used in pipe fittings....

sewing machine manufacturers did the same thing - odd pitches and diameters. I have a box full of 'singer' taps....

Then there is the BA threads that have VERY STRANGE pitches:

#3 = 34.8
#4 = 38.5
and so on.....

Re: How Can I Determine Threads

Posted: Fri Jan 25, 2019 6:02 pm
by James Powell
BA pitches are totally rational. You are expecting that they are inch. They are metric...

Yes, an imperial diameter with metric pitches.

No, I don't understand why.

https://britishfasteners.com/threads/

Re: How Can I Determine Threads

Posted: Fri Jan 25, 2019 6:55 pm
by RET
Hi,

Sometimes it helps if you know who built the model or even where it was built. If it was built in England, there is a very good chance it would have BA (British Association) threads which are quite different from what we are used to in North America (BA #0 is the biggest, a little less than 1/4" dia. and BA #12 is the smallest). The Dart I have has some BA threads, but the bits I have added have either National Coarse or (more likely) National fine threads, mainly because I can use standard cap screws which are much stronger. I also use ME threads on occasion, the ME 40 pitch series are mainly used to thread tubing.

Models built in Canada could be a mixture of the two, depending on the preference of the builder; those made in the US would likely be either NC or NF threads or some combination of the two.

For piping, I mostly use NF threads because with them I can make smaller fittings that look more as though they belong.

Perhaps this may help a bit.

Richard Trounce.

Re: How Can I Determine Threads

Posted: Fri Jan 25, 2019 9:05 pm
by rhbroehl
Thanks all for the suggestions