How Can I Determine Threads

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rhbroehl
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How Can I Determine Threads

Post by rhbroehl » Thu Jan 24, 2019 8:13 pm

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

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Bill Shields
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Re: How Can I Determine Threads

Post by Bill Shields » Thu Jan 24, 2019 9:44 pm

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).
Too many things going on to bother listing them.

rhbroehl
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Re: How Can I Determine Threads

Post by rhbroehl » Thu Jan 24, 2019 10:07 pm

Makes sense. Thanks for the suggestion

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Greg_Lewis
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Re: How Can I Determine Threads

Post by Greg_Lewis » Thu Jan 24, 2019 10:37 pm

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.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

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Dick_Morris
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Re: How Can I Determine Threads

Post by Dick_Morris » Fri Jan 25, 2019 12:14 am

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/

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Bill Shields
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Re: How Can I Determine Threads

Post by Bill Shields » Fri Jan 25, 2019 5:30 pm

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.....
Too many things going on to bother listing them.

James Powell
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Re: How Can I Determine Threads

Post by James Powell » Fri Jan 25, 2019 6:02 pm

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/

RET
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Re: How Can I Determine Threads

Post by RET » Fri Jan 25, 2019 6:55 pm

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.

rhbroehl
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Re: How Can I Determine Threads

Post by rhbroehl » Fri Jan 25, 2019 9:05 pm

Thanks all for the suggestions

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