German Thread Pitch

[tmuir]

I've just recently got myself a very large German Bing steam engine to restore.
Haven't nailed the date down on it exactly yet but its was made some time between 1908 and 1925.

Now a few of the screws / bolts are missing from it and will need to be replaced.
I would prefer not to have to drill out the threads in the castings and retap them with modern metric threads if I can help it.

I've been told that on something like this the threads will be metric but at a different pitch to ISO metric threads.
Does anyone here know if this is true and if so does anyone know where I can buy some screws or at least dies to make new ones?

Thanks

 

[GailInNM]

Determining the size of the thread needs to be the first thing to do. If there are other bolts on the engine that are the same size as the missing ones, then they can be measured easily enough to determine what you need. Next compare the measurements to a thread chart that may have that thread. The chart I use is at:

http://www.watchman.dsl.pipex.com/thread.html

It is one of the most complete showing different thread systems that I know of.

There were a lot of thread systems used in the late 19th and early 20th century so it may be difficult to pin down what you need. It may be that you will need to make, or have made, the bolts. That is generally not too difficult if you have a sample and a screw cutting lathe. Hex heads can be milled to match the style of the existing bolts.

If you don't have a sample bolt, You may be able to get an impression of the thread with silicon mold making rubber and measure that. Spray or otherwise coat the threaded hole with mold release or oil, then put a long smaller common bolt in with the head down in the hole and then pour or press the mixed mold making rubber in place and let cure. Most times you can unscrew the molded section using the bolt sticking out. If there is a lot of corrosion in the hole, the rubber may tear and have to be picked out. You still may be able to measure the pieces well enough to determine what you need.

Gail in NM,USA

 

[tmuir]

Thanks for the link I've book marked it.
I've got several thread charts but none that go down that small.
I hope they turn out to be something easy to obtain as I've yet to attempt to cut a thread on my lathe and I wasn't planing on learning how to do that yet for a while.

 

[Loose nut]

If you can't find it on that chart you are screwed,OK bad pun, there is a book bring sold, at one time by HSM I believe, that was supposed to have every screw thread made in it. You may be able to find it there if can't get the info elsewhere.

 

[Bogstandard]

If you go to here and have a look at the chart, it shows most threads used in Europe.
You can then use that to see if there is anything near enough to cut it out to.

http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item28

It should print out to fit nicely on a sheet of standard paper.

John

 

[mklotz]

I'm fairly sure that the data in the URL that Gail supplied is from a chart put together by Andy Pugh in the UK.

I took that data and wrote a small program (FINDTHRD) around it which allows you to search by either pitch or major diameter. For example, if you search for 32 tpi, it will provide you with a listing of all the thread types that are within a (user-specified) percentage of 32 tpi.

I find that this makes it a lot easier to use. The program is free and can be downloaded from my website.

 

[Hilmar]

Use Gail's method or use a piece of wood( from tooth pick to broom stick will work) screw it in for a few threads and unscrew and check it. It gets you so close that you will find the match.
Hilmar

 

[tmuir]

I find that this makes it a lot easier to use. The program is free and can be downloaded from my website.

Thanks for that I have downloaded it for when I start the restoration.
I'm pretty sure I have atleast one example of each of the missing screws so if nothing else I should be able to identify them.

 

[Jasonb]

The problem with the two charts posted is that they only cover metric fine & coarse threads but it is possible to buy taps & dies for upto 5 different pitches on some of the smaller diameter metric threads, even more on the larger sizes.

Its really a case of measuring the dia and pitch to see whats liekly to fit.

Jason

 

[tmuir]

The problem with the two charts posted is that they only cover metric fine & coarse threads but it is possible to buy taps & dies for upto 5 different pitches on some of the smaller diameter metric threads, even more on the larger sizes.

Its really a case of measuring the dia and pitch to see whats liekly to fit.

Jason

This is starting to sound like it could be harder than I thought.
Diameter is easy to measure and I've got gauges to measure the teeth which will probably be the wrong pitch but will still be ok to count the thread but I have no idea how to measure the pitch on a screw that is only 2 or 3mm in diameter.

 

[mklotz]

Having had your problem (measure pitch of tiny screw) a number of times, I finally broke down and bought a pocket optical comparator.

http://www.use-enco.com/CGI/INSRIT?PMAKA=505-1937&PMPXNO=951104&PARTPG=INLMK3

It's a quality 10X optical loupe to which one can fit various measuring reticles. The part is placed against the reticle and viewed through the optics in order to make the measurement.

It's a bit pricey for a one-off job like yours but it does work well. Perhaps you can find a friend who has one.