English or Metric threads old American clock movements

ALR guy

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I've been given an assignment by my repair mentor to refurbish an old striking clock mechanism. It's obviously been worked on previously and the screws attaching the plates to the posts are all M3 5.5 threads. For a variety of reasons I'm highly suspicious that these screws are not what were originally used. Based upon this very brief note, can anyone tell me if, perhaps, the screws should be UNF or UNC versus metric?

The attached photo shows some identification markings on the lower left corner of the front plate. At the bottom of the photo is one of the suspicious M3 cheese head screws.

IMG_1185.jpg
 

shutterbug

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I've seen that arrangement several times. It think they are original.
 
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Jim DuBois

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I suspect they are SAE threads. Metric or the various other thread patterns used in Europe would not have been used in American clocks. Metric was not implemented in the UK for example until 1965. There would have been no practical reason to import screws, taps, dies, and other associated tooling.

Measure the diameter, verify the thread pattern using a proper thread gauge and check the SAE tables for a match. You will find an SAE version very close to M3 5.5 and that will be the proper answer.
 

wow

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I've been given an assignment by my repair mentor to refurbish an old striking clock mechanism. It's obviously been worked on previously and the screws attaching the plates to the posts are all M3 5.5 threads. For a variety of reasons I'm highly suspicious that these screws are not what were originally used. Based upon this very brief note, can anyone tell me if, perhaps, the screws should be UNF or UNC versus metric?

The attached photo shows some identification markings on the lower left corner of the front plate. At the bottom of the photo is one of the suspicious M3 cheese head screws.

View attachment 658099
What is a cheese head screw?
 

ALR guy

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Thanks folks. For sure the screws that I removed are M3 5.5 and I measured them as Jim instructed. Unfortunately the female threads in the posts are buggered up so I can't make any useful measurements.
 

Willie X

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Often called gun screws, not to be confused with 'panhead' screws.

Originally the screws were #5-40. The threads are probably messed up now because someone used metric thread screws. :rolleyes:



Willie X
 
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CameraGuy32

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A M3 has a theoretical outside diameter of .118 inches. Actual diameter will be close to .115 inches.
The 5.5mm pitch is very uncommon. Most likely it .5mm (.0196 inches pitch)

A #5-40 has a theoretical outside diameter of .125 inches. Actual diameter will be close to .123 inches.
The 40 pitch is .025 inch.

Given that, it is common for someone to attempt to jam a #5-40 into a M3 tapped hole. It may be a bit difficult in steel, but with a bit of force, it will go into a brass plate.
 

Willie X

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Forget the metric measurements. No clock, made in the US, ever had metric thread fasteners.

This clock takes a #5-40 screw. Length is about 5/32". It screws into a steel pillar post. Not a complex issue ...

Willie X
 
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