It's a thread definition/specification for UNC type threads much as we have Whitworth, BSF, BA etc thread definitions. If you want details then google #5-40 threads and you can get the actual diameter and pitch for each number.

Martin

Here is a comparison of varoious thread standards. It compares Metric, Unified, BA, Withworth etc. and may help you find what metric screw could replace the Unified.

Thor

Edited By Thor on 07/05/2021 08:08:58

Getting to know the engine by drawing is a good idea. It will also enable scaling and metrication which may alleviate some of the odd screw sizes. I find most of Elmer's engines to be on the small side so upscaling can be helpful if, like me, poor eyesight and unsteady hands, are an issue. The engine you are looking at has a frame that is just 2.25" long. My last attempt at one of Elmer's was his No.19 Standby Engine, the drawing shown below:-

The final outcome was quite reasonable looking engine that runs quite well.

The details and construction are on my website – Journeyman's Workshop Good luck with the drawing and subsequent build.

John

And having done that, it may be easier to convert the American nuts and bolts into Metric equivalents.

Starting out in Model Engineering, I expect many of us were surprised at the number of different thread systems out there. They date back to when nations did their own thing and before, resulting in plans calling for difficult to find nuts and bolts, when similar would do.

In the US, despite the steady advance of metric engineering, the Unified Thread system still dominates and # screw diameter nuts and bolts are easy to buy. Also available elsewhere in world, but tend to be harder to find and more expensive because, outside North America, they're unusual. Although the internet is your friend, I wouldn't recommend American nuts and bolts to anyone other than Americans.

In the UK, industry is almost exclusively metric, but for reasons good and bad, we are stuck with a mob of obsolete and obsolescent Imperial standards as well. BA, BSC, BSF, BSW, UN, and others. Though it's possible to work in any system, it pays to major in either Imperial or Metric. Imperial is good for building models from old plans, mending imperial equipment, and if you went to school before 1970 or inherit a workshop full of wonderful Imperial tools! Otherwise go metric.

The rest of the world is metric and so is the future.

#5 is 1/8" nominal diameter (3.175mm), so either M3 or M3.5 would do. In the American system the # is often left out, so 8-18 is the same as #8-18. The first number is a gauge number, the second is the number of threads per inch. A good reference book is useful; Model Engineer's Handbook, Zeus, Machinery's Handbook etc.

Dave

Should be possible to plot it

Draw the 3/16" pin 1.25" from the pivot and then a circle 1/2" dia around that. the 1/2" is the pin plus twice the distance of edge of jig to it's ctr line. Then draw a line through teh pivot that is tangental to the 1/2" circle. mark a point 3/8" along the other end of teh line and that's your hole location.

Plot (click for larger image)

And the two dimensions taken off the drawing.

If you do want to make the engine a bit bigger and to metric standards then use the scale of 1/32" on original = 1mm

Edited By JasonB on 07/05/2021 13:26:22