What is this for a screw?

Hello dear model engineer friends,

I recently bought an live steam engine and I'm a total beginner, so its would be very kind of you if you can help me on that one:

I want to replace some worn screws, but I really have a problem measuring them.

If I measure the outher diameter of the screw thread and compare it with BA (which I guess is the screws thread) it does fit into the BA11 category, however when I look at pictures on ebay of BA12 screws which is a number smaller, the threads are much bigger. So there is something I do wrong. I attached a picture of the screw.

I appreciate your help very much.

It shows 1.57 mm on the display, or 0.062 inches.

Where was it on the engine, and what sort of beast? 11ba is not common these days (I think LBSC used them). A (set) screw wouldn't normally have a taper tip, which that appears to have, so it might be a self tapping screw of the kind used to fix to plastic- often get them to hold a battery box together- rather than a machine screw. If you think it is worn, it may be the wrong screw anyway- just because it is in the hole, doesn't mean it belongs there!

How old is the engine? could be 1/16" whitworth or a undersize 10BA as 11BA is not commonly used

Kevin,

What you need to do is look up some thread information. Forget e-bay. Suggest you google "thread pitch charts" and identify the distance between thread crests. Like Matthew, my first thought was that it was a modern self tapping screw.

Use a ruler and magnifying glass (if necessary) to determine if your screw is metric or imperial to start with. We need a better screw picture and you need to say if it is the head or the thread that you are considering worn. Does the thread look worn?

The screw need not be a BA thread.

You need to measure the female screw thread diameter - easy enough if it is not damaged - and the pitch of the thread. Refer to engineering tables for the likely real size of the screw (yours is likely undersized because it is worn?).

Otherwise, as Matthew above.

I have found taking a picture, with a digital camera, of the screw against a ruler useful. You can zoom in to the picture to count small thread pitches. The picture does not need to be great but it is best to have the camera as far away as possible and use zoom to reduce parallax errors. If you are likely to have to repeat this exercise a lot then some blade type thread gauges may be a useful Christmas present, metric and either Unified or Whitworth will cover most needs.

Martin

I think the answer is in the question ie 'worn'

They would almost certainly be 10BA at 62 though. Also it is not at all uncommon for BA screws to be rather under the specified OD.

Cheers,

Julian

Edited By julian atkins on 23/09/2016 10:32:51