Could M1.7 also be known as 10BA??

all you need to know about metric threads

**LINK**

Why M1.7?

The most useful series of fixings (or anything needed in a wide range of sizes) go up in steps of similar size, so that whatever your 'ideal' size is there is always a standard (or 'preferred&#39 size nearby.

The best example is resistors, which go up in odd values, or at least they seem odd until you realise that the values are spaced so that they go up in steps more or less regular steps of 1.5 times the previous value (at 20% tolerance). For higher tolerances they go up in smaller steps, always chosen so that there aren't any gaps between the tolerance bands.. This minimises the number of values they need to make, means there are no 'rejects', and simplifies the design process.

The steps can be any size. For BA fixings the step is such that as you go down the series each size is 0.9 times the previous one – quite close together which is why just the even sizes are more commonly used.

Other examples are things like A-series paper where A4 is twice the area of A5.

For ISO metric things there are the R5, R10, R20, R40 and R80 series which break down each 'decade' (from 1-10 or 100-1000 for example) into steps of 10, 20, 40 or 80 roughly even-proportioned steps.

Engineering ignores these steps so most metric screws you come across are rounded to the figures: M1, M1.5, M2, M3, M4, M6, M8, M10.

The final answer to your question is that M1.7 is the most useful size between 1.5 and M2.

Neil

Since you said it was for an Aster loco, Japanese made but I think designed in Switzerland, it will depend on the designer and whether he is a hands-on practical man or an academic CAD jockey. Wanting to meet scale (of a sort) the designer will have found that M1.7 is the required size and exists in the tables. Being Swiss every possible small screw will be available locally so he just specifies it no problem. A hands on man would have known what was more commonly available and adjusted. The Far East like to follow instructions so will not suggest an alternative and just build to print at slightly extra cost.

How did the friend determine it was 1.7mm. Either he measured the diameter and quoted that not actually differentiating it as metric or BA, or he has a parts list, (quite likely as many are sold as kits) that specifically declares it as M1.7 therefore not BA.

Edited By Bazyle on 12/11/2015 13:43:52

If you look at the ISO extended metric threads here you will see there is nothing odd about M2.6 at all

**LINK**

On the other hand they have been rationalised and I believe this is one is more up to date unless JIS has dropped some of theirs.

**LINK**

It pays to check dia and pitch on Japanese microscope. German too.

John

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John W 1:

"There is a site that lists metric threads that might be of interest."

Thanks for reminding me of this useful site – it feels odd that one goes to the US for such a comprehensive list.