Thread types in 7 1/4 guage

[ukjimoo]

Hi all

I am well under way producing drawings for a 7 1/4" Jinty and i am finding that a lot of the screw sizes so far, come out at around the 0BA mark.
Now, i have boxes full of M6 screws which are about the same size as 0BA and have similar head sizes to the 0BA when purchased with the smaller hex.
So do i spend a fortune on Mild steel 0BA screws or do i use the free high tensile good quality screws that i have?

My question is really - why do we traditionally use BA threads in Model Engineering? Is it because of the good range of small sizes as opposed to Metric?
Also when (as in 7 1/4" and larger) we get near and beyond 0BA, is there a standard practice for which sizes/types of thread to use?

Best wishes
Jim

[alanstepney]

If they are arond 6mm, and you want to use metric, then M6 sounds ideal to me.
The one caveat is to ensure you use reasonable quality, both fasteners and taps / dies, as there is a lot of rubbish around. Cheap, but not suitable for ME applications.
You have covered that, and added the key word (free), but worth bearing in mind should you need to buy any. (Or for anyone else reading this thread.)

There is no accepted standard range, but if you are using metric, it would make sense to stick to it throughout, where that is possible.

BA was the standard for a century, and is still an excellent thread series. For anyone who has BA threading equipment, and is familiar with BA, it makes sense to continue with it.
As you say, stocks of good quality bolts & nuts are still available.

But as you have metric already, may as well use that.

When you get to pipe fittings, etc, they you will probably be stuck with ME threads, as AFAIK, there isnt any reasonable metric alternative.

[Tel]

Free screws? Use 'em - better than Yak milk and Black Puddin'

[tcase]

In the UK, BA are the only small fasteners readily available with hex heads, allthough this is changing now.