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Old 4th Sep 2020, 10:45 am   #1
Nanozeugma
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Default Determining bolt parameters...

Morning

A small conundrum which may have a simple answer, I just don't know what it is

I have a venerable Farnell LFM2 oscillator. When acquired the perspex cursor and fixing bolts were missing.

I've obtained some 1mm clear acrylic sheet to remake the cursor, but the missing securing bolts are being a bit of an irritant.

3mm is too big, 2.5mm works but is a sloppy fit and obviously isn't "right."

Given that the LFM2 appears to date from around 1968, I surmise that the issue is that the bolts were of imperial specification, not metric.

How to determine the correct size? Any insight much appreciated.

Image attached showing one empty threaded standoff, the other occupied by a 2.5mm bolt.
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Old 4th Sep 2020, 11:21 am   #2
Craig Sawyers
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Default Re: Determining bolt parameters...

Fair guess is that it is 6BA

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Old 4th Sep 2020, 11:22 am   #3
M0FYA Andy
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Default Re: Determining bolt parameters...

I would be surprised if the threads were anything but BA sizes at this time.

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Old 4th Sep 2020, 11:28 am   #4
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Default Re: Determining bolt parameters...

More than likely a BA thread - which actually ARE a metric specification. 0BA is specced as having a 6mm dia and 1mm thread pitch.
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Old 4th Sep 2020, 11:35 am   #5
barrymagrec
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Default Re: Determining bolt parameters...

Just like M6 - except for the pitch angle - they are usually interchangeable in practice though.

M3 and 6BA are also the same pitch, which means you can sometimes rescue a stripped 6BA thread in thin material by tapping out to M3.
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Old 4th Sep 2020, 11:52 am   #6
Alistair D
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Default Re: Determining bolt parameters...

Just measured the inside diameter of a 6BA nut at 2.45 mm. Hope that helps

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Old 4th Sep 2020, 12:01 pm   #7
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Default Re: Determining bolt parameters...

6BA is midway between M2.5 and M3, size-wise. Very likely possibility!
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Old 4th Sep 2020, 2:29 pm   #8
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Default Re: Determining bolt parameters...

Thanks people, 6BA appears to be the answer
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Old 4th Sep 2020, 2:31 pm   #9
Radio Wrangler
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Default Re: Determining bolt parameters...

BA sizes still available from model engineering suppliers.

Stands for British Association. Forwards looking folk, specified them in metric units.

Must of us on here have tubs of salvaged BA stuff so if we know what ength and head shape you want someone might be able to find a couple.

David
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Old 4th Sep 2020, 5:32 pm   #10
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Default Re: Determining bolt parameters...

Quote:
Originally Posted by Radio Wrangler View Post
BA sizes still available from model engineering suppliers.

Stands for British Association. Forwards looking folk, specified them in metric units.

Must of us on here have tubs of salvaged BA stuff so if we know what ength and head shape you want someone might be able to find a couple.

David
Thanks for the thought David.
I reckon 1/4 inch slotted pan or dome head, stainless if I can find any.
If I can't find any, I might take you up on that
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Old 4th Sep 2020, 7:36 pm   #11
kalee20
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Default Re: Determining bolt parameters...

Quote:
Originally Posted by frankmcvey View Post
More than likely a BA thread - which actually ARE a metric specification. 0BA is specced as having a 6mm dia and 1mm thread pitch.
The thread is specified in metric, yes - but lengths of BA screws and bolts are usually in inches!
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Old 4th Sep 2020, 8:21 pm   #12
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Default Re: Determining bolt parameters...

Just had a rummage in the garage and I have more 1/2" × 6 BA round head nickel plated screws than I will ever need, some slotted brass, mostly cross head. PM details and I can stick some in the post FOC.

M3 and 6BA are almost the same pitch. I supplied a forum member with some M3 press nut inserts having long threaded bodies a few years ago as substitutes for unobtainium knurled 6BA nuts, and a 6BA screw jammed just before emerging. It needed running a 6BA tap through it to work. An ordinary M3 nut would loosely fit a 6BA screw.

Last edited by emeritus; 4th Sep 2020 at 8:40 pm. Reason: Typo, clarification
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Old 4th Sep 2020, 9:16 pm   #13
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Default Re: Determining bolt parameters...

Quote:
Originally Posted by emeritus View Post
Just had a rummage in the garage and I have more 1/2" × 6 BA round head nickel plated screws than I will ever need, some slotted brass, mostly cross head. PM details and I can stick some in the post FOC.

M3 and 6BA are almost the same pitch. I supplied a forum member with some M3 press nut inserts having long threaded bodies a few years ago as substitutes for unobtainium knurled 6BA nuts, and a 6BA screw jammed just before emerging. It needed running a 6BA tap through it to work. An ordinary M3 nut would loosely fit a 6BA screw.
Most kind, I shall P.M. you.
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Old 10th Sep 2020, 11:26 am   #14
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Default Re: Determining bolt parameters...

Thanks to the kind auspices of "Emeritus," I now have a quantity of 6BA bolts, nuts and washers. The bolts fit perfectly.
Many thanks.
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