3.5mm hex. head screws

The recognised metric replacement for a 4BA screw is a M3.5 x 0.6 screw. I am trying to find if such a mild or stainless steel hex. head screw is available. I have looked at Polly Model Engineering web site and some of the European model engineering suppliers' web sites without success.

Has anyone knowledge of the availabilty of such screws and nuts?

JA

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Chris Gunn

I've not come across M3.5 hex head from any of the usual suspects here or abroard. If you don't need many then not to hard to turn down some round bar, thread and then machine the hex.

I think JA wanted M3.5 Chris?

Quite a few suppliers of M3.5 screws come up on Google but hex head don't.

I think you will find them at PTS, Precision Technology Supplies. I got some M2.5 from them, usually use them for all my stainless screws.

JH, you are right, I searched for 3.5 hex head, and mistakenly believed Google took that on board, should have looked closer at the results. I thought it was too easy.

Chris Gunn

Not quite, because it is hex with a slot, but perhaps useable, Niko. **LINK**

https://www.ebay.de/itm/265158626286?hash=item3dbcaf8bee:g:YwIAAOSwHd5gnPZY

Why not use 4BA?

Fix a nut to a piece of stud and clean it up. If you use a machine screw for the stud you can run the nut up to the point where the thread finishes to have as much metal as possible filling the nut.

Martin C

Many thanks for the replies.

The ISO Metric 3.5mm Coarse thread is a standard thread and features in thread tables including the one in Tubal Cain's Model Engineer's Handbook. I know that it is used on some electrical fittings.

I asked the question because I am converting the design of a large well known model, with quite a few 4BA screws, to metric. Before going too far I thought it would be wise to check if the metric equivalent was available.

2mm, 2.5mm, 3mm and 4mm hex headed scews are readily available but I am going to assume that I will never see a 3.5mm hex headed screw.

JA

There are these:

Socket Head Cap Screws DIN 912 (ISO 4762) M3.5 in A4 Stainless Steel - Westfield Fasteners Ltd

or these:

M3.5 / 3.5mm Metric A2 Grade Stainless Steel Socket Cap Allen Bolt Screws | eBay

I’m not sure what size Hexagonal bar (metric/imperial) is available but it shouldn’t prove too time consuming to make your own?

The awful truth appears to be here : **LINK**

https://itafasteners.com/products-bolts-hex-head-anchor-bolts.php

M3.5 is a standard … but not for bolts

I agree with Martin; but might be tempted to use hex-socket grub screws as the thread … thus providing optional tightening methods.

MichaelG.

JA wants hex head not cap head. Though I have used m3.5 cap heads on models I reserve them for the modern aero and ic engines rather than anything period.

When I have made hex head bolts in M3.5 to keep them in keeping with the other model hex heads which are smaller than the usual ISO sizes I mill the hex to 5mm A/F which fits in about right with M3 with it's 4mm hex and M4 with it's 5.5mm hex. For the few I make it is not worth sourceing 5mm hex stock and the spin indexer makes machining quite quick.

No sure what engine martin is making but when doing others in metric I often find the suggested BA size a bit large for example the 2BA ones on the Stuart Voctoria, beam, etc are on the big side so use M4 rather than M5 which would be closer.

Edited By JasonB on 22/11/2021 13:30:52

M3 hex heads are 5.5mm spanner size, M4 are 7mm
It seems to me that 6mm hex bar would seem an appropriate midway approximation.
4BA AF hex size is 0.249", so not much larger than 6mm.

Both 1/4" and 6mm hex bar are readily available.

Bill

The model ones tend not to use standard hex, see my comment above and this table Similar to many people using one size smaller BA fixings.

They are also taller so more in keeping with a period engine.

.

Do you mean JA ?

MichaelG.

.

Incidentally … My reference was to Martin Connelly’s suggestion.

Yes, edited it now.

First part of my reply was to Juddy.

Though grub screws may be a bit limited on length depending on what is needed or if studs are being used for the majority of fixings.

Edited By JasonB on 22/11/2021 13:33:33

Yes, sounds good Bill.