Britbike forum Forums Thread Like Summary - Domed Head Bolts

Why are many British bolts domed headed? Perhaps airflow, or a smooth look paying homage to the bald headed executives.
All joking aside why the dome? There must be a logical solution.

I made them for my 66 A65 by mounting stainless bolt in drill press and ran it against a file. Look the part.

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I think it unlikely that these bolts were " made in the machine shops of BSA itself".
I lived and worked in the Midlands for a few years shortly after the demise of BSA and even then I was surprised and a little charmed by the existence of the traditional industries seemingly unique to each village. Wednesbury for locks, Dudley for chains, Bloxwich for metal stampings, engine blocks were cast in Wolverhampton, and nuts and bolts were made in Darleston. Each small industry provided the pieces for the large manufacturers. I worked in several, even the upholstery for Leyland Cars was made in small factories.

The demise of the large auto producers, both car and motor cycles, was devastating to not just the actual communities around the assembly plants.

I think it much more likely that the domed bolts, in all their array of threads, were bought in. In the glory days of the '60s, with little competition, the extra cost was nor particularly relevant. They were abandoned about the same time that using the "right" thread, was changed to using the "cheapest" thread.

Nice job, Beach. Looks original to me.

I have copied the circular indentation to signify "unified" treads in a similar way, by putting USA-made bolts of the proper threads in a lathe, then placing an end mill in the tail stock and cutting the circular marking that way.

Birmingham was the workshop of the world, even in the 80's when I was working in the Auto industry there were hundreds of these multi spindle machines around. My first job was to convert a shop of 20 of these machines from HSS to Tungsten carbide tooling.



I believe the domed head was for both clean lines and less likelihood of a bolt head catching on a branch etc, a lot of off road riding at weekends in the UK. Just a guess.

I have always been under the assumption that the domed bolts were for "looks"

You will find them on fenders, and various other locations in which "details" matter

For Triumph / BSA, they seem to have ditched that style around 1969-ish when hardware was switching to American

Just my thoughts

perhaps the question is the wrong way round ...... why not domed heads on oriental and American bikes?

They used domed because that is what should be used if you have a bit of pride in the way you design bikes, and how they look. A pointless expense you say? well that depends on your perspective.
They stopped using them when they had to compete with the souless bikes built to appease the gods of the balance sheet, where every cent has to be shaved, every dollar grabbed.

When rebuilding my bikes I also domed the heads of the stainless bolts using a rednecks lathe, i.e. a cordless drill and an angle grinder. Quick, simple though perhaps a tad hazardous.

I think there were a few reasons, if you are making your own in house fasteners, why not reduce the head size to save space and weight and materials.
The better quality marques like Ariel made a point of having particularly well made fasteners.
If you are starting with hex bar a reduced head Across the flats size saves a lot of machining, also allows for neater construction, many of the clearances around fastener heads are tight on a motorcycle, designers made stuff as tight as possible to look pleasing ,and save weight/ materials.
Once you have done all this you might as well add a dome/ chamfer, thats what separates machinists from the animals.

I think Irish Swede had it right .

The domed head is an artifact of the Machining process .
its easier to "part" a piece without parallel sides .
a straight Plunge parting tool is forming a Groove on 3 sides at once .
( you have to have your tooling more precisely placed to pull this off )

if you "part" as a decreasing wedge you can face the dome as the "parting " cut is made deeper and more narrow

the cutting can be done with a pointed tip and not a forming tip .
The Parting cut only becomes a parallel cut right before the piece is ready to fall off anyway

You can always take standard hardware and make them more "original" looking like others have stated

On my '68, I took standard 1/4" American fine thread bolts, then turn the head down to remove any markings then added a chamfer on the ends

Mounted in a 5C collect chuck

Then cadmium plated

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''Once you have done all this you might as well add a dome/ chamfer, thats what separates machinists from the animals.''


All my bolts should have 2 inch square heads then..................................

The old head bolts from my A65 say Rubery Owen on the head.

Last month I decided to change up “Trusty Rusty” my TR6 into a moderate “Thruxton” style model

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Don't forget the Bradleys shock mounting bolts.

Here we call those acorn nuts