Cycling UK Forum

Hi,
I need a replacement seat bolt but it's from an older bike circa 1978 I think. I am hoping that someone knows the name of what I need as I am.unsure what to buy?

binder bolts on older British frames are odd (by today's standards) thread sizes, such as 1/4" whitworth or 9/32", or even 5/16". Careful measurement and trying a few different bolts is what is required. No-one can tell you on the basis of a few photos, not with certainty, anyway.

cheers

Thanks Brucey I will start collecting.

Hi
It's a "socket head cap Allen screw", sometimes shortened to "socket cap screw". As said, the thread is anyone's guess but a "proper" engineering supplier should be able to help you there
Regards
tim-b

tim-b wrote:Hi
It's a "socket head cap Allen screw", sometimes shortened to "socket cap screw". As said, the thread is anyone's guess but a "proper" engineering supplier should be able to help you there
Regards
tim-b

I was told once by the owner of a bolt supplier that it was called a socket head cap screw and that Allen was the first manufacturer of the hex key used to drive it.

Hi

Mike_Ayling wrote:

tim-b wrote:Hi
It's a "socket head cap Allen screw", sometimes shortened to "socket cap screw". As said, the thread is anyone's guess but a "proper" engineering supplier should be able to help you there
Regards
tim-b

I was told once by the owner of a bolt supplier that it was called a socket head cap screw and that Allen was the first manufacturer of the hex key used to drive it.

Allen held the first patent for forming hexagonal socket heads in 1910 (and presumably made hex keys as well). Other more modern sockets such as Torx, etc, aren't generally referred to as "socket head cap screws" and I included "Allen" for clarity, but you're absolutely correct
The big confusion arises over when a bolt becomes a screw (and vice versa)
Regards
tim-b

Maybe if you have some bolts of the right diameter you could try, by hand only, to see if the thread is a normal modern one. If it were, SJS do that sort of thing in stainless. Obviously, don't force a bolt in if it doesn't want to go because you would just ruin the threads.

Thanks everyone.
It's on a Mercian Superlight I think it's a 1978 model.
I might ask Mercian what they used back then.
I couldn't believe that these were out in 1910.
I will go for the stainless version possibly from Clerkenwell Screws when I'm net in London.

garygkn wrote:Thanks everyone.
It's on a Mercian Superlight I think it's a 1978 model.
I might ask Mercian what they used back then.
I couldn't believe that these were out in 1910.
I will go for the stainless version possibly from Clerkenwell Screws when I'm net in London.

Mercian might have what you need.

Hi

I couldn't believe that these were out in 1910

The idea was floating around in Victorian times but it took a few more years to come up with a manufacturing technique
Patent image (link)
Regards
tim-b

Just been into the garage, the one that fits my Rogers is 5/16 by 9/16 BSF if anyone else is looking (as a starting point, no guarantee they are all the same thread! .)

My 1970's Roy Thame (same builder/workshop as Holdsworth professionals etc) uses this bolt



Which is;

- threaded 5/16" x 22tpi (5/16" BSF)
- is full-threaded
- has a threaded length of 3/4" (19mm)
- has a head diameter of about 7/16" (~ 11mm)
- has a head depth of 5/16" (~8mm)
- uses a hex key size 7/32" (~5.5mm)

It is in all respects a bog-standard 5/16" BSF caphead screw, in high tensile grade (equivalent to modern 12.9 grade).

The core size of the thread is about 6.8mm, so measuring the ID of a hole tapped 5/16" BSF is likely to give a measurement around 7.0mm. However this won't help distinguish it unequivocally from a number of other screw threads.

Standard BSF caphead screws have a smaller head diameter in relation to the shank than metric capheads; this makes them an inherently better choice for this application, because the bolt is loaded in bending as well as tension; it doesn't need a big head as a bolt loaded in pure tension might.

5/15" whitworth is also used in some old seat binders and the external head size of a caphead screw is the same for a 5/16" whitworth caphead as for 5/16" BSF, so no clues there. In theory the core size of the whitworth thread ought to be smaller but in practice it might not be.

To identify the thread pitch a set of thread pitch gauges can be used. Alternatively IME a standard M8x1.25 threaded bolt will enter a 5/16" BSF or a 5/16" Whitworth hole and go in a turn or two before it starts to bind; this doesn't much help you distinguish between the Whitworth (18tpi) thread and the BSF (22tpi) thread; 1.25mm pitch is very close to 20tpi, so they are both about equally wrong.

It is tempting to tap the fitting to M8x1.25 but the resultant female thread won't be very good and a M8 bolt head will have to be machined to fit the seat lug; it will be too fat otherwise. I does avoid having to carry an odd allen key for that bolt only though, since a 6mm key will then fit.

cheers

[quote="pwa"... Mercian might have what you need.[/quote]

That's got to be your starter for ten. Even if they no longer stock them, they must surely know the size.

A while ago I had a problem with a bolt on my lawnmower with a stripped thread. It had a square head to fit a plastic quick-release lever. I went here:-

https://boltandnutsupplies.co.uk/about-us/

Our shop located in Pudsey, includes a trade counter where helpful staff are available to offer advice.

In fact, I didn't know I had a problem in that the thread was apparently unusual. That should say "We regard identifying the type of thread as a challenge and will then source what you need." I wasn't looking for a hundred boxes of them, just one bolt but the "helpful staff" certainly helped me. There must be others like this elsewhere.

Hi,
A tip here would be to try entering any bolts you have from the left hand side directly into the thread.
Commonly the right hand side hole which is a close fit on the head of the screw is not lined up well with the left hand side, so it can appear to bind.
if you have a digital caliper or some measuring instrument like micrometre et cetera and you have a range of metal drills.
You could try gauging the hole with some drills, ideally you need drills with 0.1 mm steps.
Try the drill if you can get the right size from both sides so that it will pass through the hole.
It's possible that on the left-hand side the thread is rusted so the drill blank (use the shank of drill bit) Might not pass from the left hand side will pass mainly from the right.
If at any time you intend to clean the thread up or re-tap to a different size don't forget to use a tapping fluid.

Years ago we used to call it hexagon socket cap Head screw.
There has been discussions on bolts vs screws on these post before, I would go looking at the engineers and machinery handbook first.
I can't remember offhand, but nowadays terminology is loosely flung around so most peoples understanding might be somewhat different.

Hi,
Taken from "Machinerys Handbook"

"Differentiation between Bolt and Screw.—A bolt is an externally threaded fastener
designed for insertion through holes in assembled parts, and is normally intended to be
tightened or released by torquing a nut.
*********************************************
A screw is an externally threaded fastener capable of being inserted into holes in assembled
parts, of mating with a preformed internal thread or forming its own thread and of
being tightened or released by torquing the head.
An externally threaded fastener which is prevented from being turned during assembly,
and which can be tightened or released only by torquing a nut is a bolt. (Example: round
head bolts, track bolts, plow bolts.)
An externally threaded fastener that has a thread form which prohibits assembly with a
nut having a straight thread of multiple pitch length is a screw. (Example: wood screws,
tapping screws.)"

Edited-
https://en.wikipedia.org/wiki/Bolt_%28fastener%29