What's the best way to find out correct seat pillar size for old bike?

[atoz]

Hi all

I am the fortunate owner of my late Dad's old Baines (not the Gate). Has a steel seat pillar, but the design will allow quite limited saddle position adjustment- has a B17 narrow saddle. The bike is built out of 531 double butted tubing. What I need is a modern(ish) micro-adjust pillar to get the best position. How do I find out the correct seat pillar size to match the seat tube? NB the existing seat pillar will definitely remove, no problem.

Historical note: this bike was acquired postwar, after my Dad destroyed his existing Dayton by being dazzled by reflected sun glare and having an argument with the back of a car. The Dayton was rebuilt with a new down and head tube but was never quite the same bike. The Dayton was built out of (then) experimental chrome moly tubing, and acquired from someone in the trade.

[Mike Sales]

Measure the old pin! Needs a vernier caliper and a practised user. Pin sizes come in .2 mm increments.

[Cyril Haearn]

mm back then, or fractions of inches?
Like to see some pictures

[robgul]

Your LBS may have one of these http://www.superbiketool.com/prod4.asp? ... 17&pid=499 - but the Vernier measurement is the obvious.

It is possible to be pretty accurate by laying the seat pin on a piece of paper with a strip of wood either side and then draw a line against the inner side of each bit of wood - - then measure between the lines. Given that seat posts come in a limited range of sizes it's usually possible to ascertain the right size with this basic method.

Rob

[jimlews]

27.2 mm

[David9694]

An electronic micrometer from EBay isn’t that expensive.
If you’ve got a seatpost that fits, and presumably has no discernible size mark, that is still a major clue and the simplest, quickest and probably cheapest approach is to get your LBS to sell you a replacement. Otherwise, it’s trial and error via EBay in 0.2mm increments, starting with 27.2mm and working downwards.

[Brucey]

if it is a proper handbuilt frame in lightweight tubing there are two things that are very likely;

1) that the frame has been correctly reamed so that the old seat pin was indeed the correct size (in contrast to lots of mass-produced frames where this is by no means commonplace) and

2) that the correct size for the seat pin is indeed 27.2mm

Most likely you have something like a plain tubular birmalux seat pin in aluminium with a 7-piece clip on the top of it at present. This seat pin will be marked with the size on it but the markings may no longer be visible. If you measure the seat pin with Vernier calipers, it is a very good idea to measure the diameter where the seat pin hasn't been into the frame; the reason for this is that these (thin walled) seat pins often end up deforming into a slight oval that doesn't give a true size easily.

If the old seat pin isn't quite round then the frame may not be either, and some effort will be required to fit the correct size of seat pin.

cheers

[mig]

Measure the old pin! Needs a vernier caliper and a practised user. Pin sizes come in .2 mm increments.

why is this? always seems such a small amount..!

[Brucey]

Measure the old pin! Needs a vernier caliper and a practised user. Pin sizes come in .2 mm increments.

why is this? always seems such a small amount..!

In engineering terms 0.2mm is a country mile. Almost everything else on a bike is made to a tolerance that is ten or a hundred times better than that.

BTW most sea pins are in 0.2mm increments but there are some seat pins that come in 0.1mm sizes. For example several MTBs have been fitted with 30.9mm seat pins and (trust me.... ) nothing else will do.

cheers

[profpointy]

Measure the old pin! Needs a vernier caliper and a practised user. Pin sizes come in .2 mm increments.

why is this? always seems such a small amount..!

In engineering terms 0.2mm is a country mile. Almost everything else on a bike is made to a tolerance that is ten or a hundred times better than that.

BTW most sea pins are in 0.2mm increments but there are some seat pins that come in 0.1mm sizes. For example several MTBs have been fitted with 30.9mm seat pins and (trust me.... ) nothing else will do.

cheers

the point isn't that .2mm is or isn't a lot of tolerance (it is a lot as you say), but why make seat posts in such a range of not-quite-the-same sizes ? Screws generally come in M4, M5, M6, M8, M10 etc sizes - 1mm increments, 2mm once you'r over 8mm. 0.2mm error in a screw would be significant, but they don't do screws in 0.2mm size increments because it would be silly.
It does seem silly that seat posts are not in 5mm or 1/4" increments. Admittedly with a few things you have 25mm and 25.4mm sizes, but there's a reason for that.

[mig]

why is this? always seems such a small amount..!

In engineering terms 0.2mm is a country mile. Almost everything else on a bike is made to a tolerance that is ten or a hundred times better than that.

BTW most sea pins are in 0.2mm increments but there are some seat pins that come in 0.1mm sizes. For example several MTBs have been fitted with 30.9mm seat pins and (trust me.... ) nothing else will do.

cheers

the point isn't that .2mm is or isn't a lot of tolerance (it is a lot as you say), but why make seat posts in such a range of not-quite-the-same sizes ? Screws generally come in M4, M5, M6, M8, M10 etc sizes - 1mm increments, 2mm once you'r over 8mm. 0.2mm error in a screw would be significant, but they don't do screws in 0.2mm size increments because it would be silly.
It does seem silly that seat posts are not in 5mm or 1/4" increments. Admittedly with a few things you have 25mm and 25.4mm sizes, but there's a reason for that.

that is really what i meant. so okay we've got a 27mm seatpost and there's a reason to have a bigger one. sooo let's have 27.2mm rather than 28, 30 etc

i've always thought that it must be to match tube internal dimensions..??

[Brucey]

i've always thought that it must be to match tube internal dimensions..??

indeed. If the OD of the tube is fixed (eg by a need to fit lugs, front mech etc to it as well as any other tube manufacturing concerns) then when you use a different tube wall thickness, you need a different seat pin.

cheers

[atoz]

if it is a proper handbuilt frame in lightweight tubing there are two things that are very likely;

1) that the frame has been correctly reamed so that the old seat pin was indeed the correct size (in contrast to lots of mass-produced frames where this is by no means commonplace) and

2) that the correct size for the seat pin is indeed 27.2mm

Most likely you have something like a plain tubular birmalux seat pin in aluminium with a 7-piece clip on the top of it at present. This seat pin will be marked with the size on it but the markings may no longer be visible. If you measure the seat pin with Vernier calipers, it is a very good idea to measure the diameter where the seat pin hasn't been into the frame; the reason for this is that these (thin walled) seat pins often end up deforming into a slight oval that doesn't give a true size easily.

If the old seat pin isn't quite round then the frame may not be either, and some effort will be required to fit the correct size of seat pin.

cheers

The existing seat pin is definitely chromed steel. I am fortunate it is, since this meant it was easy to remove after decades in the same position in the frame! I''ll have a look at it for any markings. Based on the postings I am inclined to borrow someone's 27.2 seat pin to see if it will fit. Given the manufacturer was the Baines brothers, I think we can take it as read that the frame was correctly reamed.

The ride quality on this bike is quite lively, even by modern standards. My Dad had touring width 26 x 1/4 tyres in this bike, which IMHO was a bit of a waste. I think it was intended for weekend racing with a bit of light fast touring thrown in- clearances are correct for tubs for 700C. That's why I want the saddle adjustment right, so I can enjoy it for the way it was meant to be ridden.

[ratherbeintobago]

27.2 mm

Going to play devil’s advocate here.

I’m fairly certain that my 20ish year old Raleigh Pioneer has a smaller diam seatpost than 27.2mm, so surely the only way is to measure?

[thirdcrank]

I suspect a lot of us on here have been in a similar position; I certainly have. I think I first saw a version of the seatpin size checker illustrated on Sheldon Brown, but I've never seen one in the flesh - or whatever they are made of. I cannot imagine who would bother with one, although their existence means there must be a market. It's interesting that nobody has said "If you live near xxxxx you can borrow mine." Presumably, a bike shop of the type capable of replacing the seatpin in a lightweight bike would go for trial-and-error if the size wasn't marked on the old one and 27.2 didn't fit.

I suppose it's another problem caused by the internet, or at least mail order.