Shim shiminee, shim shim sheroo: seatpin advice, please.

[Nettled Shin]

I've had a friend apply his digital calipers to my seatpin and, on average it seems to be 35.4mm OD, although it is a bit ovalized in places, eek!

I've got two solutions:
Buy a 34.9mm seatpin, but I would then need a 0.2-0.3mm shim. Is there anything lying around a typical house that would do that?
Commercial seat tube shims don't seem to come that thin.

Alternatively, I could buy a 31.8mm seatpin (3.6mm narrower), and try to use a shim made for 27.2 to 30.8mm conversion (1.8mm thick). There would be a 15mm wide gap where there wasn't any shim, would that cause problems? And the shim might crease rather than spring open?
I've seen plastic shims; do they do the job better or worse? They seem to be a bit deeper.

[531colin]

What size does the frame manufacturers spec. say?

[Brucey]

35.4mm is a common ouside diameter for the frameset seat tube. Such seat tubes accept various smaller diameter seat posts. I don't think I have ever seen a seat post of 35.4mm diameter.

What kind of bike is this? Are you sure your friend measured the correct thing?

cheers

[Nettled Shin]

Are you sure your friend measured the correct thing?
cheers

It's definitely over 35mm. I measured it myself using a strip of paper wrapped around the pin, so that the two edges just butted up, then divided the length by pi, and came to 35.4 +/- 0.2mm, then my friend used Mitutoyo digital calipers, which aren't cheap, and obtained readings that were mostly 35.3--35.4mm. I didn't take him the bike, just the pin, and watched him measure it, so he couldn't have measured the seat tube OD by mistake.

[531colin]

What size does the frame manufacturers spec. say?

If you don't know the make, plan B....

What is the internal diameter of your existing seatpin? ....You might be able to use a bit of it as a shim to fit a new seatpin if the internal diameter of your existing pin matches an available external diameter of common seatpins, taking into account is your existing pin tight/slack/just right.

[Brucey]

If you are measuring +/- 0.2mm the old seat pin must be very oval. Normally even if they are oval at the bottom they stay a bit rounder nearer the top.

I quite like Colin's idea.

A few times I have been forced to make a seat pin fit because I couldn't get the exact right one.

Normally this involves turning down a thick-walled one on a lathe, or turning up a shim to fit. Maybe you could find someone local to you to do the same kind of thing. Unfortunately I don't have such good access to a really good lathe any more, so it has clipped my wings somewhat in that respect.

Mad thought; since the diameter you need is very close to the OD of some seat tubes, maybe an old scrap frame would liberate a length of tube to use as a shim?

cheers

[Nettled Shin]

If you are measuring +/- 0.2mm the old seat pin must be very oval

That was my error guestimate. The pin is even more oval than that, at one point, we measured 35.8mm and 35.0mm at right angles to each other, but this is consistent with the pin being 35.4mm OD.

Colin's idea of cutting up the old pin is neat, but it is a bit final, so I might hold fire on that idea. I haven't got a micrometre with me at the moment, so I don't know how thick the wall is.
Are plastic shims able to do the job? Most shims seem to be metal. Is it just that people aren't comfortable with the idea of plastic holding up their weight?

[531colin]

Vernier calipers should be able to measure seat pin internal diameter or wall thickness.
Plastic shims manufactured for seat pins (or handlebars) are OK, but I wouldn't cut up washing up liquid bottles and use that.....but bits of beer can are OK for thin shim.....if you see the difference!

[Brucey]

can you measure the seat pin nearer the top to get an accurate reading?

some plastic shims are OK.... but not all plastic shims....

cheers

[531colin]

Theres alloy tube in various diameters and thicknesses here...http://www.aluminiumwarehouse.co.uk/Aluminium-Round_Tube/c120_131/index.html?page=2

Google will find conversion charts from inches or Standard Wire Gauge to metric.

[T44ISKN]

I've used USE plastic shim/adapters with success in the past. My particular frame demanded a 30.0 seat pin which are not that plentiful. So I bought a USE 27.2 to 30.0 adapter and a normal sized seat pin. They do them in all kinds of sizes and because they're plastic, you don't need to grease the inside that contacts the pin, so no slippage. A combination of one of these and a standard sized pin might work for you?

I'm a trendy consumer. Just look at my XT910 using hovercraft full of eels.

[breakwellmz]

Hi.

I`ve been for some using smaller diameter than intended
(For the frames) seat posts and a shim.Usually a 25mm diameter.

More flex,more comfort

[531colin]

A couple of things relevent to this thread floated into my head......
You can use "more than half" of 2 shims to avoid a gap....I wouldn't be happy with a big gap in a thick shim.....vernier calipers will measure the wall thickness of a bit of tube sufficiently accurately for this sort of thing.....the source of error here is the ovality of the existing seatpost......It might be possible to get another measurement if its a welded frame by measuring the outside diameter of the seat tube and subtracting twice the wall thickness.......
When I had my own shop, I used to keep a herd of knackered seatposts for this sort of job, but theres not a living in it....

Could an adjustable reamer be pressed into service as a gauge?

[cycle tramp]

What's the seat post made from? If its alloy or steel you may be able to make the diameter of the seat post bigger by hitting it with a hammer and punch across its surface. The punch will distort the metal, making a dent, but the metal around each dent would be pushed up. If you continue to make punch strikes along the surface of the metal the diameter of the seat post should increase... personally i'l never tried it What i have done is simply wrapped lots of electrical tape around the seat post until the diameter was slightly larger than i wanted and then just pushed the seat post in and tighten the clamp. Under pressure the tape became a sticky mess, but the seat post was held securely

[Nettled Shin]

You can use "more than half" of 2 shims to avoid a gap....I wouldn't be happy with a big gap in a thick shim.....

I was contemplating this this very morning. The problem became more complicated as I thought about it because two halves could easily fall down the seat tube, or become lost when transporting the bike, so that led me down the line of buying four shims and offsetting the joins, then gluing it all together.

Could an adjustable reamer be pressed into service as a gauge?

I'll have to google that. My tool knowledge is very basic. TBH, I don't know any bicycle shops that would be prepared to faff around with this for me.

What's the seat post made from? If its alloy or steel you may be able to make the diameter of the seat post bigger by hitting it with a hammer and punch across its surface. The punch will distort the metal, making a dent, but the metal around each dent would be pushed up. If you continue to make punch strikes along the surface of the metal the diameter of the seat post should increase... personally i'l never tried it What i have done is simply wrapped lots of electrical tape around the seat post until the diameter was slightly larger than i wanted and then just pushed the seat post in and tighten the clamp. Under pressure the tape became a sticky mess, but the seat post was held securely

Ah, Cycle tramp, you are living up to your alias. Both of those ideas appeal to my ghetto-chic, make-do-and-mend nature, and it is good to know that insulation tape can be employed in an emergency. I think I'll put your idea to one side for the moment, along with the one marked 'deliberately make the inside of the seat tube rusty so as to reduce its diameter'.