# Seatpost size for Look steel frame



## Sweeney (Apr 29, 2007)

I just got a NOS Look steel frame. It doesn't have any model information on the frame. It does have a Columbus sticker that says
''Columbus Tubi speciali trafilati ACCIAIO Cromo GARA tre tubi''

I measured the ID of the seat tube at 26.6. What size seat post do I need, 26.5?

thank you


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## Scooper (Mar 4, 2007)

The GARA seat tube wall thickness is straight gauge (non-butted) 0.8mm, while GARA OR has 1.0mm walls. If the seat tube on the Look frame is the standard 28.6mm O.D. _and_ if Look used the stock GARA seat tube in this frame, the inside diameter of the seat tube would be 28.6 - 1.6 = 27.0mm.

If the frame uses GARA OR, an oversize tubeset designed for mountain bikes, the seat tube O.D. would still be 28.6, but the I.D. would be 28.6 - 2.0 = 26.6.

Standard practice is to use a seatpost with a diameter 0.2mm less than the I.D. of the seat tube to allow for manufacturing tolerances of both the tubing and the seatpost as well as warpage at the seat cluster from heat during the brazing process. Even using a reamer to ream out the seat tube, the 0.2mm difference is used for providing a nice fit (not too tight, not too loose). So, if all of the assumptions above are true, if the the frame is GARA, it should take a 26.8mm seatpost. If the frame is GARA OR, it should take a 26.4mm seatpost.

Seat tubes often become elliptical in shape at the seat lug after being used for a while, so measuring, even with a good caliper at several points across the opening and the averaging them, is often inaccurate. Most bike shops have stepped seatpost measuring rods that are graduated in standard seatpost diameters so the seatpost diameter for an unknown seat tube can be easily determined.

Here are the J.A. Stein stepped seatpost sizing rods.


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## Scooper (Mar 4, 2007)

Here are the tubing wall thickness for various Columbus tubesets circa 1989.










Link for higher res image

Data sheet for GARA:










Link for higher res image

Data sheet for GARA OR:










Link for higher res image


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## Sweeney (Apr 29, 2007)

Thanks for the info. I just ordered the seatpost


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