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Old 01-12-19, 11:58 AM
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Join Date: Nov 2013
Location: Near Pottstown, PA: 30 NW of Philadelphia
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Bikes: 2 Trek Mtn, Cannondale R600 road, 6 vintage road bikes

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I don't know if this helps as Motobecane was evolving to ROW standards during the timeframe. Dimensions year to year can be different. My 1979 Mirage is made with 2040 straight gauge steel. My seat post is 25.4mm and the ST OD is 28.3mm which includes the paint. I doubt I could get a 26mm post into my frame.

Can you find a machinist who can measure the ID of the seat tube, a few cm below the seat tube top lug? That would "learn ya" some good data.

Get a good flashlight and inspect the ID of your seat tube. Ensure there is no ridge which is blocking the post from fully inserting. My winter project is a 73 Grand Record. It's stem wedge had rusted badly and left a ridge on the ID of the steerer. The stem would not insert past the min insertion point. I used a ball hone which did not help much. Quality time with a round file finally removed that ridge and the stem fully inserts now. Any obstruction in your ST?
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