Advice needed for seat post sizing, Columbus Neuron tubing

I bought a mid 90s Menet (Martelly) road bike earlier this year which has Columbus Neuron tubing all round. I have only just got round to changing the saddle and found two issues:

Firstly that one half of the micro adjust saddle rail clamp on top of the (Easton) seatpost was cracked, something I had never noticed. Clearly it needs to be replaced as I assume it is not possible to buy individual components?

Secondly, when removing the seatpost it was very tight. It is clearly the wrong diameter for the seat tube. I did get the seat post out OK and the tube was quite clean, no rust etc which seems to confirm it's down to non-compatible seatpost diameter. I have measured the diameter of the seatpost with digital calipers and it is 27.25 mm. The internal diameter of the tube is probably 27.0 but on the calipers it was coming up at 27.03. However, I understand that Neuron tubing is slightly eliptical so I have tried to find specs of Neuron tubing online but have drawn a blank. The external diameter of the seat tube is 29.15mm.

Assuming that the internal diameter of the seat tube is 27.0mm am I right in thinking I need a seat post that is 0.2 smaller than the internal diameter of the seat tube? i.e. 26.8mm?

Finally, the seat post is 200mm long plus the rail clamp on top. I have noticed that most seatposts seem to be 400mm long.....is it possible to buy a 200mm post?

Try a different 27.0, if it slides in and out without to much effort, thats your post.

As for length, most get chopped down to suit but there are also plenty of 300mm posts too.

legrandefromage":17e1zy4d said:

Try a different 27.0, if it slides in and out without to much effort, thats your post.

As for length, most get chopped down to suit but there are also plenty of 300mm posts too.

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I'm sure I can get hold of a 27.0 to try. I'm sure you're right because the 27.25 will come in and out without too much of a fight now that it is greased properly.

I did wonder about chopping a 400mm post down. Is this something I can do or does the cut end need to be specially finished? I have various sized files so can cut down and file it.

I reckon a 300mm post would be fine as the cut off point is the brazings on the seat tube for a bottle cage and I can probably get away with 325mm above the bbrazings.

Great advice, thank you!

The bike shop I worked at used to regularly chop posts down so just measure where the safety line is and mark the post accordingly.

Wear a mask if its a carbon post. If you're really good you can chamfer the edges so it looks nice on alumnium posts.

legrandefromage":1pq4mrg6 said:

The bike shop I worked at used to regularly chop posts down so just measure where the safety line is and mark the post accordingly.

Wear a mask if its a carbon post. If you're really good you can chamfer the edges so it looks nice on alumnium posts.

Click to expand...

Perfect, I will have a go if I need to go with a 400mm post. I'm sure I can chamfer the edges no trouble and I guess that will help the post down into the tube too.

Thanks again..........

I can only go by my own experience so give it a mo in case some else has more info.

legrandefromage":39cmwiem said:

I can only go by my own experience so give it a mo in case some else has more info.

Click to expand...

I won't do anything until next week but everything you said makes perfect sense and it seems that I have more options in a 27.0 post than a 26.8, certainly if I want to hang out for a sub 300mm post.

Neuron seat tube was 28.6Ø, with a 0.6 wall at the top, so bore is nominally 27.4Ø, so seatpost should be 27.2 -27.3Ø.

27.0Ø is too small but it might be that your frame has not been reamed or honed out post-brazing...the tube will always distort a little.
I always insisted on having the actual seatpost which was going to be used, so that the frame could be reamed to fit.

Since your 27.25Ø post is a little tight, it's probably worth measuring up some nominally 27.2Ømm posts from other groupsets and manufacturers, as the tolerances vary wildly.

Here's the data sheet from 1994:


The outside profile of all the main tubes is all round as usual, but the tubes do have elliptical internal butting, 0.2mm different between the fronts and the sides.
This means that the tubes need to be rotationally aligned for the butting, and also for the positioning of the internal tabs for bottle bosses and front mech areas

The seat tube is round profile and does not have the elliptical butting at the seatpost end, that would be very weird.

All the best,

told you so!

danson67":1z2rpvzw said:

Neuron seat tube was 28.6Ø, with a 0.6 wall at the top, so bore is nominally 27.4Ø, so seatpost should be 27.2 -27.3Ø.

27.0Ø is too small but it might be that your frame has not been reamed or honed out post-brazing...the tube will always distort a little.
I always insisted on having the actual seatpost which was going to be used, so that the frame could be reamed to fit.

Since your 27.25Ø post is a little tight, it's probably worth measuring up some nominally 27.2Ømm posts from other groupsets and manufacturers, as the tolerances vary wildly.

Here's the data sheet from 1994:


The outside profile of all the main tubes is all round as usual, but the tubes do have elliptical internal butting, 0.2mm different between the fronts and the sides.
This means that the tubes need to be rotationally aligned for the butting, and also for the positioning of the internal tabs for bottle bosses and front mech areas

The seat tube is round profile and does not have the elliptical butting at the seatpost end, that would be very weird.

All the best,

Click to expand...

Can't thank you enough for all that information!! Believe me I did exhaustive searches to try and find more info on Neuron tubing something like the chart you included but just came up with a complete blank.

Really great to have all that information and particularly the diameter of the seat tube. Since my original post I have removed/re-greased and re-inserted the seat post a few times (after having cleaned the ST and checked it for debris/damage) and now the Easton 27.2 post is moving relatively freely. This suggests that the post must have been in the seat tube for some considerable time with little or no maintenance and was probably one step away from seizing. The cleaning and grease have obviously done the trick and confirms that the 27.2 post was the correct size after all.

I will try a couple of different 27.2 posts in the ST if there is variation in sizing. I do need another post anyway as one of the seat rail clamps is broken on the Easton but at least now I know what size to go for!!! Also good to know that the ST is not elliptical....as you say, that would be a bit weird.

Thanks again........