Grand Bois

I have one of those too, but I wouldn't use it to reduce the diameter of a seat post. Belt sanders cut too fast.

Robbie pretty much desribed my method, but I do the full length of the post.

Wogster

What about a chromed steel post? Here is the problem, some genius put too small a post in there and then cranked the bolt to the point that it was tight, leaving the ears and slot pinched together, the smallest post I could find 25.4mm does not quite fit (think the hole is about 25.3mm, so I need to widen the slot, tried a steel wedge and a BFH, but that did not work.... Thinking I might try a claw hammer claw and put my 225lbs behind it, but figure I will probably break the hammer. Maybe if I can put a claw in there and whack the other end of the hammer head it might open up slightly.

noglider 

Wogsterca, I widen the ears with a very large screwdriver.

I don't sand down seatposts. I ream the seattube with an adjustable reamer. I might buy one some day. The real expensive thing seems to be the handle for it.

kludgefudge

I just use my mad hot angl grndr skilz, yo! (srsly.)

This method is really only appropriate for beaters with missing seat posts, as the end result is not going to be "pretty". Every other method posted here is more appropriate for a true C&V bike.

gaucho777 

Bumping an old thread to add on a question....

I recently acquired a 26.8mm Campagnolo Nuovo Record seat post with the intention of turning it into a 26.6mm post. Originally I had planned to sand it down following the suggestions above. However, a good friend of mine owns a machine shop and has offered to help turn it down to 26.6mm with his metal lathe. I've never visited his shop, so this would be a perfect excuse. He wouldn't charge me, and I'm sure he can do the work with far more precision than I'd ever hope to achieve with sand paper in hand. This leads to me to my question....I don't want to turn the entire post down and loose the Campagnolo engraved logo, but I don't want a very noticeable ridge running around the middle of the post either. So, what's the best way to handle the transition from the new 26.6mm section to the original 26.8mm section? One thought is to make a couple transitions (somewhere above the seat tube entry point but below the logo--say from 26.6mm to 26.7mm, and another transition from 26.7mm to 26.8) and then smooth out those transitions by hand with sand paper. But maybe there's a better way? Or should it be possible to make a nice smooth transition from 26.6 to 26.8 with a lathe? Thoughts or suggestions?

I will pose these questions to my machinist friend, too, but thought I'd pose the question here first since a) he doesn't usually work on bike parts, and b) I don't want to set my expectations so high as to be an imposition.

Chombi

On Exxon Grafteks they turned down Campy NR seaposts from 26mm to 23mm OD and they avoided making a ridge and maintained the Campy engraved logo on top of the seatpost shaft by having a transition slope cut in within maybe an inch and a half length of the seatpost shaft to gradually come out to the original OD just below the Campy logo.
You can still kinda see the conical shape produced by the transition area, but it is subtle enough that it does not look bad at all. As you might see on this pic of what was similarly done on my Line Seeker bike....

I can take closer pictures of the seatpost on my Line Seeker later and post them up here too....
I suspect that it will be hard to notice the transition on you seatpos if you do the same, as you will not be taking off as much material as they did on mine.

Chombi

gaucho777 

^Thanks, Chombi. That's reassuring. Obviously, 26mm to 23mm transition is much bigger than the .2mm transition I would have. Maybe I'm making a mountain out of a molehill.

Chombi

I'll double check the transition size on the post when I get home. Now I'm kinda remembering maybe it wasn't that big a step down after all as I'm now remembering the 23mm size is what my Vitus Carbone uses and maybe I'm bikes and posts up. The Line Seeker/Graftek size might be more like 23.8mm....or something.....or was it 25.8......dang!, Fried food grease must be affecting my memory??! But I still think it's bigger a transition than what you are anticipating for your seatpost.... I'll get back with you tonight on this....

miamijim

.1mm per side isn't going to be too noticeable.

Chombi

OK, I checked and the NR seatpost on my bike was turned down from 26mm to 23.8mm As you can see in the pic I posted, the conical transition is hardly noticeable...
BTW, they also machined out the inside of the seatpost shaft to lighten it up, as the NR seatpost shaft walls were pretty thick. You might want to considier having that done too...

iab

I paid my local machine shop $20 to turn down a post from 26.4 to 26.2. They matched the old machine marks perfectly.

Road Fan

Keep in mind, Campy two-bolt posts can be expensive. You might be reducing the value of an expensive part.

Chombi

This is a good opportunity to take advantage of buying a not so expensive used NR post that might have deep scratches and gouges at the bottom half of the shaft. As that surface will be machined off anyway, the final product could look just the same as one might have bought a more pristine/more expensive example.....

gaucho777 

Thanks, everyone, for the feedback.

I'm coming around to this. Still, even a .1mm ridge will be noticeable (and would bother me), so I'd like to figure out the best way to smooth out that transition--be it on the lathe or later by hand.

Interesting. I hadn't considered any machine marks. My post is going to be polished smooth, so it's not a concern in this case but I'll keep that in mind.

Yes, understood. However, I got a great deal on the one I have, and I'll likely keep the post with the frame for the foreseeable future. Any reduction in value to the existing post will be negated by the cost of securing a new 26.4 NR two-bolt post.

Thanks for your input and for the comparative measurements.

But I already have a NR post and an offer to turn it down by a professional machinist, so it's not going to cost anything versus the cost of securing a new 26.4 Capagnolo post ($$$!). It's not so easy to find a 26.4 Campy post in any condition, let alone a "not so expensive" one.

DrJames

For posterity, specifically for those who lack patience or are in poverty. I just turned down a 27.2mm aluminum seatpost to 27mm using a cordless drill. I chucked up the longest bit I had and then taped a shop rag around the bit to a diameter just larger then the ID of the seatpost. From here it was just several stages of sand paper, finished off with some steal wool. Cheers!

m_sasso

This started out as an inexpensive $12.00, 27.0mm seat post purchased through the C&V Sales forum. Hand sanded, polished and painted it to a 26.8mm post for my 81 Team Miyata.

Tbone5

Im from the more power is better school. Belt sander, followed by finer grit (by hand). Check repeatedly by inserting into seat tube. Once I get the fit close enough, off to the polishing wheel/buffer. Good enough for the week end warrior.

rootboy 

That's interesting. I'm wondering how you keep the post perfectly concentric...round..using a belt sander. Just eyeball it?

rootboy 

When I've contemplated this proceedure, reading other threads, I'm real curious how the machinist would hold the post in order to turn down the whole length. Would love to learn how they did it. Of course, competent machinists have all kinds of tricks up their sleeves.

SJX426 

WRT the post diam reduction, ditto for both [MENTION=29368]rootboy[/MENTION] comments above
WRT the ST cleaning. I have used the three block hone with success. There are several "grits" available. The slot isn't big enough to really make a difference at speed as long as it is deburred. I have also used a 12 ga bore brush, with an extender, to knock off the loose stuff.

Tbone5

Eye balling the fit after belt sanding seems to be good enough. THe key is it to take it slow and check against the seat tube fit often. Looking over the other responses, I think taking a cylinder honing bore to the seat tube is probably the best and cleanest route to go…still keep in mind to be conservative and do a little at time… you can't add metal back to the seat tube once its been removed.

rootboy 

Not in my world. No way. I'd use a cylinder hone to clean up the inside of the seat tube, but never to remove much metal. Not on a nice frame, anyway.
Makes more sense to alter a replaceable item like a seat post, to me. If you have to.
I tend to just obtain the correct size.

Velognome 

I've honed a post using a file & sandpaper. Worked fine, go slow. It took about 3 hours to remove 1.5mm

CroMo Mike 

I put a cheapo post in the lathe and turned in down .2 mm. Chucked it by the bottom 1" and then cut off that last 1" when I was done. I would never do that to a great post.