jberenyi

My De Bernardi frame takes a 27.0mm seat post. Is there a way to hone out the seat tube for a 27.2mm which is only .008 larger? Has anyone ever done this before?

bindibadgi

Can't you find a 27.0mm seat post? I really wouldn't want to be taking anything off the inside of the tube.

They are usually engineered without much leeway, and 0.1mm of thickness makes a big difference in strength, especially on a part which is taking your weight some of the time. Also, even if there is enough meat there, if you didn't get it perfectly centered (all the way down - and you'd need to go fairly deep) then you'd really be risking a very weak seat tube.

Coyote2

Don't try it.

bassplayinbiker

better you than me

Sheldon Brown

Yes, it's done with an adjustable reamer and a fair amount of elbow grease. Better bike shops will have this (quite expensive) tool and know how to use it.

Sheldon ".1 mm" Brown

Ben the bike

Seat tubes are reamed to produce a good quality surface finish and a precision fit with manufactured seat tubes. A seat tube reamer for 27.2mm seat posts should be stocked by good bicycle mechanics or bike builders, or an adjustable reamer can be used.

bindibadgi

I take it back. If the master says it can be done, who am I to argue?

Nice to know actually, but I think I'd still rather find the right size seatpost if I could.

Rowan

Barnett's Manual deals with this. If you can find the relevant illegal pdf...

Wino Ryder

You're talking about permanently modifying (possibly ruining) a good frame just to accomodate a seatpost. I think i'd be more inclined to take an old 27.2 seatpost to a machine shop and have them mill it down.

just my thoughts.

HillRider

As Sheldon has noted, it certainly can be done. Mechanically it's a pretty straight forward procedure with the right tools. However, assuming your frame is a high quality thin tube one, there is very little excess wall thickness and the reaming would have to be at least 75-100 mm deep. I wouldn't do it.

27.0 seatposts are readily available and probably less costly than having the frame modified. For example, Thompson's Elite and Ritchey's WCS seatpost are made in 27.0 and there are lots of others.

neil0502

I'm in HillRider's camp:

I'm sure it can be done. I don't doubt that it's done relatively frequently.

I wouldn't do it to one of my bikes, though.

YMMV.

Sheldon Brown

It's only a tenth of a millimeter!

Sheldon "Not A Thompson Fan" Brown

tellyho

I was forced to do it on my fixie frame, a Trek 420 dump rescue on which someone had overzealously clamped the binder. Didn't have to enlarge the whole tube, just take down a bump where the binder had been overtightened. I'm not sure I'd do it if I didn't have to, but it's doable.

neil0502

I guess my reservation, Sheldon, is that your average shop is ... well ... your average shop.

This seems like the kind of process with very little margin of error. Not knowing how abrasive the hones are, I'd be wondering how much damage could be done, by an inelegant push or pull, in a split second.

Sheldon Brown

The proper tool for this is an adjustable reamer, a very precise, slow-cutting hand tool.

I agree that a power-driven hone would be inappropriate.



Sheldon "Reamer" Brown

Thumpic

I'm confused....why wouldn't you just get the right part??....I did a quick search from Google and saw bunches.........

rmfnla

Exactly, and 27.2 posts are much more common than 27.0

I love the doomsayers who think removing a hair's breadth of metal is an invitation to catastrophe.

HillRider

I'm not really a doomsayer and Sheldon is correct, it's only a 1/10 of a mm. However, thin wall steel tubes have walls as little as 0.5 to 0.7 mm thick in the butted ends. So that 0.1 mm means removing 15 to 20% of the total metal thickness. Here is an example from the Columbus Tubing web site:
https://www.columbustubi.com/eng/4_4_1.htm

On a cheaper, heavy wall frame I wouldn't hesitate to ream it to 27.2 but I wouldn't do it on thin wall, light weight frame.

anti.team

But a seat tube isn't .5 to .7mm is it? None that I can think of are that thin, seems like they are more like 1.0+ although I don't have a bike in front of me to look at.

rmfnla

I hear you, HR; all I can say is I have done this numerous times (yes, on good frames) and never had a problem.

J T CUNNINGHAM

We had better keep this thread quiet, the 'weight-weenies', myself included

will be getting 'ideas'!


Regards,
J T

HillRider

Actually you do have the info in front of you. Follow the Columbus Tubing link I posted. Their Spirit tubeset seat tube is 0.5 mm wall at one end and 0.6 mm at the other.

Sheldon Brown

Tubing used for bikes is sized by outside diameter. Thin wall tubing has a bigger hole than thick wall tubing.

Steel seat tubes are generally 1 1/8" (28.6 mm) thick. Reaming one out to 27.2 leaves a wall thickness of (28.6 - 27.2)/2 = .7 mm.

Older French tubesets had 28 mm O.D. seat tubes. You wouldn't even want to go as large as a 27 mm seatpost with one of them. The old metric tubing is basically extinct, has been for years.

Sheldon "Do The Math" Brown

HillRider

Excellent point and the Columbus Spirit tubing reference I showed appears to require a 27.4 mm seat post (28.6 - (2*.6) = 27.4) unless the builder brazes in a 0.1mm shim to allow a 27.2 mm seat post.

My point was that thin wall tubing has little excess material and just reaming out what seems to be a small amount can represent a fairly large fraction of the actual wall.

Sheldon Brown

My point was that a frame that uses a 27 mm seatpost is not built with "thin wall tubing." ;-)

Sheldon "Numbers" Brown