Shaving 0.05 mm off my seatpost diameter

[arik]

I am building up a new steel frame and my seatpost tube seems a little tight. The post is a 27.2 x 350 mm PMP titanium, calipers measure it as 27.20 mm on on the button, I can get it about 25% of the way in and then it seizes. Everything is greased with nixfrixshun NFS red. I even tried copper anti seize for fun and it still gets stuck. When I used a cheap control tech "27.2" aluminum post, which actually measured 27.14, it fits pretty tightly but is acceptable, easy to move, and it does not slip. I bought a cheap 27.0 (actual 27 with calipers) seatpost for research purposes and it fits perfectly but it slips when the seatbolt is tightened, so 27.0 is clearly too small. Should I try to sand the titanium post? How would I do it?

[SPOKE]

Use a flex hone to smooth and slightly enlarge the seat tube ID.

[Mark McM]

The first obvious question is, which you've probably already considered, is there something in the seat tube that is blocking the seatpost (such as a water bottle screw or other frame mounting)?

Since the seat tube is steel, have you tried a running a seat tube hone down it? This will knock off rust, burrs, and any gunk what might have gotten down the seat tube and hardened.

Is the PMP titanium seatpost going beyond the minimum insertion length before getting stuck? Typically, you only need an insertion depth of about 3 times the diameter. If the seat post is longer than needed, you could cut off the end of the seatpost.

And if the Control Tech fits, and serves every function that seat post is supposed to do, why not just use it?

[KonaSS]

Use a flex hone to smooth and slightly enlarge the seat tube ID.

Yep, super easy.

[Red Tornado]

Use a flex hone to smooth and slightly enlarge the seat tube ID.

The first obvious question is, which you've probably already considered, is there something in the seat tube that is blocking the seatpost (such as a water bottle screw or other frame mounting)?

Since the seat tube is steel, have you tried a running a seat tube hone down it? This will knock off rust, burrs, and any gunk what might have gotten down the seat tube and hardened.

Is the PMP titanium seatpost going beyond the minimum insertion length before getting stuck? Typically, you only need an insertion depth of about 3 times the diameter. If the seat post is longer than needed, you could cut off the end of the seatpost.

And if the Control Tech fits, and serves every function that seat post is supposed to do, why not just use it?
Both of the above are good advice.
Can you tell if the interference is down inside the seat tube, or up close to the top. Where are you measuring the seat post diameter (one spot or multiple places)? Agree that a seat post hone is the best choice. It's going to be difficult to take 0.05mm (0.002") off the diameter evenly (unless you put it in a lathe & indicate it in, which could damage the finish).
I would try to find out where the interference is first, then take it from there.

[duff_duffy]

+1 hone it. Wrap top tube with something soft just incase you slip and tool comes crushing down on your custom painted frame (speaking from experience;)). The honing fixed the post problem but the scar to the top tube drove me crazy on an otherwise beautiful bike!

[arik]

Thanks for the advice so far. Regarding the control tech seatpost, the frame is italian, and I wanted a mostly italian high end build without a lot of carbon, so the PMP titanium seemed like a natural pick.

Regarding the flex hone, is this the right one?
https://www.amazon.com/25mm-Flex-Hone-Cylinder-Silicon-Carbide/dp/B002MOS9OQ/
Also, is the cutting oil mandatory?

[oldpotatoe]

Thanks for the advice so far. Regarding the control tech seatpost, the frame is italian, and I wanted a mostly italian high end build without a lot of carbon, so the PMP titanium seemed like a natural pick.

Regarding the flex hone, is this the right one?
https://www.amazon.com/25mm-Flex-Hone-Cylinder-Silicon-Carbide/dp/B002MOS9OQ/
I guess I will have to do my best to dry out the inside of the seat tube, it is lubed up pretty good right now

What SPOKE and Mark said but the flex hone needs to be 27.2 or perhaps a wee bit larger, 1 inch(25.4mm), too small. 28.6, 1 1/8inch.

https://www.amazon.com/29mm-Flex-Hone-Cylinder-Silicon-Carbide/dp/B002MOW8YS

Use a drill, be careful, the tube will get HOT.

[yinzerniner]

Thanks for the advice so far. Regarding the control tech seatpost, the frame is italian, and I wanted a mostly italian high end build without a lot of carbon, so the PMP titanium seemed like a natural pick.

Regarding the flex hone, is this the right one?
https://www.amazon.com/25mm-Flex-Hone-Cylinder-Silicon-Carbide/dp/B002MOS9OQ/
Also, is the cutting oil mandatory?

I'd be willing to go halvsies on the flex hone as have a similar issue on a Ti bike I purchased a month ago. Previous owner left a carbon seatpost in there unattended for a few years and some of the antiseize ended up bonding to the inside of the tube.

[arik]

Ok I bought the 1 1/8" flex hone with some cutting fluid, might be good to have around the workbench for future problem solving anyway. The quill stem and steerer tube were actually a tight fit as well so I might hit that too. I guess they put my frameset in the dryer too long.

[ultraman6970]

U dont even need a flex hone... just get like a stick... cut a slit and fold like a 500 grit sand paper and stick it in the slit.

Then put that stick in a drill... not at super speed but go little by litte cleaning and testing the seatpost. Thats the cheapest and quicker way to do it w/o spending money :D

[arik]

You must be a young man, never do something thriftily when it can be done by buying cool tools.

[tctyres]

Ok I bought the 1 1/8" flex hone with some cutting fluid, might be good to have around the workbench for future problem solving anyway. The quill stem and steerer tube were actually a tight fit as well so I might hit that too. I guess they put my frameset in the dryer too long.

The quill stem should be a tight fit. Otherwise, you will get additional flex. Dave Kirk posted on the slip fit recently.

[David Kirk]

FWIW - a 27.2 post should not measure 27.2 mm. Odd I know but a 27.2 post is meant to slip fit into a seat tube that has a 27.2 inner diameter. This of course means that a post labelled 27.2 needs to be smaller than that to fit into a 27.2 seat tube.

A post labelled "27.2 mm" should actually measure 27.15 or so. If it actually measures 27.2 it is in fact too large. The fact that your 27.14 post fits pretty well speaks to the fact that the seat tube is properly sized.

I point this out because if you make the ID of the seat tube larger to fit your oversized post a properly sized post (~ 27.15) will fit loose. You will be making it so that only your out-of-spec oversized post will work.

If it were my bike I'd leave the frame alone and use a post that was made to the proper tolerance and make a lamp out of the one that's too big.

dave

[Louis]

^^^^^^ Yup.

Rule of thumb - modify the easier to replace / less expensive item - especially if the more expensive one is built to spec.

[arik]

Thanks for your very timely reply Dave Kirk. It didn't sit right with me that this new custom frame from a very respected builder had trouble with something basic like this, but it makes a lot of sense that this seat post was off, it is not the only weird problem I have had with this post.

[Dino Suegiù]

Assuming your calipers are good, check the seatpost diameter at several circumference points and at several circles vertically up to your insertion line.

It may be that the seatpost is out of round/ovalized somewhere, rare but not extremely uncommon with titanium seatposts, even new ones.

It is certainly worth your time to check:
I had a new titanium 27.2 seatpost, bought directly from PMP, which would not fit into a frame. I measured; saw the seatpost was ovalized; sent PMP some images of the caliper readings per their request; and they sent a new (perfectly round) seatpost immediately.

[arik]

The plot thickens, I think I have stumbled onto an international seatpost diameter conspiracy, this is the response from the retailer:

"Most of the frames I have here measure between 27.25 and 27.35 and we ourselves produce frames with a 27.3 internal diameter for the seatpost.

I’ve also decide to double check seatposts form other manufacturers, Deda alloy, Deda carbon and 3T carbon, they all measure more than 27.2, please pictures attached.
Is not uncommon that cheaper seatpost might measure less than 27.2 but that’s due to the higher tolerances expected during production of cheaper components.

From my experience, the right way to solve the issue is to have the inside of the seat tube of the frame reamed and clean form excessive paint.
This one her his the specific tool used to do that https://ciclicorsa.com/it/shop/seat-tube-reamer/

Also note that we sell roughly 15 Pmp seatpost per month since some time now, and we haven’t had this kind of complaint before.

If you prefer to have the seatpost shipped back for a refund, sure that is possible and I’ll be happy to cover the cost for the return shipping, but I believe the right way to solve the issue is to have the frame properly prepped."

He also sent me a picture of the posts measuring 27.24, 25, 27 :bike:

[tctyres]

Thanks for your very timely reply Dave Kirk. It didn't sit right with me that this new custom frame from a very respected builder had trouble with something basic like this, but it makes a lot of sense that this seat post was off, it is not the only weird problem I have had with this post.

The plot thickens, I think I have stumbled onto an international seatpost diameter conspiracy, this is the response from the retailer:

"Most of the frames I have here measure between 27.25 and 27.35 and we ourselves produce frames with a 27.3 internal diameter for the seatpost.

I’ve also decide to double check seatposts form other manufacturers, Deda alloy, Deda carbon and 3T carbon, they all measure more than 27.2, please pictures attached.
Is not uncommon that cheaper seatpost might measure less than 27.2 but that’s due to the higher tolerances expected during production of cheaper components.

From my experience, the right way to solve the issue is to have the inside of the seat tube of the frame reamed and clean form excessive paint.
This one her his the specific tool used to do that https://ciclicorsa.com/it/shop/seat-tube-reamer/

Also note that we sell roughly 15 Pmp seatpost per month since some time now, and we haven’t had this kind of complaint before.

If you prefer to have the seatpost shipped back for a refund, sure that is possible and I’ll be happy to cover the cost for the return shipping, but I believe the right way to solve the issue is to have the frame properly prepped."

He also sent me a picture of the posts measuring 27.24, 25, 27 :bike:

Because this is a new, custom frame, email the manufacturer and see what the response is. At that point, I'd probably go with a recommendation from the manufacturer.

My guess is that they will say the same thing as David Kirk, above.

[Mark McM]

FWIW - a 27.2 post should not measure 27.2 mm. Odd I know but a 27.2 post is meant to slip fit into a seat tube that has a 27.2 inner diameter. This of course means that a post labelled 27.2 needs to be smaller than that to fit into a 27.2 seat tube.

A post labelled "27.2 mm" should actually measure 27.15 or so. If it actually measures 27.2 it is in fact too large. The fact that your 27.14 post fits pretty well speaks to the fact that the seat tube is properly sized.

I point this out because if you make the ID of the seat tube larger to fit your oversized post a properly sized post (~ 27.15) will fit loose. You will be making it so that only your out-of-spec oversized post will work.

If it were my bike I'd leave the frame alone and use a post that was made to the proper tolerance and make a lamp out of the one that's too big.

dave

Pondering this, I wonder if this makes sense? In order to lock telescoping tubes (like a seatpost and a seat tube), either the inner tube has to expand, or the outer tube has to contract. So which tube should start out at the standard nominal diameter? Logically, the constant diameter tube should be at the nominal standard diameter, and the tube that expands or contracts should change shape to meet this diameter, because there is generally a larger tolerance allowance in the flexible part. So for a seatpost/seat tube joint, shouldn't it make more sense for the (constant diameter) seatpost to be at the nominal dimension, and for the (contractable) seat tube to start at a slightly larger diameter, and be able to contract down to the nominal diameter?

[David Kirk]

Pondering this, I wonder if this makes sense? In order to lock telescoping tubes (like a seatpost and a seat tube), either the inner tube has to expand, or the outer tube has to contract. So which tube should start out at the standard nominal diameter? Logically, the constant diameter tube should be at the nominal standard diameter, and the tube that expands or contracts should change shape to meet this diameter, because there is generally a larger tolerance allowance in the flexible part. So for a seatpost/seat tube joint, shouldn't it make more sense for the (constant diameter) seatpost to be at the nominal dimension, and for the (contractable) seat tube to start at a slightly larger diameter, and be able to contract down to the nominal diameter?

I hear you.

One could say that we already have this but that it's labelled in a funny way. You could say that the seat post is a fixed 27.15mm and that the flexible part (seat tube) is oversized at 27.2 to allow the 27.15 post to slip in.

I'm not so much defending the way it's done but instead explaining the industry standard....and the acceptable industry standard is that the ID of the seat tube is 27.2 and the post that fits in it is 27.15 OD.

Maybe we should change it?.....but the current standard is what it is. And of course this also carries over to other posts sizes in that they are slightly smaller than the labelled size so that they will slip into a given ID seat tube.

dave

[arik]

Fwiw, I picked up a nicer control tech titanium "ti-mania" post for this frame, measures 27.15 mm.

[smwillis]

You can have the frame reamed out

[rst72]

i have a steel bike (Columbus Life...thin walled) and i am guessing the ST was reamed a little too much. My frame uses a seat collar vs an integrated binder bolt and it developed a crack starting at the ST slot. A shim from a coke can leveled it out and it's good now, but just throwing that out there.