Undersized seat tube
 

My new-to-me 1990 Sirrus had a slipping post, and all the reference suggested that it should be a 27.0. It came with a 26.6, and the coop seat post sizer wouldn't go past 26.8. The slot did not appear narrower at the top. A 26.8 went in nicely and doesn't wiggle or rock.

My question is - given steel lugged construction, would you correct this? If so, how?

The two obvious techniques are flaring open the slot with a large screwdriver, or finding/making a shallow taper tool that would be greased and lightly tapped into the opening. I don't think reaming is appropriate.

What you think?

I would use a telescoping hole gauge to see if it gets bigger further down the tube. If it does, I would ream it. But if the post doesn't slip, I'm not sure I can see any reason to do anything.

What's to correct?

From your description, 26.8 seems to be the right size, comfirmed by snug, no rocking fit, with the clamp slot not closing much at the top.

You didn't say, but I gather that it's not slipping anymore. If so, it's another example of reality trumping a published spec.

Not sure if a telescoping gauge is going to be available, but I could make something on the same principle. How large should a hole for a 27.0 seatpost measure?

Well, I doubt the multiple references to 27.0 are all completely in error. The bike was definitely used with a 26.6 post for awhile, so some deformation is expected. I just don't know how much a .2mm difference between the top of the collar and several cm down does to effect post fit over time and stress on the pinch bolt.

But if people think it is going to be fine pinched down .2 at the top, that's okay by me.

I would use the correct sized part.

an undersized post may slip in use, and deform lug in attempts to tighten to stop slipping.

I don’t know the seatlug pinch type or hardware to comment about bringing back to spec. I do like to use steel seatpost of correct size to reform back to spec.

hey you are a pro you should already know

It's never come up before in this subtle a manner.

Because I'm a pro, I ask rather than assume.

you might want to use something like park assembly paste to add a little peace of mind

my custom frame's maker suggested that

I’ve run into this subtle major many times, I’m surprised a pro like you has not.

Maybe fewer of my clients were idiots? I can't say why I haven't seen more of these - probably because the shops were so high end that fewer stolen seats post swaps occurred.

But I am very good, because I ask questions and remember the answers to them. So your attempt to shame me just sounds dumb.

I’d carefully pry the ears open and see if the “right” size slides in easily. If it doesn’t, I’d keep what’s there and call it a day.

Take a set of dial/vernier/digital calipers and measure a few times around but it sounds like 26.8 is the right fit. I am surprised you haven't run into this issue before though, it is pretty common I usually measure a 3-6 seatposts a year at least and sometimes more. Not super often but frequent and common enough. Stein still makes their sizing rods at least of a couple years ago. I would suspect any level of shop would run into someone with a seatpost sizing issue but I guess if you only work on purely high end newer bikes maybe less of an issue?

I think this is a good option. I bought a beat up Mercian bitza that came with a too small seatpost as well - a 26.8 as I recall. A 27.0 post seemed to fit well enough, but I emailed Mercian, who confirmed the seatpost should be the standard 27.2 for the 531 frame. I pulled out a spare 27.2 seatpost, greased everything well, and with a little poking and rocking that post turned out to fit even better than the 27.0 post.

First clean it out with abrasive paper in a slotted rod and a power drill, as far down as you expect the seatpost to go. Then open the slot up slightly with a cold chisel or large flat screwdriver.Then try to gently flare the tube by inserting a scrap seatpost only slightly and levering/turning it in all directions, insert further and repeat until you've reached the bottom of the slot - there should be a slight taper in the top of the tube, good roundness and no projections. Grease generously and try the 27.0 seatpost again. I agree that a reamer has no place in this procedure.

I've run into the wrong size post damaging the frame many times, and have fixed them. The thing that is different here is that the damage is subtle and the next size down fits so well I can see not messing with things. So I'm trying to figure out what does more harm - a slightly wrong post, or re-bending the lug to get back the missing .2mm.

You can use a quill stem (and shims, as necessary) to jack open a seat tube a little more gently and gracefully than prying with a screwdriver. Learned about that here, and it's worked great.

For every .2 mm of the diameter you need to multiply it by pi for the circumference. If you have the wrong post size it can be measured at the clamp.

As far as why yours differs from the publish spec there could be a number of reasons, maybe the spec changed mid run, maybe they used the wrong tube, maybe they put a butted tube backwards, or perhaps the published spec is just wrong.

That is a great idea! I am going to keep it in the memory banks.

I am still unsure if the 26.8 post is slipping. First rule is do no harm, try assembly paste.

These types of threads are hard to truly ascertain completely. Like how deformed are the sear binder ears, as example. I used different methods to open up seat tubes but as I referenced they start with the least intrusive, like post size and assembly compounds. Next up might be careful augering with a "post that's a tool", if I sense an oval lug I might do a light reaming. Or not, running a hone before any real material removal is highly suggested. There's a touch and feel process and "better than before" is sometimes enough. Andy

I have a 27.2 quill tool to take dents out of seat tubes, but I think it's too big to use on anything smaller..

do not use "carbon paste", "gritty grease" or other snake snot. worthless.

mark the post with a sharpie to define what goes in the frame

then in the clamping area (below your sharpie mark) use your automatic center punch to make some divots / indentations on the post to set up some interference, some "bite" when you tighten the binder.

I have the wonderful J Stein knurling tool, works great

/markp
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https://cimg9.ibsrv.net/gimg/bikefor...78c294ac67.jpg

The post isn't slipping, despite the fact that I am 100% sure it is the wrong size by .2mm. The original Strong 27.0 seatpost was likely stolen, and the owner substituted the clamp and seatpost from a mountain bike with a 26.6 diameter, causing the top of the seat tube to distort enough to measure 26.8 at the top and hold a quality 26.8 seatpost well.

I would imagine that if the substitute seatpost was 28.6, the tube would still measure 27.0 with the binder loosened. And if they had used a 26.2 the distortion at the split would be obvious to the eye.


The quandary is whether it will be more damaging to correct it than the potential damage of just leaving it alone and riding. It clamps firmly, but obviously isn't going to be supported inside the tube like a 27.0 would be. However, plenty of people are riding around on bikes with poorly dimensioned seatposts that move easily in the frame, so I don't know if .1 all the way around the bottom of the post matters if I can't feel any movement when I use my mighty gorilla paws to try and wiggle it.

You asked earlier what the id of the seat tube should be. Tbh, that's not a number I keep in my head, so I went and measured a collection of seat tubes. For 27.2 seat tubes, they ranged from 27.32 t(nova) o 27.4 (columbus). I recall Richard Schwinn telling me that .2mm was okay, but my memory is faulty. And I was fixing one of his bikes that broke because the seat tube was too big, I think. If I braved the cold in the garage, I could probably dig up some more seat tubes. I have Deda and True Temper out there. No wonder seat post makers can't decide how big to make seat posts.

I tried measuring with my calipers just to see if I could. I have some Mitutoyo digital calipers, and I failed miserably. My numbers were all too small. My telescoping hole gauge was definitely worth what I paid for it on ebay

The bit of framebuilding lore I have read suggests that .6 and .7 walled tubes distort from brazing heat and require reaming and a seatpost of .2mm smaller than tube spec as a result. But thicker butted tubes, like double butted 9/6/9, take a seatpost at nominal tube dimension since they don't distort as much. 28.6 -.9 -.9 = 26.8 seatpost, rather than 26.6.

Long story short, for 9/6 or 10/7 tubes, the pre-braze dimensions are generous compared to thicker walled stuff, so you need to measure the frame itself.

Nice tool but I almost chocked at the price https://steintool.com/portfolio-items/knurling-tool/

You're overthinking this. You said you've repaired frames with worse damage. Therefore, repairing this very minor damage won't hurt anything.

I would try to fit the "correct" post, but it doesn't really matter. There's enough variability in seatpost OD and seat tube ID that a post that's "wrong" by 0.2mm may also work. As long as it doesn't slip and you're not deforming the ears when you clamp it down, it's fine.