[chiefsilverback]

I won't mention brands, but I have a frame and dropper combo where the 30.9mm diameter seat post feels like a near friction fit in the 30.9mm seat tube. The O/D of the post is relatively easy to measure and it comes in right around 30.89-30.9mm, but the seat tube appears to be exactly the same O/D, or maybe slightly undersized, and also feels very rough on the inside as if maybe it wasn't reamed to it's finished I/D?

Which part should be the actual diameter, and which should have the 'tolerance'? Should the tube be reamed a couple of hundredths of a millimetre over the nominal size, or should the post actually measure a couple of hundredths under?

 

[lvdukerider]

I'd borrow another 30.9 seat post or two just to see how they fit. That would determine if it's the seatpost or the frame, before I attempted to permanently modify my frame.

 

[Cleared2land]

That would be my advice too

 

[006_007]

Also confirm where the tight fit is. 1" down? 2" down? Etc.

Could be where the top tube is welded to the seat tube it distorted the tube slightly making a real tight fit.

If this is a new frame consult the manufacturer before honing the seat tube.

 

[TandemBear]

I'd borrow another 30.9 seat post or two just to see how they fit. That would determine if it's the seatpost or the frame, before I attempted to permanently modify my frame.

Seat posts diameters vary considerably, despite being stamped with the same dimension. Sounds like you're using calipers, so you can reference the ID and OD somewhat accurately - or at least make solid comparisons.

Before reaming, I'd suggest flex honing the seat tube. This will remove any rough spots or burs within the tube. Honing doesn't really change tube diameter, but just smooths surfaces, whereas "reaming" does indeed change diameter. I'd follow up a reaming with honing if it were my frame. Also, once you ream the tube to a new ID, you can never go back! And there's less material there as a result. So approach with caution!

You failed to mention what material we're talking about here. So before doing anything, consider the material and take appropriate corrective action.

Also, doing anything to the frame yourself may put any future warranty claims in jeopardy. For this reason, I'd refer you to the shop who sold you the bike or an authorized dealer for the brand. If there isn't an LBS in the picture, then contact the manufacturer directly.

Given the choice, I'd take a too-tight seat post issue before a loose "slipping" seat post any day!

Good luck!

PS Oh, one more thing! If this is a metal frame, then check your clamping area. I've seen lots of frames where the seat binder bolt has been over tightened in the past (due to various factors, one being incorrectly small seat post usage) and permanently deformed the top edges/corners of the seat tube where the slot is cut. Be sure this isn't your problem. Be sure there's no issue here before assuming the entire ID is too tight for your post. I've bent many a seat tube back out to its original position and found seat posts no longer bind or get marked up by the tube ends & edges.
If a carbon frame, this obviously does not apply.

 

[bmwpowere36m3]

I haven’t run into this problem across multiple carbon bikes, sure some are tighter fit than others. However, I did need to cleanup up some overspray, runs and burrs on the RM blizzard fatty I bought this winter…


The fit was terrible, ok for a rigid post… but not good for a dropper. Ended up using a Dremel with mini sanding drum to cleanup the ID.

 

[chiefsilverback]

I’m not planning to ream anything yet, my question is whether seat tubes are normally reamed as part of the manufacturing process,or do manufacturers tend to rely on their tube supplier providing stock with the correct I/D?

For reference this is the body of my dropper post having removed it from the frame today…

 

[Roge]

Yeah, thats way too much persuasion to remove the post. As TandemBear said, use a hone. You can get a brake cylinder hone from an auto parts store for under $10.

 

[chiefsilverback]

I just noticed in the "related threads" below this one I see a question about 27.2 posts, and within it suggests the nominal ID on a seat tube should be 0.2mm over the 'standard size", to give 0.1mm clearance all around the seat post? So a 27.2 tube should have a 27.4 ID and presumably a 30.9 should be 31mm?

I do have an email in to the frame manufacturer.

 

[Mountainfrog]

Yeah, thats way too much persuasion to remove the post. As TandemBear said, use a hone. You can get a brake cylinder hone from an auto parts store for under $10.

I've honed more than a few in the 90's. The three shoe brake hone worked rather well for me.
I plungered them to give a consistent cross hatch. Cleaned up and lightly greased. No more wobbly, tight, or boogered posts.

 

[Cleared2land]

A brake hone is a good idea that I would not have thought about!

 

[bmwpowere36m3]

That’s how the stock post looked after first removal out the Blizzard… for me, the problem was obvious and mostly near the clamp. Not deep down the seat tube.

Ball or flex hone are good ideas if the general ID needs to be cleaned and opened up.

 

[chiefsilverback]

Presumably a 1.25" flap wheel would work, if such a thing existed(I can find 1", 1.2" and 1.5" but not 1.25")?

 

[Mountainfrog]

Presumably a 1.25" flap wheel would work, if such a thing existed(I can find 1", 1.2" and 1.5" but not 1.25")?

On the the top edge of the clamping area then maybe yes, a hone does a cleaner job overall however, it only takes a small amount in most cases.

 

[TandemBear]

A brake hone is a good idea that I would not have thought about!

This is excellent advice because when I procured a hone for 27.2 seat tubes, it was really expensive at $50 or something. That said, you do want a quality product that performs as intended by properly smoothing the inside without removing too much material. In other words, one that utilizes the proper grit abrasive that when used with appropriate honing oil, does the trick.

I'll add that in addition to my comments above I've also addressed many a seat tube/seat collar area where very rough edges and burs were left. (High volume production simply cannot address these issues, I've found.) Again, this refers to metal frames, so I would use a fine metal file to round all edges at the seat collar and tube slot, then sand with appropriate silicon carbide sandpaper. Thereafter I'd hone to achieve excellent results. No more marred, scratched and mangled seat posts. Proper fit, clamping and smooth insertion and removal. What a joy!

Must say, I am not familiar with the appropriate approach to carbon seat tubes. Time to bone up on that! That said, I highly doubt it's much different. Mechanical high-grit abrasion is almost certainly the solution here. Otherwise whatcha gonna do? Heat and epoxy? Heck no! Again, this where the manufacturer would come in and offer appropriate advice.

 

[TandemBear]

Presumably a 1.25" flap wheel would work, if such a thing existed(I can find 1", 1.2" and 1.5" but not 1.25")?

Not in my book. I also wouldn't recommend a Dremel with sanding drum or abrasive wheel. There's simply not enough precision in these tools. And that advice applies to metal frames that don't abrade nearly as fast as carbon would. NO WAY would I use ANY of these on a carbon frame. You could easily burn through the wall diameter, thereby rendering your frame done and void, functionally and warranty-wise. Sure, it could be reparable, but if so, just go to a carbon expert PRIOR to doing damage with a Dremel or rotary tool in the first place!

"Do no harm" also applies to bikes - especially carbon frames!!!!

 

[chiefsilverback]

I'll add that in addition to my comments above I've also addressed many a seat tube/seat collar area where very rough edges and burs were left.

On this particular frame it's quite smooth around the top of the seattube, but as I reach my finger down inside i can feel it gets much rougher.

If I do have to go the hone/sand/abrasive route then I guess the challenge will be finding something to go deep enough into the tube...

 

[chiefsilverback]

NO WAY would I use ANY of these on a carbon frame.

For reference, it's an aluminium frame, with pretty beefy wall thickness.

 

[TandemBear]

One more comment...

The condition of your seat post could be from a totally other cause: galvanic corrosion. Unfortunately, this issue has reared its ugly head with carbon frames and dropper posts in my recent experience. Although the picture the OP provided doesn't indicate this, it's still a possible cause. Once you get corrosion between the two materials, removal is difficult or impossible. And if you do remove it, the corrosion removes the anodizing and mars the seat post considerably.

So this needs to be considered as well.

 

[TandemBear]

On this particular frame it's quite smooth around the top of the seattube, but as I reach my finger down inside i can feel it gets much rougher.

If I do have to go the hone/sand/abrasive route then I guess the challenge will be finding something to go deep enough into the tube...

FYI (so much for "one more comment"), I find the standard hones to be too short for many seat tubes. For this reason, I use a drill bit extender on my hone and it allows full insertion into the seat tube for thorough honing of the entire tube. This isn't necessarily needed for your situation, but it sure has worked wonders for me to remove surface corrosion from steel frames in the past. (Just don't mangle the hone on the bottle bosses if there are any.)