Seatpost sizing worries on British made Proteus frame

[Wandering_Lark]

Hi - I have what I understand to be a British made Proteus frame (photo below - maybe I'll post properly about the frame separately as I'd love to know more about it.) and I'd love some help around the seatpost diameter.
TLDR: It came with 27.2 which was hell to remove or put back greased
bought 27.0 also hard to put in thuough in an uneven way.
will I break my frame if the 27.0 is actually too small and its just that the lug is warped/ deformed?

More detail:
right now I have an issue with the seatpost. I needed to adust it and it was extremely hard - but not impossible - to move. I finally got it out (by bracing a foot against the bottom of the triangle and corkscrewing for about 20 minutes). No aparent corrosion. the post was a modern alloy 27.2 that I assume the previous owner had put in.
I greased this up and tried putting it back - fet like hell so I stopped and tried with another post I have that's the same size - also hell.
also tried gently prying the slit open a bit - no help.
I wondered about a warped lug?? (saw that on a forum)
Anyway I ordered a 27.0 post
This is hard to push in at the top and then fairly smooth for about 150mm and then seems to hit somethingthing jaggy inside the tube that makes it very hard to move (and scratches the post? or maybe thats a jaggy bit where the seat and toptubes meet - also the jaws of the clamp slit scratch the hell out of posts)

I would just force the 27.0 in and call it a day BUT I'm worried that if this IS too small will the 0.2mm play cause damage to the frame over time? (I'm a faily llight rider about 75kg incase that factors in)

I guess I'm asking what are the possible causes of all this awful and will I break the frame using a 27.0 if its really 27.2 but the lug at the clamp is defrmed or some other pokery.

Thank you

 

[Nob]

I’d get the seat tube reamed so it’s properly circular - a frame builder should be able to do that for you or an engineering shop start with a 27mm reamer then try the seatpost then only if you are still forcing it put a 27.2mm reamer down. You won’t damage the frame this way. You could also emery the seatpost as another option but measure the diameters of seatpost & internal diameter of the seat tube as your starting point.

 

[bikeworkshop]

Because the post fits unevenly, the internal bore is not cylindrical.

So as @Nob recommends, reaming is essential (unless the tube is too bent☠)

Old school bike shops should have a 27.2 reamer.
If the 27.2 post goes in and out at all, that is probably the original, intended size.
A 27.2 post won't go into a 27.0 seat tube.

If the (27.0) post is tight though, and the clamping slot/ lugs aren't distorted by adequate clamping to stop post slip, then you're unlikely to do any further damage.

Seat tubes were usually only reamed from new to a certain depth, and seat posts used to insert only 4-5 inches or so max, so sometimes a longer modern post will jam in an older frame surprisingly quickly.

The seat tube is often bent by a larger (height&girth) rider having too few inches of post in the seat tube, and too much leverage on the saddle.
Sometimes you can see this bend.

Then also the seat cluster can be distorted by stress and fatigue.

I think you've got all 3 here - but cleaned up if it looks ok it should be ok, as long as any bend isn't too great.

If the seat tube is bent though, @Nob 's suggestion of sanding the seatpost down might be a necessity.

 

[Nabeaquam]

I’d get the seat tube reamed so it’s properly circular - a frame builder should be able to do that for you or an engineering shop start with a 27mm reamer then try the seatpost then only if you are still forcing it put a 27.2mm reamer down. You won’t damage the frame this way. You could also emery the seatpost as another option but measure the diameters of seatpost & internal diameter of the seat tube as your starting point.

I’ve made my own reamer out of a much smaller seat post. It easily fit inside the seat tube. I put a threaded rod through the seat post held with jam nuts at both ends. I wrapped 80 grit sand paper around the seat post reamer, held in place with spray on contact glue. I used an electric drill to power the reamer. Up and down and I used a lot of oil. As the sandpaper wore out, I put more on again and again. Eventually I was able to put on two layers. This took all afternoon and 2-4 sheets of sandpaper that I cut to fit the homemade seat post reamer with old scissors. There was a tight spot, from a slight bend, part way in that I was able to eventually ream out. You could still feel the ripple in the tube but I got enough out to use the bike. You can’t see the ripple, it takes hardly any tubing distortion to make telescoping parts useless. This was 10 years ago and I still have the bike. I’ve done the same thing on fork tubes that were distorted from over tightening the quill jam wedge. Not fun but what else can you do to eat up time and have cheap fun when you retired. I could watch TV but I’ve been retired 16 years and nothing good has been on TV in all that time. So, file, grind and hammer away.

 

[jim haseltine]

My SOP for a new-to-me frame is to firstly run a round file vertically down the inside of the seat tube against the clamping slot to chamfer the edges of the slot. I also never, ever, use grease on an alloy seat pin, I always use a medium or heavy oil because I've seen the wrong sort of grease react in the alloy/steel interface and jam a pillar.

 

[Nob]

I’ve made my own reamer out of a much smaller seat post. It easily fit inside the seat tube. I put a threaded rod through the seat post held with jam nuts at both ends. I wrapped 80 grit sand paper around the seat post reamer, held in place with spray on contact glue. I used an electric drill to power the reamer. Up and down and I used a lot of oil. As the sandpaper wore out, I put more on again and again. Eventually I was able to put on two layers. This took all afternoon and 2-4 sheets of sandpaper that I cut to fit the homemade seat post reamer with old scissors. There was a tight spot, from a slight bend, part way in that I was able to eventually ream out. You could still feel the ripple in the tube but I got enough out to use the bike. You can’t see the ripple, it takes hardly any tubing distortion to make telescoping parts useless. This was 10 years ago and I still have the bike. I’ve done the same thing on fork tubes that were distorted from over tightening the quill jam wedge. Not fun but what else can you do to eat up time and have cheap fun when you retired. I could watch TV but I’ve been retired 16 years and nothing good has been on TV in all that time. So, file, grind and hammer away.

You should have been on Blue Peter……

Of course you can use a honing brush on a cordless drill for the seat tube…

 

[Nabeaquam]

You should have been on Blue Peter……

Of course you can use a honing brush on a cordless drill for the seat tube…

I had to take way too much off for a brush. It took hours.

 

[Wandering_Lark]

This is all amazing info - I love this forum!

Sounds like it maybe that it's not reamed deepenough in and with the 27.0 I'm hitting the unreamed section.

Also sounds like it may have warped or wabbled? but also might just be rough inside.

Am I right in thinking that as long as I can tighten up the 27.0 in the tube and it seems resonably snug in as its going in that for now this will be ok and not mash the frame?

I can tidy up the rough bits I can reach and cut the seat tube so it doesn't hit the unreamed section for now? Just until I can afford to get a frame builder to ream it to 27.2.

Also any tips on London frame builders to approach much appreciated.

 

[hamster]

I would go and talk to Condor, a proper shop and framebuilder.

 

[Nabeaquam]

Should be OK with a 27. As long as it doesn’t move or twist when riding and cranking. You can always do a yooper fix on an ill fitting post.