The_Joe

I bought a vintage frame that I thought was Columbus SL. It won't easily take a 27.2 post so I'm thinking it may be the beefier SP. Now, it's only a 55cm frame and I'm barely 160lbs. Knowing that SP is just a slightly thicker walled.version of SL, would.it be terribly unwise to try and hone/ream it larger?
My eyes think maybe it's a bit pinched. Cheap calipers show it a bit over 27 but not 27.2. A 27 seatpost feels sloppy. I figure on a 40 year old I should probably run a flex hone down there anyway just to make sure it's clean.

So, if it were SP and I decided to ream it out .2mm wouldn't that just bring to the safe thickness of SL? Crazy?

Nessism

It's most likely SL, with the tube pinched in at the top. I'd use a reamer to clean it up. Not a dingoball hone. And if it is SP, a reamer will easily open it up to 27.2. The metal is pretty soft.

Andrew R Stewart 

The concern is where is the seat tube/lug deformed (presumably where will the reamer take material off at) and how thick a wall is there at that spot. What I might do is to run an adjustable reamer in the seat tube expanding it till it just begins to nick off some tube. See where this first contact is WRT the tube's walls below the lug and make a judgement of post fit size against wall thickness.

I suggest chamfering the binder slot's interior edges to reduce their catching on a reamer blade. Flex honing won't remove much material compared to a reamer. An interesting data point can be found sometimes with this adjustable reamer set to just touch method as one can slide the reamer deeper into the seat tube and past the seat binder/lug area. Then one really begins to see what their seat tube might be ID wise before the brazing.

I agree with Nessism in the tube likely being SL. Andy

The_Joe

Thank you both! Very informative.

unterhausen

You can get a telescoping gauge and check the dimension at various radial directions. It could well be pinched. I would try to fix that first. Can't fix it without measuring.

I suggest finding a Starrett 579C used somewhere. I bought a cheap set on amazon and regret it.

bulgie 

I strongly suggest trying to remove as little metal as possible. On a frame where the ears have been bent in from mounting a too-small post, you need to gently bend the ears outward befor reaming. I like to bend them just enough that the reamer isn't touching there, or touches barely. After doing that, you may find that the reamer has very little metal to remove elsewhere.

Another advantage of bending the ears out is it can prevent the known issue of the reamer blade catching in the slit. That problem can also be avoided by using a spiral aka helical reamer, but those are more expensive and hard to find in an adjustable reamer. Most bike shops and home-mechanics, if they have a reamer, it'll be a straight-blade style, that tends to hang up in the slit unless you deal with it one way or another.

Mark Beaver

I’ve had pretty good luck spreading the ears of a seat lug that’s been compressed by folks using a too-small seatpost; by using a 1 1/8” ie 25.4 diameter quill stem, strategically locating it where the metal needs to be bent out, tighten stem, relocate slightly, repeat repeat. Works well if you’re careful.

Mark Beaver
Halifax, NS, Canada

BTinNYC 

So obvious...now that you've told me!

I'm on the verge of opening a 22mm Peugeot to 22.2 and did not/could not figure out how to manage the binder slot.

repechage

I purchased a 51 year old Colnago in a 49 cm size and it had a 27.0 post, I really doubted it was SP. measured below the joinery and corrected it with a reamer. I did have to open up the lug ears a wee bit.

in 1975 I ordered a track bike with PS tubing, bought all the parts I would require including a 27.0 seatpost… “we reamed it out to 27.2 for you” Grumble.

50PlusCycling

Rather than ream out the frame, I would machine the 27.2 seat post down until it fits properly. Seat posts are more easily replaceable than frames.

t2p

the seat tube on this frame was reamed from 26.6 to 26.8 (to accommodate greater selection of seat posts)

frame builder did this - been a while but I believe he did first check / determine there would be little / no chance of increased failure due to the change … the bike had a number of years use after this (no issues)

this frame 26.6 seat tube id - the next year the similar frame seat tube id was 26.8

have a road bike with Columbus SL / SP (seat) tube mix - can’t recall seat tube id - and if was reamed to 27.2 if it was 27.0 … (curious - will need to check)

The_Joe

I'm actually using a vintage Shimano seat post in very good condition so I'd rather not turn that down, as silly as.it sounds.

Either way, I was able to open the ears a bit and run a quick cylinder hone to clean up some gunk. 27.2 fits perfectly now.