noobinsf 

I just picked up this frame, and I am trying to figure out the seat post size. I have this random 25.x post that gives me different readings depending on where I put the calipers, but the most consistent seems to be 25.5-25.6, and it seems to be nearly the right size, though a smidge too small. Does anyone know for sure?





QR clamp is closed in this pic

Not butted?

johnnyace 

Unfortunately, the Laprade seat post (original) on my '84 Alpina Uno is worn where the sized would be, so I can't offer a definitive answer. It is made with the same hi-ten chromoly, so I would guess not butted.

noobinsf 

Oooh, could you maybe measure the upper part, just for a ball park? At least you’re starting with the stock post.

cheffyjay

well, my alpina uno has a 26.6 but I don't know what year it is

T-Mar

A post diameter of 25.6mm is very small for a CrMo frame. It's right in the middle of the the range of the two most common hi-tensile sizes, 25.4mm and 25.8mm. The tubing decal does not mention butting, so presumably it is plain gauge. Most Univega of this era were contract manufactured by Miyata, whose plain gauge CrMo set tube typically used a 26.8mm post. Based on the Taiwanese origin, this one obviously isn't Miyata sourced. The most common Asian plain gauge CrMo seat tube during this era was Tange #5, which typically used a 26.6mm seat post.

However, that's significantly larger than 25.6mm and the seat tube cinch slot would be severely pinched at the top, which is not evident in the photo. Clean off the jaws of your calipers, close them, re-zero the setting and measure again, making sure that it's on a clean, unmarred section of the seat post.

johnnyace 

Yup, I get 26.6mm.

noobinsf 

What’s weirder now is that I measure the outer diameter of the seat tube at 28.6 with a wall thickness of 1.4-1.5, so the inner diameter must be 25.8 or 25.6 (outer diameter minus wall thickness x2). I validated the caliper accuracy with a measuring tape. There is no apparent damage to the seat tube, no evidence of past over tightening. Dunno — I’ll just have to get something that fits, I guess, like a 25.8 or thereabouts.



QR clamp is open here, no tension, no apparent damage.



Here is a 26.8 seat post I had on hand for reference. It has a small bevel on the bottom, and that is the only part that fits into the opening.

noobinsf 

Okay, I think the mystery is solved, now that I tried to fit the 22.2 stem I was planning to use into the steerer, which didn't fit, and then noticed that the head tube looked pretty broad. It looks like these are the measurements:

Steerer tube: 25.4 outer diameter
Stem (steerer inner diameter): about 21.1
Crown race: 26.4
Head tube inner diameter: 32.7

These dimensions, plus a 25.4 seat post, are common to... wait for it... BMX!

Here is some data from Saint Sheldon's headset crib sheet, emphasis mine

Standard: BMX/O.P.C. bikes
Frame Cup Outside Diameter: 32.6 mm (varies)
Crown race Inside diameter: 26.4 mm (varies)
Steerer O.D in mm: 25.4 mm
Threads Per inch: 24
Stem diameter, Steerer I.D.: 21.15 mm (.833")
Notes: Used mainly on bicycles with one-piece cranks, also some early mountain bikes.

noobinsf 

Just confirming, now that I have the headset mounted with a stem adapter. BMX headset fit like a glove, as did the .833 stem adapter. Still haven’t bought a seatpost, but soon… Gratuitous shot of wipe down and wax…

noobinsf 

I determined that 25.8mm is the right size, but the post I bought is too short by at least a cm.

While I wait for the new, longer seat post, I am focusing on the wheels the same guy sold me. They started out looking pretty ratty, but they spun smooth when I tried them and the rims looked hardly used, so I figured they would be worth the effort.

noobinsf 

Closer inspection revealed that they’re Cyclone, but the cartridge bearing model.

noobinsf 

Spot test with wet sanding is promising:

zandoval 

26.6 v/s 25.8 v/s 25.4... Much different sizes for what you would think is a common frame. I had the same problem with my 83 UNIVEGA Gran. It was supposed to be 26.6 but I had a hard time fitting the post. I eventually took it down to the machine shop and had it turned down to 26.4. A perfect fit.

I have quit trying to reason with seat and stem post standard sizes on vintage bicycles. I just get something close but a little larger and have it turned down to what I need. I determine the post size by adding tape around the end of a smaller pipe till I get a snug fit. Then I measure the diameter of the tape. I don't trust just the diameter of the receiving tube at its head. Also... I don't remove more than a millimeter of the thickness of the posts/stems. This technique has been especially helpful with French frames.

noobinsf 

I ended up setting my calipers to 25.8 and gently rotating them inside the tube with the clamp undone, and I got consistent light contact all the way around. I went with 25.8 as the first test, and it fit like a glove. (But, I was attracted to a NOS Sakae seat post and bought it without checking the length, and it is just a bit too short for me, so now I have a longer Kalloy post on the way.) I've been lucky so far with seat posts on the bikes I've had -- this was the first one that brought a little confusion.

noobinsf 

More work to be done on this one, but I finally had a free day to connect everything and make it roadworthy.

noobinsf 

This bike took a back seat for a while because the 21.13mm threadless stem adapter I found just didn’t cut it — I could not get the quill nor the stem cinched enough to stop them from rotating at the slightest bump. Luckily, I found this riser stem with exactly the same dimensions as the threadless stem that fit well.

Now that my kiddos are riding their own bikes with no training wheels, I can no longer walk alongside to keep up, so I had to up the urgency on this ideal dad pursuit bike. After an afternoon of adjustments and fine tuning (as well as a couple of cork spacers to get this awful basket to stop rattling), version 1.0 is finally ready to go. Also will be good for Sunday morning bagel runs.

tgot 

Looks good!

Can you make it back from the bagel shop without spilling the coffee on the SF hills?

KHutson

I would just like to pause and say look how cool the fork is! Love that 'biplane' style, I have a keirin bike with that similar style fork.

noobinsf 

Thanks, yes, it was one of the main factors in making me jump at the frame!

john903

That is looking great. This is something I am trying to put together as well. I looked but may have missed it. What handle bars are those?
Have fun.

noobinsf 

Thanks! These are Nitto Albatross bars. To my knowledge, the Soma Oxford bar is nearly identical, so that is another option.

merziac

Great score, so cool.

Nowadays, any biplane fork and or frame is worth the effort.

We have a later non biplane step through one of these for my sons girlfriend, also red, one of the cutest bikes in the fleet.

And yes it ended up with 700c fenders.