&@$#%& Seatpost! No, not stuck like one would think.

I am in the midst of rehabbing the Univega Viva Sport I picked up earlier this Spring, giving it a total freshening. Now to dial in my fit. Went to adjust seat height, so I had to really lean on the 5mm Allen binder bolt but thankfully it loosened - and the seatpost dropped right down. Wow!, I say, not stuck. So I pull it up a tad and it comes right out. Someone had chopped it off right at the max height line! So now there's not enough post to grab with the clamp when adjusted to the height I want. Now I gotta find a new seatpost!!!! Grrrr!!!!

I grabbed my calipers and measured the old post at 1.051". by my calculations that works out to be 26.7mm So I go looking and see 26.6 and 26.8, but no 26.7. With as loose as the post was since it dropped down on it's own weight I think I am safe to assume that 26.7 is a bit small.

Any of you gurus out there know what size seatpost I should be looking for? Bike is '84 Univega Viva Sport, Miyata-built frame, with double-butted CrMo main tubes.

Thanks in advance!

Your Viva Sport sounds just like the one my wife rode when we were first dating, that one had a 26.8 seatpost. I know this because I upgraded hers to a Thomson for better seat angle adjustment. I saved the Thomson when I sold her Viva Sport to her cousin, I put the stock seatpost back and now the Thomson is on my '88 Schwinn Cimarron LE. I did check Sheldon Brown's seatpost database but the Viva Sport was not listed. That's always a good place to check for seatpost size questions.

Good luck!

-D-

When I made seat posts, the outside diameter of a 26.8 was 26.75 and the seat tube was supposed to be reamed to 26.8. This gives 0.025 (0.001") clearance all around to allow insertion. The difference between your measurement of 26.7 and 26.75 is within reading error if you are using calipers.

Miyata (who made most if not all Univegas) did actually have certain models that used a 26.7 size post. It's a size that only they used, AFAIK. This topic has come up before and the usual prescription is to ream the seat tube so it takes a more-common 26.8.

I don't know what the factory spec seatpost size was. 26.8 is probably correct, or possibly even 27.0 or so. The reason I say that is because how tight the PO had cranked the binder bolt to hold the seatpost, and the way the seatpost dropped right down under it's own weight when I loosened the binder bolt - no seat on the seatpost either.. I've NEVER had a seatpost do that before! I mean the one in there that I removed could have been a slightly too-small seatpost from his parts bin. I'm hesitant to measure the seatpost opening with calipers since it may have been distorted by the seatpost binder bolt.

So, maybe the question I should have asked last night was: What is the factory seatpost size for a '84 Miyata/Univega with CrMo Double butted tubes???

I see from the '84 Miyata catalog that their CrMo double-butted tubing has wall thickness of 0.9/0.6/0.9mm. So would I be safe to measure the OD of my seat tube and subtract 1.8mm to get the ID?? Or did they do a finish-reaming of the seatpost tube to accept a certain spec-size seatpost?? What about paint adding thickness... Aaaauuuggghhh!!!

I never had this problem when I changed seatposts on my Fuji years ago - because I bought that bike new so I knew no parts were replaced with wrong-size parts before I bought it.

Yes, you can assume that the unfinished seat tube ID should correspond to the OD minus the wall thickness and then minus a fraction (usually .02mm) to allow for the sliding interference fit required. It's safe to assume that the manufacturer (or the shop that did final assembly) did some reaming of the ID to insure that the seatpost actually did fit and to remove any paint or other foreign matter inside the tube.
The best bet (if you don't find the confirmation from Sheldon or other internet published spec and can't wait for somebody here to give you 100% positive info): take it to an LBS that has a seatpost sizer gauge or use a few marked, known-sized posts in the age-old "go/no-go" test, but keep them greased and don't force anything that resists.
Cut to the chase: 90% certain that the correct size in a still-round seat tube opening will be either 26.7, 26.8 or 27.0 and between these 26.8 and 27.0 are the most common for Japanese seat tubing of this era...have to throw in the oddball 26.7 just cause it is a Miyata/Univega.

OK, went out and measured the OD of the seat tube and got 1.127" = 28.62mm

The wall thickness is supposedly .9mm, so .9 x 2 = 1.8mm

28.62 - 1.8 = 26.82mm for tubing ID


I also tried my hand at measuring the ID with the calipers in several points around the opening and came up with an average of 1.057" = 26.85mm


So, both methods agree that the correct seatpost is 26.8mm.

Problem solved. Now to find a decent seatpost! Any suggestions???

OP, I'll trade my Trek 520 with the totally stuck seatpost for your Univega.
As for a new seat post, your LBS should be able to provide what you need for under $20. While that diameter is not common today, it's not exactly rare either.

The 'chain' LBS near me looks at me like I'm from another planet when I say I ride lugged steel, ride on 27" wheels, and still use freewheels. If it isn't CF with brifters, they lose all interest in making a sale. They don't even stock any 27" tires - they have to be 'special-ordered'. No components - they have to be 'special-ordered'.. They have bikes, clothing and limited accessories, period. If that is what I have to put up with, I'm better served to avoid the LBS scene altogether and order stuff myself. The only LBS around here that had any 27" tires or wheels only had the cheapest generic Chinese crapola, and they went out of business last year. Of the three LBSs within 18 miles, none have a used parts bin - they'd rather sell on ebay.

I will be checking out a different (and independent) store tomorrow in a college town 20 miles away... I'm not holding my breath.

26.8 My 83 Univega Sportour has that size and yours should be too. Good luck with the search. Did the seatpost that was chopped have the odd rail attachments, kind of like a cross between a modern post and the old clamps attached to the side ?

I came up with 26.8, 26.7 with my dialed calipers. I got a 26.8 seatpost worked perfect.. From your readings, I would get a 26.8 seatpost.f

Here is the one I got, kind of cheap, but it turned out perfect.. a black 26.8 seatpost.

https://www.ebay.com/itm/300789520168...84.m1423.l2649

I put it on this..

https://www.cehoward.net/gpyy56.jpg

The Internet is your friend; but if you find a local LBS that stocks what you need, more power to them. The best ones, I've found, are the "Recycled Bicycles" type of shops, that are doing the same thing you're doing, as a business. In addition to vintage-compatible new parts, they can be a gold mine for actual vintage used parts.

Yes, when not clamped it should be smooth slip-fit, requiring hand force to raise and lower. I'm going to guess you are two steps low, but it's just a guess.

I would caution: problem is not solved. Possible solution has been identified, and needs to be tested.

ooooooo.

Agree with Roadtire, very nice.

Thanks much, that Grand Prix is one of my prizes.. Would you believe that this bike was $50 find off Ebay..Oh, it has since passed through the hands of Robbie Tunes, and you can see what his magic did to it.. Come tell, I got another 85 Grand Prix, and that one is being built too.

Back to the $15 black 26.8 seatpost. It was perfect for the Grand Prix.. It was all black which really stands out, and the price of $15 shipped.. Come on now, that was Christmas!!