Seat tube sizing
 

Hi folks, I picked up '74 Sekine a few weeks back and have been planning on swapping out the seat tube. Current one has a separate clamp, and just want something something easier to adjust and a bit slicker looking. But I'm having a bit of trouble with sizing.

The existing tube is marked as a 25.4, and measuring it this morning with a micrometer puts it pretty much at exactly that. Digging around earlier posts on this site, it looks like the tube should be closer to 26.4 - 26.8 in sizing. Looking at the top rear of the tube, at the gap at the back it looks like it's been "squeezed" together to accommodate the smaller post. (Can't post photos unfortunately!)

I guess my question is what the best course of action is here. I was planning on trying to stretch the opening out so the gap at the back of the tube is roughly parallel, and go to the LBS to see what the tube size really is. But not sure if having such a smaller post in there would mean about the integrity of the tube. Bolts and fittings are quite stiff so want to know what I'm doing before trying to wrench things apart. With regards to replacement posts, ~26.6 seems like an easier and cheaper size to come by - and would be "correct", not that that matters too much.

Thanks for any advice!

are you meaning that you have a steel/chrome seat post?

Seems like steel to me

99% sure you have a chrome post happening there, but stick a magnet to it if you would

chrome posts are always smaller in diameter than alloy. can't say for sure without seeing it, but it's likely safe to go ahead wedge open the seat post collar....that gap in the seat tube. use something with a broad edge...broader than like a flat head screw driver. i'd use a chisel. anyway, get the gap to open enough to remove the post. then you can set about opening the gap proper like to get a correctly sized post in

edit: i should say....a good way to go about using said chisel is not to pry so much as twist to get the gap open. get the chisel head in the gap and clamp the handle with some pliers. use the pliers to twist the chisel thereby forcing the gap open. try best as you can to keep the chisel head flush to the post stuck in there so the edge of the chisel doesn't pop out of the gap and mar up the metal/paint

[MENTION=528872]aker[/MENTION] - I was initially confused with your post until [MENTION=175113]thook[/MENTION] got it. Terms in the bike world are always fun. We call the tube you refer to as a seat post but we sit on the saddle! The post is often referred to as a post but also as a pillar. I think a subtle difference is that if there is a clamp that clamps the pillar and the saddle rails then it is a pillar.
https://live.staticflickr.com/2832/1...82005f8b_b.jpgOLYMPUS DIGITAL CAMERA on Flickr

The modern or even more recent design includes the saddle rail clamps as part of the post.
https://live.staticflickr.com/65535/...26898174_b.jpgP1040421 on Flickr

We are also guilty of calling the fork "forks" when there is only one. Also, the rear block or cluster of gears are referred to as cogs when they are really sprockets. Cogs are gears with interlocking teeth like in a planetary gear set found in IGH hub.

Seat post colors are usually found on newer bikes. the part you need to spread is called the seat lug (saddle lug?). The slot is created after the frame is assembled. thook is describing applying the chisel edge or screw driver edge against the post and in the slot. Most chisels I have have an edge that is tapered toward a sharp edge at a very steep angle. Screw drivers are flat on the edge and would provide the best engagement with the edges of the slot. Be gentle.

If you can have someone pull up while you "Open" the gap, it might slide out easier. Use a penetrating lubricant to help, In a pinch you can use WD-40, otherwise Liquide Wrench, PB Blaster or KROIL will work too.

Once you get a correct post/pillar for it, lubricate before inserting to prevent corrosion between the AL and steel. Not all pillars are steel. the one in the first picture is AL.

"forks"...haha! it always gives me a chuckle when folks call it that

wood chisels have sharper/flatter edge than metal chisels. though. i suppose i could've mentioned that :)

i do agree a flat head driver, could work fine. but, in my own experience with prying delicate broader surfaces apart, the broader the edge the of a prying tool seems to work better without as much slipping or marring. whatever works..and whatever you have...certainly

kroil is awesome!!!!....if you can find it...lol. nowhere around here locally has it, though. gotta get it online. and, it's pricey. because i've worked on vehicles so much, mixing ATF with acetone is a near substitute. just an fyi ;)

My bad about the parts confusion - typing too fast and clearly got mixed up!

The setup is like SJX426's first photo. The issue is that the two lugs at the back are pinched together to fit the smaller post. The seatpost itself seems to be alloy in the end.

I'll try and pry open the gap as you've both suggested. I've also read about sliding a coin in the gap and prying it open by pressing against it with the binder bolt - slow and steady in any case, and we'll see how it goes. I'll dig up a seatpost closer to the "right" size and see how it fits in the tube once the lugs are a bit more open. Will post an update - and hopefully will have enough posts to attach a photo by then!

Thanks all

rock'n'roll!!!!

The OP's gallery shows a red frame having full length brake cable housing secured to the frame via brake cable clamps. Given 1974, that would mean the bicycle is either an SHS or SHT, both which used the same frame. The frame tubing was Tange Champion double butted CrMo which should use a 26.6mm seat post. If you want confirmation on the model, post a full, drive side picture of the bicycle.

As noted, gently pry the cinch slot open using a wide, flat bladed tool, until it is slightly wider at the top of the slot, than the bottom. The inside of the seat tube should also be resurfaced to remove any burrs or marring created by the under size post. Ideally you would use a reamer or hone but a rat tail file and sandpaper will also work. Do this with the frame upside down, so any removed material does not fall down, into the bottom bracket. Clean out with a lightly oiled cloth to remove any remaining debris. As noted, lubricate the new post with some light grease before insertion. This will prevent the new post from seizing in the seat tube, which is a big issue with aluminum seat posts in steel frames.

https://cimg5.ibsrv.net/gimg/bikefor...45632e80e1.jpg
https://cimg7.ibsrv.net/gimg/bikefor...33895a8b74.jpg

oh...hey...that's no big deal :)

what t-mar said, but i'd alternatively suggest stuffing a paper towel down the seat tube, use the rat tail/half round file, and then an extendable magnet (or whatever magnet on the end of your driver/chisel) to get filings out. just a thought

edit: alternatively, again, you might instead use an automotive brake cylinder honing tool. they insert into any drill and can be rented at most auto parts stores. might oil that paper towel, too, so filings will cling to it. and, again....magnet

Thanks very much T-mar for helping with the photos, and for everyone's advice. After I finally got a seat post in the correct size, I got to work figuring this out. The first problem was the seat post bolt - the nut was only able to screw off half way before getting stuck. The bolt was a carriage bolt, so couldn't get any purchase on unscrewing it further. Luckily the lugs were spread out far enough, and after some delicate hacksaw work (and a couple unfortunate but minor paint losses...) managed to saw the bolt in half. Then it was a matter of carefully prying out the cinch slot opening, doing it as slowly and evenly as possible. I had to hit the inside of the tube with a half-bastard file to even it out and get the last bit of width out. There was a bit of initial stiffness in inserting the new (greased!) seatpost, in the first inch or so, but after that it went in smoothly - so as I suspected the sizing was right, and the tube was likely crimped at the top. The one remaining problem was the brake hanger, which was still fitted to the size of the crimped lugs. I carefully spread these out using some needlenoses - bit of a hack job but it worked well enough. A new 5/16" hexbolt and nut are doing duty holding it all together.

I've put a picture on my profile/ albums if anyone wants to see how it looks now. I want to go back and even out and deburr the sides of the slot, as well as clean up the new seatpost (which has some marks from the previous owner), but otherwise it's now an anxiety-free ride.

Regarding bike ID, after research from past posts from the resident Sekine experts on this forum, I'm 99% sure it's an SHS - Titlist derailleurs, Fingertip shifters, Tourney brakes, Shimano hubs, alloy Araya rims, SR crank, Sakae stem and bars. I've put a photo up for how it looked when I picked it up. The peddles are a replacement, some touring Shimano ones from the 80s, and the brake levers might be later as well. The seatpost that was in there was unmarked, but it was a straight post with a separate clamp for the saddle. It's in pretty good nick all-told. Debating doing some upgrades to some period Dura Ace, but that's long term and it's a great ride as it is. Thanks again for everyone's help!

From reading verktyg/Chas. C's posts, I have learned to love running a brake cylinder hone into seat tubes and steerer tube interiors. Cleans up burrs and scrubs off corrosion that comes with previous owner's having fitted undersized seatposts, smooths up the interior of Nervor steerers to allow Nitto Technomic stem to slide smoothly into place. The last two bikes I bought, both 1973 531-tubed Raleighs, had undersized seat posts fitted and the binder bolt lugs cranked in. In both cases I got to gently pry them back out into approximate shape, do a quick and gentle deburring of the seat tube mouth and then ran the hone on an electric drill down the first few inches of the seat tube. Dramatic improvement!

It's definitely an SHS model, as it has the SR Silver crankset with the large chainring swaged to the crankarm. The SHT used the upscale SR Royal-5 with an integral spider and removable large chainring. The SHS was 3rd in Sekine's hierarchy at the time and was considered lower mid-range.


A few words of caution about the SR Silver, as it uses an atypical 3 degree taper, making replacement spindles harder to find. Also, with the SR Silver, Sekine generally used BB shells that were closer to 70mm than 68mm, making many owners think they need a spindle with the wider spaced bearing races found on spindles for a 70mm BB shell. However, the cups used by Sekine with the SR Silver had very thick faces and they used the narrower bearing race spacing found on spindles for 68mm BB shells. With all these deviations from convention they were at least standard English threading.