IsleRide

I'm just finishing a rebuild on a '79 Trek 310. This is a bike that was never listed in Trek's catalog in 1979. It was built with Ishiwata butted 0625 Hi-tensile tubing. So in that regard it is basically a TX 300.
Although a lower spec for Trek it is built up with Campy NR, Nitto, Ofmega, Edco, Barelli, Mavic, Brooks etc. to a high standard.

All other Treks of this period have a seat post of 27.2. This one is supposed to be a 26.8. It came to me with a Campy NR 26.2 sized post. It was clamped down hard. I have that post in it with a thin shim for now but want to get the right size post.
I tried putting a 26.8 from another bike in it but it was a no go. I did not agressively try and spread the top of the clamping area outwards.

So after 40 years of seat binder abuse should I try and get the 26.8 in there by spreading the top of the seat lug or will a 26.6 be a better idea?

63rickert

I just assume any used steel bike has a damaged seat lug. If a previous owner has replaced the seat post it will always be the wrong size. Worse is the guy who puts the seatpost way up high not knowing that old bikes had shorter posts. That damage can be fixed too.

If you are very sure 26.8 was original size start bending some metal. Once done the stresses are lower. If nervous about metal bending type mechanics get some help. Be assured this type of work is done all the time.

IsleRide

That's just it. Not entirely sure about the original size. It is mentioned on the Vintage Trek site and Sheldon's as 26.8 but since it's such a low production model that could be wrong.

Not afraid of a little bending but I guess my real question is I know the 26.2 is too small and enough to close the gap when clamped down but is .2 mm smaller OK?
I have had "same size" posts fit one bike but not another. (both being unmolested bikes) And, "same size" posts measuring differently with a caliper.

Maybe I can track down some other TX 300 owners on BF?

Gresp15C

I have used an inside caliper -- the old fashioned type that looks like a compass -- to check out the condition of seat tubes. I have also used one to actually measure for a seatpost, but would not recommend trying that. But running it up and down the inside of the tube by feel will tell you if the clamping area has been distorted. You're using the lower portion of the tube as a standard for the upper portion.

gsa103

It'd be a miracle if a 40yr seat tube is anything resembling round to 0.2mm. That difference is miniscule. Use a 26.6 or 26.8 whichever is more readily available. There will be zero difference in performance.

IsleRide

gsa103,
You saw exactly where I was going with this. I bought this bike from the second owner and the smaller seat post was in the bike when he bought it.
He owned it for more than 20 years.
I just don't want the clamping area to bottom out like it does with the 26.2 post. I also don't want to over-manipulate the old metal just to prove that the original size can be made to fit.

3alarmer 

.
...firstly, it's hard to overmanipulate High tensile steel. It's pretty forgiving.

More to the point, the way a seat post is supposed to work, is that the frame maker does a final pass with a reamer, to make the interior of the seat tube round down to a certain distance. This usually takes care of any issues that arise during the brazing process, like distortion of either the seat lug or the tubing from heat, or braze that gets lost in there in the process.

So what you're really shooting for is the post that most closely fits into that final reamed interior, and gives you a nice slip fit all the way down the length of the seat post. You can get a smaller post to work. Obviously someone has already done that. But it will never work well, and there's always some issue with rocking back and forth in the interior, with an undersized post that is held in place primarily by the seat lug only.

Most people who want to do a good job of repairing the damage that you've inherited will pry apart the seat lug at the cut, using something like a pry bar with a chisel cut tip. Measure the seat lug to see if it is ovalized after prying it open to the point where the slot sides look like they are parallel. Don't pry on the ears much if you can help it. Sometimes they break off. Just go at the slot if you can. Then measure the diameter across in several spots. If it's ovalized, about the only repair that works is to re-ream it. Having decided what you can fit in there, make sure you take some sandpaper or a round file and remove any burrs you might have raised while working on the expansion of the lug. If you don't have an adjustable reamer, and the seat lug is ovalized, you might need to just go with that smaller 26.6...fitting a post is more art than science.

Here's one on a 1950's bike I reamed, because it had been road hard and put away wet. The original seat lug was so distorted I had no other choice...I wasn't going to paint and restore it unless I could ride it with a secure post and saddle.

63rickert

In 1979 Trek was still a completely handbuilt frame. That is a good thing. The Burke family, who owned Trek, knew less than nothing about bikes except how to sell them. (And they still don’t.) The hippies in the barn had a lot of leeway. There is nothing catalog spec for your bike. If there were catalog spec there would be no reason to believe it. Your bike is supposed to be Ishiwata hi-ten, the seat tube could be a substitution and it could be anything at all. Start with you have a bike in front of you.

Everything 3alarmer stated is correct. If a little bending and a little filing and sanding doesn’t make it round enough you do want to ream. If you do not want to buy or own an expansible reamer this is a good reason to head for the LBS. If the LBS does not own an expansible reamer you are in the wrong place.

Good enough might be good enough. Mostly depends on if you intend to ride it. As you have already discovered the bike is rideable with wrong parts. Lots of mechanics would say if it ain’t broke don’t fix it. The possibility of breaking the frame seems remote. I use my bikes. I have broken a frame due to an incorrectly fitted post. I see mechanics who think sort of maybe good enough is plenty good enough as vandals. But if you only want a nice garage ornament do whatever is convenient.

One final way to know if it was 26.6 and not 26.8 will be the reamer takes out a lot of metal. If that happens no problem. Bike was overbuilt in first place. You now have a good round straight hole. A straight hole a size large is much better than a wobbling post.

IsleRide

Great advice all. Thank you.

I do intend to ride this bike so will approach it with the attitude to make sure it's right.

Edit, Sunday 1/31
After carefully opening the gap to parallel and making sure the opening wasn't ovalized at the clamp area a 28.6 post from another bike was able to be inserted. (fairly easily I should add with the use of a spring loaded pry tool I use to spread the ears) There is some minor corrosion at about 150mm depth but any vintage road post I get for this will be no more than 200mm overall. (I did clean and used a rust inhibitor on the internals before I built up the bike)

I can see now how bad the 26.2 post fit. Certainly possible to rock in place while flexing the seat lug. While I don't see one size under (26.6) being a problem I feel better knowing that I can use the size that was originally intended for this seat tube.