twowheelies

Sheldon Brown has it at 27mm, which is what i bought. But it's super tight on my '93/'94 Daimaonback Master (see photo). Is that just all the corrosion that has accumulated over almost 30 years, making the hole a little smaller? Or do i have the wrong size? It will go in about an inch, but i'm afraid to push too hard for fear it'll get stuck.

Any input greatly appreciated!

veganbikes

Have you tried using some calipers? Those work quite well on Diamondbacks or any bike you might need a seat post on. If you have an old seatpost and it doesn't list size use some calipers and if you don't have a seatpost then use the calipers in the seat tube. If you have a lot of corrosion you might want to clean that out maybe go down to your local shop and have them hone the seat tube, they can also measure it and get you the correct seatpost if that one is not correct. Of course you loosened the seatpost binder bolt before putting in the post so that wouldn't be an issue.

twowheelies

I hear you, i couldn't measure because the previous (stuck) seat post was destroyed on removal. Thought i could avoid any more hassle if someone just knew the right size... but yeah, if i can't find that info i will resort to LBS.

veganbikes

You know the right size, it is super easy you loosen the binder bolt and you measure with a pair of calipers on the inside. It is good to measure for yourself you never know what someone else was doing while measuring. Plus if you have a honing brush you can clean out corrosion and make the seat tube nice and smooth.

onsay99

One of my vintage bikes is a '91 Centurion Master TG which is in
the same family as the Diamondback Master. My alloy post
size is 26.8mm. Confirm yours with calipers as stated above,
but there is a better than even chance that yours also needs 26.8mm

Andrew R Stewart 

The right size post is the one that fits well and can be tightened up. This is not always what the spec list shows as frames become distorted during construction (or after) and the factory or shop might not do/be able to do the slight reaming then honing that a well fitted post often wants. BTW it is the fit below the seat lug that is most critical. Caliper jaws won't fit that deep, sizing sticks steps are usually too short to reach down into the seat tube far enough. One can do all kinds of regretful damage to a post and/or frame when good technique isn't followed. I suggest trying a 26.8 post before doing anything that's not reversible. It's a rare Asian bike that really is intended to use a 27mm post, 26.6 and 26.8 are far more common fits. NYC should have no lack of bike shops that might have testing posts for the purpose of finding the best fit size. Then buy the replacement post from them as they earned it by finding out what that best fit was. Andy

Papa Tom

It's interesting that you are having this problem with a Diamond Back frame because I bought (what I was told was) the "right" post for a Diamond Back that I inherited, slipped it in about half an inch and then hit a wall. By this point, I already couldn't get it back out, so after about two hours of frustration, I decided I was just going to pound it in to the right height for myself and ride it that way forever! Unfortunately, I couldn't get it in that far and now it's permanently lodged at an unusable height. The frame is sitting in my basement and makes me sad (and embarrassed) every time I walk by it. Let me know if yours ends up stuck, too, and how you get out of it without looking as stupid as I feel!

Andrew R Stewart 

PT- How far in the seat tube did you hammer the post in? One sure fire way is to use a hack saw to cut a slice down the length of the post's ID. Once the post wall is sliced it's wedging action inside the seat tube will relax, perhaps with help from a crushing clamping of some big plier type tool. Cut the post off about 10mm above the top of the seat tube and run the hack saw blade (held in a jab saw handle) and start cutting away taking care to not continue cutting into the frame tube. Not a quick or pleasant method but very reliable. Andy

fredjustison

I was having the same problem with a older Specialized (Crossroads) bike I picked up cheap at a Thrift Store. The seat post was missing and I had a old seat post from a Trek 800 mtn bike. It would only go in about an inch so i took it out sprayed some Liquid Wrench and 3-in-one oil in seatpost tunnel. I then greased the post with Marine grease and it went in with a little effort.