sdimattia

I have a 1983 Trek 520 Touring frame. I can't figure out the seatpost sizing.

The other '86 560 Pro I have takes a 27.2mm seatpost. With this '83 520 frame, it seems to be somewhere between a 26.0 and a 26.4mm post. The 26mm slides in super easy and can be rattled around. The 26.4mm post is super hard to get in and only goes in about an inch. It seems like a 26.2mm post would be perfect but I don't think that's a thing. Should I just shim the 26.0 post or take the whole frame to my LBS for reaming?

I've tried sanding the inside of the tube too. The 26.4 post gets gouged when I try to put it in but the 26.0 just seems too loose. Thanks in advance.

If it helps, I have 26.0, 26.4, 26.8, and 27.2 seatposts on hand.

Kuromori

26.2mm seatposts are a thing but usually for something like French straight gauge Vitus 888. It is very unusual to need it on a Imperial dimentioned frame, especially one with quality tubes.

SurferRosa

Let's see a pic or two of the seat lug/collar. Does it look overly pinched?

sdimattia

this is the 26.0 post, seems too big to me


I repainted the bike, taped off the seatpost. I stupidly got rid of the original seatpost but even that was a b**ch to get out. I ordered a new binder bolt that should be here sometime in the next few days.

sdimattia

The tubes are Tange Mangalloy 2001 if that helps at all. All the other measurements for parts of English so I agree, it doesn't make sense for the tube to be dimensioned at 26.2mm.

jdawginsc 

Could the seat tube have somehow gotten slightly out of round at the top, and the 26.0 is the correct post size? Does the 26.0 rattle at the top, or also at the lower end when inserted deep into the tube?

I actually have an oddball Nuovo Record in that size, oddly enough, so it is definitely a size. BMX bikes, some older SR posts...definitely available.

The Golden Boy 

I would lay money on a (any) 500 series Trek having a 27.2 seat post.

merziac

This has been kludged at some point, very careful finessing will be the fix, take it very slow and feel your way through the process.

Consulting a framebuilder would be a good idea.

SurferRosa

That seat lug looks really good...

I know what you mean, but sometimes things just don't add up.

I looked online in the Trek documentation about the '91 400 I've been refurb'ing. The doc said it was a 27.2 seatpost. I had one in my bin that measured closer to 27 after some very necessary polishing. It still didn't fit. With 80 grit, I took it down to 26.8, and it's now perfect.

sdimattia

It's definitely looser at the top, less so when the post is fully inserted at the bottom. I'll have to wait to get the binder bolt to see if it'll clamp hard enough to hold the post in place. It's too cold (and I'm too lazy) to go down the parking lot to borrow a binder bolt from another bike.

sdimattia

I agree that something got shoved in at some point . . . Luckily the tube isn't bulging or cracked but that tube that came installed on the bike took way too much effort to get ou .

merziac

Being too tight points to something too small was tightened down on.

Really doesn't look like it from here so who knows, many possibilities and very little to go on.

This can/will be a valuable learning experience if you proceed with caution.

You could take it to a machine shop and have them gauge it a couple inches down inside the ST to be sure or get a gage at HF.

Mr. 66

I have an '83 520 it's 27.2, and the 560 that I had was 27.0.

sdimattia

I'll take it to my LBS to see what they can do. Thank you everybody for your input thus far!

merziac

No disrespect, but very few of them care enough to know what they are doing with this nowdays, non integrated SP clamps as well as threadless HS's and cartridge BB's have been the norm for far too long now, "we don't fix that, we just replace it".

Any doubt whatsoever about their ability should eliminate them for this.

sdimattia

I completely understand. They'll probably tell me they can't do anything - they tend to err on the side of caution when it comes to any "alterations" or diy jobs.

Would it make more sense to see if the clamp will hold a 26.0mm post? The only thing i don't want to do is overtighten anything if the post is too small.
Alternatively, maybe the framebuilder would be a better bet. Only limits are money really.

merziac

The clamp or tube is likely already compromised in some way, it needs to be sorted. If you go with the seemingly too small setup it will continue to be a problem in some way like wiggling loose for not being supported for most of its length, the clamp should not have to be overtightened to hold the post.

It depends where you are at but you need a skilled C+V guru for this. A frame builder shouldn't charge much to evaluate and maybe rectify or explain how to proceed.
It would still be money well spent.

A co-op may be able to help as well

If you were in PDX or Seattle, we would have you sorted in no time.

If you tell us where your at, there may be a member that could help, you just have to ask.

sdimattia

I'm in NYC so nowhere near Portland or Seatle unfortunately. There is an REI near me, not sure about certified framebuilders in the city though. Would REI be of any help?

merziac

Nope, I'm talking about a co-op where you can go in, use tools and a stand and get help from old hands or sharp lads with tons of experience for a small fee.

There are plenty of builders in NY, you need a shepherd, paging noglider , Tom should have some words of wisdom.

As I stated before, this can be a good chance to learn a skill or learn about a skill, there are a few scenarios like this where it is invaluable to know how to proceed, be it hands on or who to get help from.

Framebuilders don't have to be certified, many older ones are/were self taught or apprenticed oldschool, these are the guys you want when things go south.

sdimattia

Perfect thanks. Perhaps certified was the wrong word to use. I guess "knowledgeable" would be more appropriate. And I'm also looking to learn new wrenching skills!

merziac

excellent, that's why we're here, we will answer ?'s and help guide you as best we can.

The co-op is a great way to get started so you can become aware and familiar with tools and processes, then get your own tools, don't skimp, good tools work better when you are learning, they are not cheap and many here do not fully subscribe to the Park tool program but they are the most prevalent and most of them will get the job done.

sdimattia

I've slowly been building up my park tool collection, have most of the basic stuff. Why pay off student loans when you can buy bike tools?

merziac

Well you're certainly on the right track then. I would still seek out a co-op to get help on kinky stuff like this as there are several things on bikes that require the right tools and a different mindset than other mechanical venues.

Bicycles seem simple and they are, until they're not and you can find yourself upside down in a NY second trying to figure out why it has turned into such an epic
a** whoopin.

T-Mar

The Trek catalogues state the 1983 model 520 to be Reynolds 501, which would take a 27.2mm post. Tange Mangalloy 2001 is thicker and would typically utilize a 26.8mm post. However, if inadvertently installed upside down, with the thicker butt at the top, it would require a 26.4mm post. It's not uncommon for tubes to distort somewhat during the brazing process, though conscientious manufacturers typically ream they back to OEM spec. If the tube was installed upside down, distorted slightly and wasn't reamed, a 26.2mm post is not out of the realm of expectations. Personally, I'd just have the seat tube reamed to fit a 26.8mm post, which is typical for that tubeset. You've got plenty of material to work with.

sdimattia

Update 03/01

Stuck a 21mm binder bolt to see if that would make any difference with a 26.0 post. Originally, the 26.0 post had about 2mm of play around the top of the tube but not farther down the tube.

Now my concern is, does that bolt look too tight in the collar? I had to tighten it down a good amount, not at the gorilla fisting stage though. If the saddle gets knocked hard, the post will rotate a bit (ex: if the bike fell over).

If it looks too tight, I'll go up to a 26.4 post and find someone to ream the tube . . .