is there a standard 531 seat tube size?
 

My 74 Ellis-Briggs is what I'm trying to figure out a post for. It's full 531, so I was hoping there was a standard seat tube size for 531, or at least that time period. The guy I bought the frame-set from said it was 27.2, but that doesn't seem to be the case. A 26.8 post I have goes in, but seems like it May be a little too small. Is there an even 27 that I need? Being a newbie, I could well be mistaken and 26.8 might be the proper size, so once again I turn to you guys who always know more than I.

Both of my 531 British bikes take 27.0.

Here's uncle Sheldon's seatpost database;
http://www.sheldonbrown.com/seatpost-sizes.html
Look at other 531 bikes of the era and see what you come up with. My straight guage 531 Raleigh is 26.4 fwiw.

For a British 531 bike, the assumption would be 27.2

Now, many British bikes have fastback seat stays. These often distort the seat tube.
Even traditional style stay attachments can do this, so I would check the lug ears to make sure they are not closed, ream the tube to check roundness and then select a seat post diameter.

27.2 is most common, but 27.0 is not out of the question. There's often a .2 difference depending on whether or not the seat tube is reamed after brazing.

Actually, I was going to try to track you down and PM you if I didn't get an answer with this thread. What size is yours?

I've had a few 531 British bikes in 26.8...

Thanks, Chuck. I guess I'll have to figure out why the 27.2 didn't go and figure out what needs to be done to fix it. Hopefully I can figure out a way to just pry the opening apart more without hurting the paint around the edges. I don't have anything to ream the seat tube.

Both of my Reynolds 531 bikes ('72 Raleigh International, '71 Schwinn Paramount) use 27.2mm seatposts.

Part of the reason for the varied answers is that Reynolds offered the 531 butted seat tube in different gauges. One took a 27.2mm post, while the another took a 27.0mm post. There was even a very heavy gauge seat tube intended primarily for women's frames that used a 26.4mm post. Beyond that, you could get all kinds of variations depending on build-up, distortion and how agressively the tube was reamed/honed.

My new Walter Croll project is built from 531c.

Takes a 27.2.

http://farm6.static.flickr.com/5245/...508b472a_b.jpg

Do NOT attempt to do this yourself unless you are trained in the use of the proper tools and have them. I have seen way too many perfectly good frames ruined by DIY'ers. If you take it to a shop and they pull out an adjustable reamer take it to another shop. The chances that they might actually know how to set up an adjustable reamer at a bike shop is infinitesimal.

If you want to clean the inside of the tube use a non-abrasive like scotchbrite and figure out some way to clean out the trash you make before you clean it. Certainly before you stuff in a seatpost.

I had a BobJackson that some moron had originally assembled and it was a major effort to undo the mindless damage he had done. Steel is not impossible to repair but it ain't easy and it ain't cheap.

An inside micrometer will tell you what the diameter is way better than cramming in a buncha different seatposts.

Charlie

Yes, and it can also depend on how or with what it's brazed, according to Tony Oliver.

27.2 is not a universal assumption.

To answer the original question as asked: No. There is no standard 531 seat tube size. 27.2, 27.0, 26.8, 26.6 (some 531 Professional metric) 26.4, 26.2 and - in one of Neal Lerner's Super Course cases, 26.0.

I will not try to explain why this is the case, but I think these sizes are sufficient proof that you cannot second-guess seatpost size in a 531 frame.

-Kurt

I've seen these neato stepped rods some bike shops have, each turned step is a different seatpost size, from smallest to largest. It drops in to and stops to tell you the right size. I want one...

Won't work on a seatlug that's been spread.

-Kurt