JoeBass

I was using a post that was marked "26", which I'm getting rid of, but the post was VERY tight in the tube, and even though it was marked "26" my calipers say it's 25.5. Anyone know the correct size for the bike?
Thanks much,
Joe

unworthy1

Have to see what frame since there WERE some lesser-than-all-531 DB frames sold by Allegro, such as "city bike" and Mixte style...but the vast majority or road frames would have been built with METRIC 531 DB which means a seat post of 26.4 or 26.6 are typical sizes, mine is a circa 1975 and takes a 26.4 IIRC.
Show us some pics, especially of the seat cluster and rear slot, maybe had some squashing of the tube,

juvela

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I would think 26.6. Have had three or four Neuchatel products come through.

Tip - On crusty old beasts like yours I have a set of expanding blade reamers I use to remove any rust, dirt, paint or burrs at the lug edge. Tool used lightly to ensure a clean & round tube interior for pillar.

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Andrew R Stewart 

The correct size post is the one that fits best. Simple to say but hard to know without actually trying. many bike shops have a range of posts for this very purpose, to test fit before scratching a new one. Of course the two way street suggests that the right post is then bought from the shop. Also this shop should be able to advise about any frame concerns WRT post fit and solid clamping. Andy

rustystrings61 

My LBS has a tapered seatpost gauge that has proven to be pretty reliable. My Allegro, a c.1974, has a 26.6 mm seatpost, but looks to be very similar to yours, from the photo I have seen. More photos would be good (they would be good, anyway!).

On the one in my workshop, there is not a lot of obvious clearance on the lug ears when they are tightened down - it is a very close, precise fit.

JoeBass

It's one of the higher end 531 models with a serial number of 235xxx, which I think dates it to around 1970, although I haven't been able to get an exact date.

rhm

Mine is older, about 1960, but also butted 531. It came with a 26.0 post and a shim. These appear to have been original.

FBinNY 

The best and most reliable gauge for correct post is the seat lug slot.

The slot is cut into the finished frame with a slotting saw, leaving a constant width and parallel sides. Since the correct post will be a snug running fit into the tube, we'd expect the slot to close only very slightly when clamping.

So, first of all, a post must fit the tube (below the lug) with a decently snag running fit. Then when tightened, the slot will have closed very little, and certainly no more than to half the width.

Before trying, use a reamer and or files to deburr the slot, and clean up the tube, removing only the least amount of metal possible (you're not trying to increase the ID).

As noted, a metric 531 frame will take a post larger than 26mm. If you have a pose smaller than that and it's tight, you know for a fact that the frame has thicker walled tubing, though there's not enough info here to know exactly what tubes it's built with.

rustystrings61 

Mine, sn 241247, was tentatively identified as probably being from 1974 by Craig Griffith, who runs the Swiss Bicycles site. Yours looks almost exactly like mine, except you have what looks like original paint and a headbadge. I can't tell for sure, but those look like the Bocama model 14 pattern II lugs one mine - which is 60 cm c-top with a 59 cm c-c top tube. And the fork crown appears to be the same flat topped Vagner PL I have, as well. And mine has a 26.6 mm Zeus seatpost in it, which fits perfectly.

Odd about the seatpost diameter, though.

Lovely bike, btw!

JoeBass

Ah! I was carefully examining the wall thickness of the seat tube when I noticed that the tube was indeed shimmed! It looks original, with a little lip on top to lift it out, so it seems this bike may have the same setup as rhm's, with the shim and 26mm post. The only thing still confusing me is why the post I pulled off, which was marked "26" is measuring 25.5mm with my calipers.

unworthy1

'nominal' and "actual" are often two different things, including the number of mm...