seat pin wear/mismatch & shim length/depth

[scottg]

.........if I buy another 2nd had one I'll go for 0.2mm larger....and polish down to size!!)

there's absolutely nothing preventing you from 'polishing' the seat pin so that it works perfectly with a readily available beer can shim.

I prefer to buy an oversize Kalloy seat pin and StF.*

*Spin to Fit

[Brucey]

……
…..,,,
On the ovalisation, I can see that could stop rock...but is that the main cause of seat post slippage....

Seems to work that way

It may interest you to know that subsequent to devising the 'ovalising' scheme I have designed an add-on which will work with most seat pins and has the potential to suppress rocking, even in frames where the seat tube is sleeved, and the sleeve is 'rocking'. To fit this gizmo, the top part of the seat pin should be drilled approx. 3mm dia, axially. The hole is to accept a modified spoke nipple, which will be tightened after all the other adjustments are completed.

[willowbike]

With respect to the length of the shim needed, it's worth checking what the profile of the inside of the seattube looks like. It's pretty common for seattubes to be a thicker wall at the top of the tube before opening up a bit further down. In that case, a shim longer than the length of seattube that is actually the relevant ID would be unnecessary. Not that it would do any harm.

Having said all that, a seattube profile like that is less likely on a frame with a seatpost diameter of 27.0/27.2.

[hoogerbooger]

have decided seat tube is essentially uncrushed by clamp, as with seat pin out the slot in the actual tube is 0.75mm for near all, with just a slight pinch at a tad under 0.7mm close to top. The 0.3mm widening below is actually where the lug is cut, but the tube slot more or less stops.

(I am fairly sure the tubing is Reynolds 531 ST ( Special Tourist) or equivalent earlier tubes off the shelf( built circa 1982-84) . Which if correct suggests they didn't ream it as the seat tube would be butted be 0.81mm/0.56mm/0.81mm.....it's a tight fit on the 27.0mm post)

( only recently realised that the early 80s 531ST (Special Tourist) is not the same as the late 80s/90s 531ST (Super Tourist), the earlier is for light weight touring, the latter for fully loaded touring, with all frame tubes thicker walled with a 0.91mm seat tube)

[Brucey]

it is my understanding that it is not uncommon to find an early 531ST frame with a 27.2mm seat pin. Presumably this was achieved via reaming.

[willowbike]

I thought ST seattubes were 1.0/0.7? Which would be a 27.2 post? That's certainly the spec on some of my old Reynolds data sheets.

[hoogerbooger]

Links via
https://kuromori.home.blog/reynolds-531/#80s

But re seat pin doesn't it kinda depend on the reamer used....or if they did.

[hoogerbooger]

Despite the measurements of 27.0mm at the collar and the 27.0mm seat pin being stiff to insert and not rocking with the clamp loose....the just over 0.6mm slot in the seat tube closes up fully on tightening !!!

0.628mm of circumferance = 0.2mm of diameter...err .... but slot in the seat tube looks and measures close to parallel...within 0.1mm.

Confused as if vernier is out, it'll be similarly out ID of seat tube and OD seat post...so how does that 0.6mm slot close up ?!

[willowbike]

Links via
https://kuromori.home.blog/reynolds-531/#80s

But re seat pin doesn't it kinda depend on the reamer used....or if they did.

Ok, I see, it looks like the later, 90s ST (Super Tourist, not to be confused with Special Tourist) is 1.0/0.7 and the earlier Special Tourist is a heavier wall.

While you can in theory ream to any size, assuming there's enough material. Best practice is to use a seat post that's the same size (or at least a very close) match to the original ID. Reaming is normally done to make the seattube round again, post-fabrication, as the ID will distort a little through welding/brazing.

Reaming just to change the size of the ID isn't something normally done in fabrication. It would be easier in some of the more modern double-butted seat tubes where the seatpost ID section is relatively short before opening up to the thinner-walled centre section.

But anyone who's spent any time reaming newer heat-treated tubes will tell you that you want to limit the amount of reaming you have to do at all.

[hoogerbooger]

useful link from previous discussion on Reynolds and seat pins, with 1985 catalogue pages for Special Tourist:
viewtopic.php?p=1516874#p1516874

[ 28.6 - (0.55x2) = 27.5 .......they must have spent a long time bronze brazing my frame to get the seat pin size down to a tad over 27.0mm !! . ALSO RE my original query on shim depth, brazing can't have altered the full depth of pin insertion]

[Brucey]

Despite the measurements of 27.0mm at the collar and the 27.0mm seat pin being stiff to insert and not rocking with the clamp loose....the just over 0.6mm slot in the seat tube closes up fully on tightening !!!

0.628mm of circumferance = 0.2mm of diameter...err .... but slot in the seat tube looks and measures close to parallel...within 0.1mm.

Confused as if vernier is out, it'll be similarly out ID of seat tube and OD seat post...so how does that 0.6mm slot close up ?!

the hole might not be round.

At this point, most folk would ream the hole until it is definitely round and then either use a slightly larger seat pin or a shim.

[hoogerbooger]

Well, shimmed it up to 27.2 and post is holding. It will gouge the <i>[inappropriate word removed]</i> out of a 27.2 post....(as it has to the 27.0 modern one)...but if I can find a shop that will ream it, I may well.

1st proper ride on it....and it's a cracker. Who needs modern stuff ( err..except the tyres n saddle !)

[rogerzilla]

For what it's worth, this is a very common problem with the old Campag aero posts. Thet're quite old, so they get tarnished, people polish them up for sale, and suddenly a 27.2 is only a 27.0.

[rogerzilla]

Well, shimmed it up to 27.2 and post is holding. It will gouge the <i>[inappropriate word removed]</i> out of a 27.2 post....(as it has to the 27.0 modern one)...but if I can find a shop that will ream it, I may well.

1st proper ride on it....and it's a cracker. Who needs modern stuff ( err..except the tyres n saddle !)

20260716_190757.jpg

That's a good-looking bike. Those SR posts were on everything in the 1980s and they suit older frames. I have one on a different Harry.

(why does the forum software rotate vertical photos?!)