Seat post woes 27.2 too big - 27.0 too small!

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tabze3
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Seat post woes 27.2 too big - 27.0 too small!

Postby tabze3 » 15 Nov 2010, 5:35pm

Hi, I have recently brought what I think is a mid eighties steel road frame with reynolds 531c main frame tubing which I am in the process of building up with many of the old parts for a previous 531 galaxy frame which had to be retired after it developed a cracked down tube.

I had been using a 27.0 seat post in the old 531 frame which fitted well/worked fine. I assumed/hoped I would be able to swap the same seatpost over to the new frame but the fit is too loose in the 531c frame. Even with the seat post bolt well tightened up with the seat post slot sides nearly touching when tightened, I found on the first test ride that the my old 27.0 post was twisting about in the new 531c frame. When not riding the bike and with the seat post clamp undone, you can feel some play in the fit of the 27.0 post in the frame even with the post (which is quite long at 400mm x 27.0) is pushed all the way down into the frame (it is 400mm long). I assumed that I therefore needed a 27.2 seat post and purchased another USE 400mm x 27.2 post (cheap at £16.00 but have used these posts of several other bikes and they have been problem free) but now find that this post wont' go into the seat tube past the first 4cm or so even with a good film of grease and having cleaned up the inside of the seat tube as far as I can.

It seems that I actually probably need a post of about 27.1 but I don't think this size is made? Do you think I need to get the seat tube "cleaned up" at a bike shop? I gather good shops often have some kind of special tool that they can use to clean up the inside of the seat tube which might help the 27.2 to fit better/to slide down further as it should or could I perhaps alternatively purchase a seat post shim to use with the 27.0 post which would add the equivalent of .1mm diameter that I seem to need?

From googling about most info I can find seems to suggest that the 531c seat post diameter should be 27.2 but there is not way that the 27.2 post that I have will fit in as it is and by feeling down the tube as far as I can with fingers it seems very smooth inside with no rust. I have measured the diameter of both the posts that I have (one stamped 27.0 and the other 27.2) with callipers and taken several readings and averaged them both out - the 27.2 post actually measures on average 27.098, the 27.0 post averages 26.27 which makes the actual difference in diameter between the two 0.828mm. I thought if I could find a shim of that would add about .4mm diameter this would hopefully be a midway point that would work/hold tight. In googling for shims, I see that some are designed to make 27.2 seat posts "bigger", the next size down seem to be shims that start at 25.00 and go up in increments of 0.2 mm or so and therefore was assuming that if I buy a shim sized 25.0 - 25.4mm this might work? Any advice/anyone else's past experiences of getting round this as cheaply as possible would be much appreciated as this is now the last part I need to fit before I can start using the bike. I did wonder about making a coke can shim but worry about this getting lost down the seat tube and ending up causing more problems!

Thanks in advance for any advice or thoughts.

Robert

tooley92
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby tooley92 » 15 Nov 2010, 5:43pm

Sounds like you need a frame builder to run a 27.2mm ream down the seat tube (your 27.2mm post going in by as much a 4cm is a good sign that it should be 27.2), where in the country are you :?:
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snibgo
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby snibgo » 15 Nov 2010, 6:09pm

When sliding a post inside a tube, the average diameter of the post doesn't matter -- the maximum diameter does.

If you know a friendly framebuilder, that's great.

Or you might look carefully into the tube, and tackle any high spots with emery paper glued to a stick.

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531colin
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby 531colin » 15 Nov 2010, 6:28pm

Your 27.2mm post fits the first 4cm, so it is the right size. But something has happened to the tube 4cm down, so the post wont go all the way in. (Usually they bend the lugs for the binder bolt, so it wont go in at all.)
Can you see any damage to the tube? According to Tony Olivers book, 531c seat tubes are 0.8 / 0.55mm. wall thickness, and I wouldn't want to shave much metal off 0.55mm!

thirdcrank
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby thirdcrank » 15 Nov 2010, 7:02pm

On the home-made shim question, it's a workable solution which I've used more than once in my less-well off times (ie most of my cycling lifetime.)

You do say that the seat-lug slots are almost touching when tightened. What sort of bolt does it have? I had a broadly similar problem with a new frame where the seat-pin was slightly loose (gradually slipped down when riding) because the two elements of the allen key type fixing were running out of adjustment before the frame was fully tightened up. I solved that by simply buying a new one with more adjustment. (Obviously, this won't apply if the bolt arrangment is fixed at one end - eg slot holding the bolt head in one side of the lug, or a bolt screwing directly into the frame.)

The reason I am looking for a solution like this is that as it's an old frame, something must have worked in there before and it's hardly likely to have been sold new with a shim or a slipping seat-pin. If BF and I had been used to force one in, it would be stuck in there. The measurements involved in something like this are minute.

Gearoidmuar
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby Gearoidmuar » 15 Nov 2010, 7:47pm

Why not sand the seatpost itself? I haven't done it, but the amount you would need to do it could be tiny?

tabze3
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby tabze3 » 15 Nov 2010, 8:14pm

Thanks for replies/advice. Maybe I might try sanding the "new" post as suggested - I have not sanded metal before - what sort of sandpaper is best - wet and dry stuff?

tabze3
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby tabze3 » 15 Nov 2010, 8:18pm

Sorry, forgot to respond in my previous reply to your suggestions (thirdcrank) about the type of bolt/adjustment. it is one of those slotted type bolts and the sides of the seat post slot/cut out parts were extremely close together with the 27.0 seat post well tightened so worried about damaging the lugs by tightening any more. Will try the sanding for now and post after I have given that a go to let you know how it goes. Thanks

thirdcrank
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby thirdcrank » 15 Nov 2010, 8:37pm

One other thing which has occured to me is that while it's obviously pointless trying to force a seat-pin in that's too big, it's fairly common, I think, with steel frames that they are not a slip-in fit: used seat-pins from steel frames often bear the zig-zag scratches to prove it.

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Phatman
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby Phatman » 15 Nov 2010, 9:32pm

I've found there's 27.2 and 27.2! I bought a lovely red anodized 27.2 seatpost that was very snug and would only go in with the zig-zag treatment thirdcrank mentions. Happy to do that on a less precious post but not on this one. I then bought a Shimano post on here that went in like a dream.

Maybe try another post first if you can!

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andrew_s
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby andrew_s » 15 Nov 2010, 10:06pm

You haven't got a seatpin binder bolt that's the wrong size?
They are often a sleeve bolt like chainring bolts, and the inner part can tighten against the outer part before it's clamped the seatpost properly.

Try the 27.0 seatpost with a couple of washers under the head of the binder bolt.

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Phatman
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby Phatman » 15 Nov 2010, 10:19pm

Andrew's last comment is a good one - it reminded me that I did that once on a binder bolt that was part of the lug. It worked a treat on a post that did what your 27.0 is doing.
Last edited by Phatman on 16 Nov 2010, 10:31pm, edited 1 time in total.

tabze3
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby tabze3 » 15 Nov 2010, 10:58pm

Thanks everyone, really helpful. Will have a look at the seat binder bolt again tomorrow and also try the washers. If that doesn't work will try sandpaper on the 27.2

The Mechanic
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby The Mechanic » 16 Nov 2010, 9:00am

Very interesting. I recently purchased a circa 1977-1983 Revell bike with a full 531 frame. The seatpost that came with it was marked 27.2 but was very difficult to remove. It was not siezed in the frame but was a very tight fit and came out with the tell tale zigzag marks after a lot of tooing anf froing. I am reluctant to force one of my better posts in there and certainly not a carbon one. As the post was actually in there to start with, I don't think a 27mm post would be enough. If the seat tube was 27mm, I don't think the 27.2 post would not fit at all. Did Reynolds make 531 DB seat tubes in 27mm id?
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Edwards
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Re: Seat post woes 27.2 too big - 27.0 too small!

Postby Edwards » 16 Nov 2010, 9:12am

If the 27.2 is not a snug fit for the first 4cm? I would look for an obstruction in side the seat tube with a torch. It could be just a small raise in the side of the tube.
The worst thing I can think of is it should be 27 and the first 4cm of the frame has been worn or altered by a previous owner.
Keith Edwards
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