Seatpost tolerance data point

[572cv]

I love Thomson seatposts. I have them on all my bikes, mostly elite, and one masterpiece. Most of them are 27.2 and one is larger.

I recently decided to swap out one of them from silver to black (the black looks better in this case) and got a square deal on an elite from the classifieds. When I went to put in the new post, it was much tighter than the one I took out. Both were 27.2. So I got out the calipers, and sure enough, one was a hair larger. I have an old Ritchie 27.2 in the parts bin, checked that. a third diameter. Then another Thomson 27.2 from another bike. And it turns out that there is some small variation in diameter, but, I suspect, likely well within tolerances.

All of the 27.2 posts would 'fit' into the seat tubes, but the issue for me is that the one I was changing out is my travel bike, and the seat post gets pulled fairly often. I didn't want it to be a struggle to pull it in and out each time. Fortunately, one of the other black ones on hand was at the narrower end of the tolerance allowance, and fit fine, and the larger diameter post fit fine in the donor bike. Said another way, the tube in the donor bike was bigger than the tube on the travel bike by a hair, so the difference in post diameter was exacerbated.

I haven't contacted Thomson, because my reasoning makes sense to me, and in that case, it isn't really their problem. I am delighted to have their elegant machined posts available. The quality and clarity of design is inspiring. I would have just gotten another one had that been necessary.

I just thought one of you might benefit from knowing that this minor variation existed, and knowing that, you would not be frustrated if it is encountered.

[spartanKid]

Just curious, what was the variation?

I could see machining tolerances like 0.005" or even 0.01" due to the high volume and increased cost savings on large scales.

[beeatnik]

That minor variation will cause creaking on an alloy frame (mainly Ti) if the post is greased with carbon paste. On my Moots I spent months chasing a creak until it occurred to me to wipe down the Thomson.

[djg21]

I destroyed a nice frame in the mid-90s by overtightening a brazed on seat-post clamp while trying to secure a Dura Ace post that was stamped 27.2 but actually measured just over 27.0. I still use the frame on a trainer today, but with a Thomson post shimmed with coke cans and a jerry-rigged seatpost binder made from an old quick release. Typically, you don't have to worry about quality control with anything made by Thomson.

[David Kirk]

A 27.2 post should fit into a 27.2 bore.....which means a 27.2 post needs to be smaller than 27.2 to fit properly. Typically they are 27.15ish.

dave

[pdmtong]

its worse with carbon

I stuck a s-works post into my moots CR with integrated binder as a donor post for the shop to clamp on, we almost couldn't get it out.

meanwhile the moots ti post I spent the benjamins on went in/out PERFECTLY

YMMV - DK has commented on this several times before

[Peter P.]

It could be the anodizing adds to the diameter. Actually, anodizing may not be the correct term for the black finish on their seatposts. It could be whatever the finish is adds to the diameter.

I know Rivendell spec'd a proprietary stem and forgot to factor in additional thickness due to the nickel plated finish. I believe I read they had to either toss the stems or have them reworked so they would fit the steerer.

[572cv]

Interesting comments and thoughts. Thanks all.
Regarding the anodizing, I did gets black one to fit comfortably. So, that doesn't seem to add enough to make a big difference.
As to the questions about the actual difference in diameter, I havetogeta better for the caliper to see the digital readout. I'll work on that, and let you know what the variance is among posts.

[David Kirk]

The anodizing on an alloy post should not add to the O.D. as it is as much a stain as it is a coating.

If the post is painted or powder coated that will of course make it bigger in O.D. but the maker should account for this if they are going to give it a paint/powder finish.

I had a black Ritchey come through and I went to slip it into the frame and it wouldn't even start in. Pulled out the calipers and it was 27.3. I sanded a bit of paint off the bottom and remeasured and it was 27.16. So they didn't do their job and make the finished product the proper size and the post was useless.

Typically Campy posts run on the large side and Shimano run on the smaller side. Many carbon posts are not machined but instead they are molded and the size can and will vary a shocking amount when you go up and down on any given post. The best carbon posts are machined to size so that they are round (many aren't) and the same size top to bottom.

As hard as it is to believe there is no industry standard for how big a post should be....crazy I know. But it is what it is. Like I said earlier a good quality 27.2 post should be about 27.15 O.D. so if you put calipers to it and it seems small because it's not 27.2 on the nose remember that it's the I.D of the frame that is 27.2 and the post just needs to slip into that.

Funny bidness.

dave

[572cv]

Post diameters measured. Feedback Sports digital caliper used. Some variance in diameter depending on how it was held, these seem reasonably close.

Silver elite: 27.16 or so

Black elite 27.24 or so

Black elite 27.40 or so

Black masterpiece: 27.20 or so.


The fat one fits in an alu frame, but not in the others. So, nearly 0.2mm variation. YMMV.

[lonoeightysix]

the thing that kills me with Thomson seatposts is their somewhat unique machining. it effectively removes 50% of the seatpost/seat tube I.D. contact area. i've had more problems with slipping Thomsons than any other brand.

[JeffWarner]

I love Thomson seatposts. I have them on all my bikes, mostly elite, and one masterpiece. Most of them are 27.2 and one is larger.

I recently decided to swap out one of them from silver to black (the black looks better in this case) and got a square deal on an elite from the classifieds. When I went to put in the new post, it was much tighter than the one I took out. Both were 27.2. So I got out the calipers, and sure enough, one was a hair larger. I have an old Ritchie 27.2 in the parts bin, checked that. a third diameter. Then another Thomson 27.2 from another bike. And it turns out that there is some small variation in diameter, but, I suspect, likely well within tolerances.

All of the 27.2 posts would 'fit' into the seat tubes, but the issue for me is that the one I was changing out is my travel bike, and the seat post gets pulled fairly often. I didn't want it to be a struggle to pull it in and out each time. Fortunately, one of the other black ones on hand was at the narrower end of the tolerance allowance, and fit fine, and the larger diameter post fit fine in the donor bike. Said another way, the tube in the donor bike was bigger than the tube on the travel bike by a hair, so the difference in post diameter was exacerbated.

I haven't contacted Thomson, because my reasoning makes sense to me, and in that case, it isn't really their problem. I am delighted to have their elegant machined posts available. The quality and clarity of design is inspiring. I would have just gotten another one had that been necessary.

I just thought one of you might benefit from knowing that this minor variation existed, and knowing that, you would not be frustrated if it is encountered.

I had the same issue when I replaced a 10 year old Thomson 27.2 Elite post with a new Thomson Elite post on my Vamoots. While the old post would slide in/out pretty easy, the new post was a bit tighter fit.

The old post measured out with a caliper at 27.13mm. The new post checked out at 27.18mm, which was in spec (+0,-0.03mm) per the documents on Thomson's site. Called Thomson, they told me they changed the tolerances on seat posts a few years back to push them a little closer to nominal (27.2mm).

[guido]

Curious. I have had 5 Thompson post (1 masterpiece, 4 elites) and have never had a fit issue...