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Old 09-21-08, 06:25 AM   #1
jlillie
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Miyata seat post q

I'm looking for some help with a vintage Miyata frame. The bike is a 1987 215 ST. This is the low low end touring model for that year. The bike did not come with a seat post. The posts that I happen to have lying around are to small to fit into frame by about .010 of inch. The opening in the frame is about 1.060.
the modern seat posts I have are all 1.070. Did they make smaller seat posts/frame openings back then?
Thanks for any help.

I'm rebuilding 2 vintage Miyatas. One for myself one for my daughter.
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Old 09-21-08, 06:50 AM   #2
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Seat posts come in many sizes. I am not sure what size is used for the Miyata 215. I think you should take to bike to your LBS, so that they can measure the seat tube and get the correct size seat post.
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Old 09-21-08, 06:52 AM   #3
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Miyata's butted frames typically use 26.8mm (approx. 1.055") seatposts. During this period, Miyata manufactured their own tubesets and chose to use a double butted seat tube, unlike most other tubing manufacturers, who used single butted seat tubes. Single butted tubes are butted (i.e. thicker) only at the bottom of the tube, where it inserts into the bottom bracket shell, because of the higher stresses at that location. Miyata's extra butt at the top of the seat tube, means it uses a smaller seat post than most single butted tubes.

The advantage of having the extra butt, in conjunction with in-house tube manufacture, meant that Miyata could control the top butt diameter, so that they only had to stock one size of seatpost, regardless of the thickness of the un-butted section of the tube. Bicycle brands who used tubing with single butted seat tubes could also stock a single seat post size, but to do so they had to ream or hone the seat tube to size of the thinnest gauge tube, an operation which added extra cost. The only other major tubing manufactuer who regulalry used double butted seat tubes was Tange.
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Old 09-21-08, 07:01 AM   #4
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Concur with T-Mar. All 5 Miyatas that I've owned took 26.8mm seat posts.
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Old 09-21-08, 08:13 AM   #5
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Originally Posted by top506 View Post
Concur with T-Mar. All 5 Miyatas that I've owned took 26.8mm seat posts.
Which OP's measured opening size corresponds to. 1.060" = just shy of 27 mm, and subtract 0.2 mm for clearance, you have 26.8.
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Old 09-21-08, 11:10 AM   #6
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Concur with T-Mar. All 5 Miyatas that I've owned took 26.8mm seat posts.
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I have a mid-eighties ladies' Miyata 310 with the seat post stamped 26.7, about 220mm long. The sticker on the seat tube says "MIYATA Cr-Mo CHROME MOLYBDENUM DOUBLE BUTTED TUBES" but I don't have any gauges handy to see if the seat tube is butted at the top. The "upper downtube" on these frames joins the seat tube just below the mid-point. I expect ladies' frames are the exception to the rule.
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Old 09-21-08, 11:36 AM   #7
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and if T-Mar's explanation isn't enough, Sheldon Brown's database of seatpost sizes shows every *steel* framed Miyata (except for 2) takes a 26.8 post.
The exceptions are 1:
a Mtn. bike and 2:
a 610 which takes a 26.7! (and has the exclamation mark indicating how rare a size that is! I have never seen a seatpost marked 26.7, he doesn't say if it's a female frame or not)
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Old 09-21-08, 05:36 PM   #8
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Originally Posted by OLDYELLR View Post
I have a mid-eighties ladies' Miyata 310 with the seat post stamped 26.7, about 220mm long. The sticker on the seat tube says "MIYATA Cr-Mo CHROME MOLYBDENUM DOUBLE BUTTED TUBES"
I have a 1984 men's Miyata 310 and the seat post is also stamped 26.7; very strange size.
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Old 09-22-08, 07:01 AM   #9
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Any any seat post ending in a odd number is rare. Seat posts are normally manufactured in even numbered, 0.2mm increments. A difference of 0.1mm from the 26.8mm standard is not significant, as the binder slot would only have to cinched an extra 0.3mm. I can only speculate on the reason for these odd sized posts, but I doubt the seat tube's top butt is 0.05mm thicker than the standard. More likely, the post was manufactured slightly undersize and rather than rework them the manufacturer stamped the true size, knowing than the difference would not affect fit or function. Knowing the Japanese approach to quality, I suspect they probably even notified Miyata and received approval before shipping the posts.
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Old 09-22-08, 12:14 PM   #10
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26.8mm Roger that! Off to nashbar.....
Thanks guys.
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