Seat Post Size Question ...

[Carbon]

Picked up a sweet late 40's Rollfast twin bar boys bike with a springer fork this past weekend. I can't figure out what this seat post size is, it is just a smidge bigger the 13/16 and measures around 21.5 or so in millimeters, 7/8 is a touch to big. Any ideas what this size might be? Thanks.

 

[deorman]

Are you measuring the actual seatpost, or the frame tube it goes into?

 

[Shrunken_Head_Pedaler]

Picked up a sweet late 40's Rollfast twin bar boys bike with a springer fork this past weekend. I can't figure out what this seat post size is, it is just a smidge bigger the 13/16 and measures around 21.5 or so in millimeters, 7/8 is a touch to big. Any ideas what this size might be? Thanks.

A pair of calipers would get you exact measurements in decimal then change to fraction. Should be able to find something in the range you need.

 

[Wildcat]

It should be 13/16ths. In the early 40's it was 5/8ths, then when they went larger it was 13/16ths. I've had several models of both, they didn't use any other size that I know of. Between 13/16ths and 7/8ths would be 27/32nds, that wasn't a size used by any American bikes back then.

 

[deorman]

Something like this may solve your dilemma.
https://www.modernbike.com/wald-905-seatpost-shim

 

[Carbon]

I'm measuring the actual seatpost with a slide caliper, getting 21.6mm, 13/16 fits wabbly loose in the seat frame tube. I've tried several different sizes with no luck. It's about 1mm shy of being 7/8" which would be to large for the hole.

 

[Carbon]

It literally measures dead center between 13/16 and 7/8, weird, never had this problem before.

 

[AndyA]

It literally measures dead center between 13/16 and 7/8, weird, never had this problem before.

Hhmm, I know that Wildcat says there ain't no such thing, but it sounds like 27/32. Guess you need to go with a shim. If the Wall shim is too thick, try a piece of aluminum soda can. Have fun!

 

[Wildcat]

Will the collar tighten enough to hold the 13/16ths stem securely?

 

[Carbon]

No, it pinches the frame cut notch together to much and I would have to modify the seat post clamp that slides on the frame seat tube.

 

[Carbon]

Oh yeah did some nosing around over at the cabe and figured out the serial for the year, its a 55 model.

 

[Carbon]

I put a new set of really nice double wall coaster wheels with kenda tires, new stem and bars and changed over to a snow flake sprocket. Repacked all the bearings, this thing rides better then new.

 

[rrtbike]

Perhaps someone has reamed out the seat tube of the frame. Or perhaps you could to accept a bigger post. Good looking bike!

 

[64Pete]

Just wondering how round the seat tube is?
If it's been ovalized the 7/8" post won't go in.
OR................
See below, it's happened before, might help.
https://ratrodbikes.com/forum/threads/whats-the-difference-between-7-8-and-7-8.109322/#post-1112865

Pete

 

[BillCigar]

I had the same problem with my 74 Huffy Easy Going cruiser. 13/16 was too small and 7/8 was too big. I tried the 13/16 - 7/8 shim and( while it did fit well ), the steel 13/ 16 post bent the first time out.
So... I purchased a 7/8" 6061 aluminum solid rod. Took a sharp rat tail file to the OD, and slowly filed it to fit. I kept rolling the rod in my hand to try to keep it roundish. After fitting, I used Scott Brite on the whole rod to give it an even finish. Can't tell where I filed it, now. I did not file the whole length of the rod, only the section that had to go into the seat tube. Worked good, took about 20 min. My seat post extends about 12" out of the seat tube, and is 6" into the seat tube. I am 6' 3" 210 lbs., and it has not bent in over a year of frequent riding.
Go with the aluminum rod. It is really an easy fix. BTW... the 18"x 7/8 brand new 6061 rod was only about $13 at my local metal recycle yard.

When you install it, be sure to grease the seat tube and aluminum rod well. Aluminum and steel do not play nice. That rod will freeze in place over time without grease. With grease, it will be fine...no worries.

 

[Carbon]

That's a great idea BillCigar, I just ordered me up a 16" piece of solid 6061 aluminum rod. Thanks for the tip and for all the other recommendations fellas

 

[Tallbikeman]

I had the same problem with my 74 Huffy Easy Going cruiser. 13/16 was too small and 7/8 was too big. I tried the 13/16 - 7/8 shim and( while it did fit well ), the steel 13/ 16 post bent the first time out.
So... I purchased a 7/8" 6061 aluminum solid rod. Took a sharp rat tail file to the OD, and slowly filed it to fit. I kept rolling the rod in my hand to try to keep it roundish. After fitting, I used Scott Brite on the whole rod to give it an even finish. Can't tell where I filed it, now. I did not file the whole length of the rod, only the section that had to go into the seat tube. Worked good, took about 20 min. My seat post extends about 12" out of the seat tube, and is 6" into the seat tube. I am 6' 3" 210 lbs., and it has not bent in over a year of frequent riding.
Go with the aluminum rod. It is really an easy fix. BTW... the 18"x 7/8 brand new 6061 rod was only about $13 at my local metal recycle yard.

When you install it, be sure to grease the seat tube and aluminum rod well. Aluminum and steel do not play nice. That rod will freeze in place over time without grease. With grease, it will be fine...no worries.

Seatpost fit issues are a bear sometimes. I have a early 1990"s Worksman LGB bike that has a nominal 13/16" seatpost but a little loose. In its case I use a thin shim to make it fit. A 7/8" post will not fit. We also have a newer Worksman that is 7/8" for its post. BillCigar has a good workable solution.

 

[BillCigar]

Seatpost fit issues are a bear sometimes. I have a early 1990"s Worksman LGB bike that has a nominal 13/16" seatpost but a little loose. In its case I use a thin shim to make it fit. A 7/8" post will not fit. We also have a newer Worksman that is 7/8" for its post. BillCigar has a good workable solution.

Thank you, Sir.
I just wish I could take credit for the idea. Like all my best cycle work-arounds, the original idea came from an experienced gentleman here on RRB forums.
If I could remember who it was, he would get full credit for sure. I am just glad I could pass it on. Thanks to everyone here on RRB's for all your help.