tmh657

I snapped the seat post binder bolt so I will order a new one. This ad has them in 18,19 and 21mm sizes. I don't have anything to measure the existing broken bolt. It seems every one I have seen in my early 80's to mid 90's steel frame bike looks the same size. Is there a standard size or do I need to go to a shop with a micrometer? Hmmm, if I did that they might have the bolt in a drawer.

Problem Solvers Seat Binder Bolt > Components > Seatposts > Seatpost & Clamp Parts | Jenson USA

HillRider

A metric ruler is very cheap and useful for a lot of things besides bikes. I recommend you get one.

Retro Grouch 

1. There are roughly 24.5 mm in an inch so the difference between 18 mm and 21 mm is only 1/8 inch. That's not very much.
2. If it didn't matter, Problem Solvers wouldn't have the binder bolts in 3 sizes that are so close together.

Bottom line, one accurate measurement is worth 1,000 assumptions. If you are planning to do very much work on bikes an accurate metric ruler is one of the best tool investments that you can make.

tmh657

25.4mm= 1inch. So I didn't provide enough information in my question. Are the 3 lengths mentioned in the ad for the length of the bolt or the diameter? The website does not say. Measuring the length is simple, diameter not quite as easy to get it accurate.

Slash5

Google "measure seat post bolt"
Found this:

The 19 refers to the measurement size of the bolt tightened completely as is, so it will go smaller than 21mm in order to tighten the seatpost. A 22mm will not tighten down because it can't go smaller than 22mm.

tmh657

Got it, thanks.

Bike tinker man

The diameter of the bolts should all be the same, the bolt head has a spigot / line up so it does not turn, on the nut end there is a SS flat washer to help in even clamp force out, & stop scratching the paintwork, if the bolt is too long obviously no clamp force, as mentioned above you really need to measure with a simple cheap steel ruler what lenght you have, remembering its possibly fully compressed now if you sheared the present thread, if the bolt is too long when you buy & try it, then you can always add another washer as a fix.

CycleryNorth81

The numbers (18,19, 21) refer to the length of the bolt. The length of the bolt is measured within the two inner face of the bolt.

Mr.Wonderful

Thank you for this post. This helped me solve my problem too.

I want to ask a further question:Regarding "the bolt head has a spigot / line up so it does not turn," on my 30 year old bike I noticed the area where the spigot goes in (on the seat tube) has worn out a bit because I never knew about the spigot on the bolt head until today. It still is ok and it does not turn but if this was totally worn out, are there other options to prevent the seat from collapsing into the tube? Can one put a bolt and nut to just tighten it (not good cosmetically) and use something else to keep the seat from moving?

Thank you.

jccaclimber

A little wear where the rib goes doesn’t matter, it’s just there so the bolt doesn’t spin. Some seatpost bolts have a hex opening in both sides, so it’s even less necessary. Also note, Campy seatpost bolts don’t have that rib, so a bike designed for them won’t even have the slot. I only know that as my 80’s Davidson is that way. When I got it it had a normal seatpost bolt and several washers jammed in there. At some point someone pointed the issue out to me, and of course the Campy bolt (also a bit shorter) fit perfectly.

Mr.Wonderful

Thank you. Appreciate it.

tkamd73 

Had a similar situation recently, early 90s Tange Prestige road bike frame. Needing a replacement, I found this on Amazon, which took some of the guesswork out, got it quick, much nicer then the original, and cheap enough to just buy all 3 if you don’t feel like measuring. Higher end road bike frame, 19mm will probably work.
Tim

Mr.Wonderful

Thank you. I saw that too.

Maybe this will work too. Or maybe one of these might be easier.

CyclingFool95 

I just put the 19mm into a Series 7 PDG. Seems good. Hope it doesn't snap under load.

Andrew R Stewart 

Not yet mentioned is why the old bolt broke. This info might be nice to know to possibly avoid the replacement from breaking too.

A common contribution is that the seat binder barrel/collar/lug ears become stretched and their end faces end up being longer are parallel to each other. The result is that as you tighten the bolt it's shoulders try to stay flush against the angling faces of the binder. This places a bend in the bolt and as you turn it to tighten this bend is changing, think about bending a paperclip wire till it fractures.

A common reason why the binder stretches is that a too small a post was used and/or poor finishing of the seat tube ID.

Back to which bolt is best. I have found that while a measurement is worth a thousand assumptions, a trial fit is worth a thousand measurements This is a classic reason to visit a LBS and size the bolt before you buy. Andy

tkamd73 

installed the 19mm binder bolt on a series 5 PDG frame, same frame as yours, great fit, I doubt you will have any issues.
Tim