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Old 11-12-08, 11:12 AM   #1
guygadois 
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Bob Jackson - 1975 - Seat Post sizing?

I have a 1975 Bob Jackson and I can't seem to get the post tight enough to stop moving. The tubing is Reynolds 531 and the Factory confirmed it was made in 1975. A 27.2 fits in the tubing well but when I tighten the binder bold all the way it doesn't put enough pressure to hold the seat firmly in place (it is tightened to the end). The binder bolt if fairly short and I don't know if I can get a smaller one.

Do I need a bigger diameter post? Should I add a washer on the binder bolt area to increase the pressure?

Any help is appreciated.



-GG-
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Old 11-12-08, 02:24 PM   #2
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Something is wrong, that's for sure.

It is almost certainly not a seat post problem. My '75 also takes a 27.2 mm post*. You need to diagnose what's wrong with the frame because without more info/photos we cannot tell.

When you tighten the binder bolt over the seat post, do the lug egdes touch each other?

Does the split in the seat tube closes down all the way?

Do the binder bolt halves bottom out on each other?

Is the binder bolt somewhat stripped, holding together but not able to hold enough tension?



* What color is yours? What paint scheme? Is it factory paint or repainted?
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Old 11-12-08, 02:38 PM   #3
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I would guess that the binder bolt is too long. The way you describe it, the binder bolt is bottoming out before the seat tube is pinched far enough in. These thing frequently get lost while diassembling and packing up a bike for shipment. The prior owner probably just grabbed the first one he found.
First make sure the bolt is going to be long enough to engage the female threads after you trim it, then take a hacksaw or Dremel tool and cut off a few millimeters of the bolt.
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Old 11-12-08, 02:46 PM   #4
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Does it even use a conventional pinch bolt or does it have what I've heard called a "Dutch" bolt where one Allen-head bolt fits into a countersunk socket on the RH side but the LH side is a tapped/threaded ear? Cause my Merlin (a BJ product and approx the same era) uses the Dutch type.
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Old 11-12-08, 04:25 PM   #5
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This may be an example of the "Dutch" style bolt which unworthy1 referred to.

Being a British made bike, the recessed Allen-head screw on my own Bob Jackson accepts a 7/32" [around 5.5 mm.] hex-key rather than a more common metric size. So, possibly the threads are BSC too -I never thought to check. If that were the case, perhaps the threads on your bike were stripped if someone had once attempted to fit an incorrect metric bolt into the threaded left binder "ear". Although it is "possible"... I would think that a seatpost larger than 27.2 mm on a steel frame from your bike's era would be extremely unlikely.

Bob Jackson... c.1973 or 74



Serial Number 10109



(...don't worry folks, I have a more suitable saddle and seatpost on this bike now...)
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Old 11-12-08, 04:53 PM   #6
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Here is a pic. I can get a better pic without the binder bolt loaded tonight...

-GG-

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Old 11-12-08, 06:58 PM   #7
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well, it's not a "Dutch" style binder bolt, and I agree that it *should* be 27.2 assuming they didn't substitute a seat tube of 753 and forget to tell you ...maybe you should try a single layer of thin aluminum-can-shim and see if it works the trick. If it does then maybe shop for a 27.4. Could also try some other 27.2s to see if you have one on the small end of the tolerance and can find one on the large end that works.
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Old 11-12-08, 07:58 PM   #8
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OK, here are some photos with the binder bold out. Note that each side has a cut out for a binder bolt with a notch on it.





Sorry about the bad photos. Send complaints to steve.jobs@apple.com

Also, the beer can solutions worked. It was a rare win-win. I dranka beer and the bike works. I just am not sure the binder bolt works and I am not doing it more harm.

-GG-
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Old 11-12-08, 08:44 PM   #9
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I wonder whether the binder bolt itself may simply be too long? The bolt might be tightening together as much as the the un-threaded portions may allow, but when closed together completely the bolt may not actually be closing the seat lug enough to actually grasp the seatpost sufficiently.

Perhaps try a simple fine-thread hex-nut and hex-head bolt something between 1/4" or up to 8 mm. in diameter should fit... (add flat washers at each outside end of the opening to protect the painted finish).

If your bolt has a tab under one side to fit into a key-way notch of the frame, the small tabs are sometimes not long (deep) enough fit into the intended notch, and prevent that side of the bolt from rotating while tightening the other side with an Allen wrench.

If it used a serrated cone at the "fixed" side, these are sometimes inadequate to firmly hold in a rusted or worn down lug.

Also, the "turning" side of the binder bolt is usually fitted with a thick washer (usually the same diameter as the bolt head)... but, I saw none in your first photo.
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Old 11-13-08, 11:18 AM   #10
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Fuel my fire of envy please...

I've targeted a Bobby J as next year's pursuit and this one...mwah! Is it a particular model or would I just be hunting a '75 Jackson roadie? Great looking bike. That seatcluster is the tops!
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Old 11-13-08, 11:20 AM   #11
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Quote:
Originally Posted by stronglight View Post
I wonder whether the binder bolt itself may simply be too long? The bolt might be tightening together as much as the the un-threaded portions may allow, but when closed together completely the bolt may not actually be closing the seat lug enough to actually grasp the seatpost sufficiently.

Perhaps try a simple fine-thread hex-nut and hex-head bolt something between 1/4" or up to 8 mm. in diameter should fit... (add flat washers at each outside end of the opening to protect the painted finish).

If your bolt has a tab under one side to fit into a key-way notch of the frame, the small tabs are sometimes not long (deep) enough fit into the intended notch, and prevent that side of the bolt from rotating while tightening the other side with an Allen wrench.

If it used a serrated cone at the "fixed" side, these are sometimes inadequate to firmly hold in a rusted or worn down lug.

Also, the "turning" side of the binder bolt is usually fitted with a thick washer (usually the same diameter as the bolt head)... but, I saw none in your first photo.
Exactly. If you can't find one elsewhere, measure your current binder bolt, and look for a shorter one. Loose Screws used to carry them, and might still.
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Old 11-13-08, 11:41 AM   #12
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I added a wash to the 'turn' side and, combined with the been can, the seat is in solid.

Thanks to everyone.



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