1935 Shelby Seat Post Slipping

[TexWest]

I am probably out of my league here, but I often get more modern ('50s-'80s) racing and touring bike frames where the post is incorrect. It is usually smaller than what the frame would initially have used. My experience has been that the slot should have each side being roughly parallel after tightening when using the proper sized post. I have regularly received frames where the slot is pinched together more at the top from the use of a post too small for the bike, aside from those frames where the clamp area was out of round though usually from using too small of a post. Every frame I have fixed where I made the slot sides are parallel has had a proper size post fit without and clamp down without slippage.
That slot appears larger at the top which may indicate that some had pried it apart to remove a seized post. It may be that you have the correct post but the hole has been widened from previous efforts. Again, I am not all that familiar with these older bikes and their eccentricities.
I would certainly be cautious and if concerned at all about damage then use a shim. I have intentionally used a smaller post before (while not having or waiting for one), but I never clamped down on it where the slot was pinched at the top. I also have never had a post clamped into the frame where the slot was wider at the top with the exception of previous damage from being pried open.
I use "Shim In A Can". They have brass, stainless steel, steel, and copper at various thicknesses. I regularly use the brass shim for many uses without any issues thus far.

I appreciate your time and the information you provided. Some very useful information that I hope can show me the way here. I’m almost certain that I’ll need a shim to make this right. Thanks so much for passing this along

 

[SKPC]

Not absolutely sure, but the frame should take the 13/16th size post. As mentioned the seat tube appears ovalized around the top. Before resorting to a 5/8th inch post and a shim, you might try to use a soft copper rod/drift with a big hammer and pound the top of the frames' ovalized area back into place bit by bit while the loose post is inserted until you take out the 1/16th flaring. Again, leave the post inserted while you do it so you don't collapse the frame tube too much. It may solve your problem as now the frames' re-formed seat tube top would clamp onto the seat post much more positively when the bolt is tightened. Worth a try.

 

[WillWork4Parts]

Guys, these older brazed frames are not like modern slide on clamp seat tubes. They weren't all finished perfectly square. During manufacturing, after the slot was more than likely opened up by hand, the deburring was definitely done by hand, so you'll probably see some taper there. This one does have a couple of screwdriver marks, but it's not bad. I've seen much worse damage from screwdrivers and folks trying to repair one closed up. The stays brazed on back there are usually quite thin and easy to crush if you try and work it from the outside with a hammer or pliers.

Best thing to do at this point would be to Positively ID the maker, model, and year, then find someone with an original that hasn't been monkied with. Have them pull dimensions with a caliper of the post OD, shim wall thickness(remember, its diameter expands), and maybe the unclamped tube ID from a couple places(ID would be an estimate if it looks oval or pinched). Post OD + 2x shim thickness should equal just thousandths of an inch under the ID of the seat tube where it is uncut. Sourcing the right size parts may be all you need. Hopefully it's a common enough maker and the post will be an easier find. OR you can use your current post once armed with the knowledge of original post and shim size and make your own thinner shim. If that doesn't fit, a rod with slight taper may be needed to insert and work the pinched areas outward, Carefully.

 

[Archie Sturmer]

From the first picture, it looks to me that the clamp has some room to be tightened; (about an 1/8" gap at the vertical slot?).

Also, I do not always have all-originals parts, like down to the bolts & nuts, (might often choose Wald seat posts).
It looks like the nut (hex thumb nut?) may have been replaced, and a split spring type lockwasher added.
I believe that 5/16-24UNF threads might be said to allow greater tightening torque than 5/16-18UNC.
I also like the narrow AN960-style flat washers.

Sometimes the clamping arrangement and forces will tend to bend the binding bolt so that it will not drop out.
That is why spherical-cone washers are sometimes used; (but would be overdesign for a seat post clamp).

Some binder bolts that I remove from old frames are observed to be bent.

 

[WillWork4Parts]

From the first picture, it looks to me that the clamp has some room to be tightened; (about an 1/8" gap at the vertical slot?).

Also, I do not always have all-originals parts, like down to the bolts & nuts, (might often choose Wald seat posts).
It looks like the nut (hex thumb nut?) may have been replaced, and a split-type lock washer added.
I believe that 5/16-24UNF threads might be said to allow greater tightening torque than 5/16-18UNC.
I also like the narrow AN960-style flat washers.

Sometimes the clamping arrangement and forces will tend to bend the binding bolt so that it will not drop out.
That is why spherical-cone washers are sometimes used; (but would be overdesign for a seat post clamp).

If you're bending binder bolts, you're doing something wrong. It means the ears of whatever you're clamping down are no longer parallel, likely from the clamping diameter being too large for what it is being clamped onto.

 

[Freqman1]

I'm pretty sure the seat/post/shim are original to my bike but here are pics of more '35ish Shelbys. We can keep bench racing or you can just go get a 5/8" post with a shim and be riding!

 

[SKPC]

Below is my 34 interface. Ignore the silicone glue that prevents water from getting into the frame that I squeezed into the slot. Notice the interface between my 13/16th post and the frame: clean. There are a few options that will work as others have mentioned. Not messing with the flare and using a shim is the easiest but you may be limited by the overall lengths of 5/8" solid seat pillars if you are tall like me. A shim setup was as noted by Shawn original to the bike.

On a side note if Mark sticks any animal parts or multiple noise makers on that Red camelback, I may never forgive him!