Seatpost binder bolt with threads in the frame

[Simon Masterson]

Hi all,

I'm building up a frame ('70s, English - Ken Bird) with threads in the frame for the seatpost binder, which I've never had before. It didn't come with a bolt, and I'm trying to find the right size to go in it - and to be honest am a bit stuck. I have a small collection of allen bolts to hand. It's been suggested to me that it's likely metric, but I've so far tried: 6mm (definitely too small), 7mm (sold as M7, threads measure 7mm on my caliper), 8mm (too big), and a 1/4 inch.

Out of all of these, the 7mm feels like it's a hair too big, and the 1/4" feels like it might be roughly the right size, but it doesn't want to bite. The threads don't look particularly mashed, though it's hard to tell.

Does anyone with any experience of frames made like this have any idea of what size of bolt they'd expect on a frame like this? Ta.

 

[Midlife]

When I worked in a bike shop we had a tin of assorted seat binder bolts. Metric M7 were common but there were oddities out there. Holdsworth were a main culprit for strange sizes.

You might have to get the threads chased out. Is there a local frame builder?

 

[Simon Masterson]

When I worked in a bike shop we had a tin of assorted seat binder bolts. Metric M7 were common but there were oddities out there. Holdsworth were a main culprit for strange sizes.

You might have to get the threads chased out. Is there a local frame builder?

Thanks, unfortunately not. I am going to have it resprayed, but that will be end of this year earliest. Worst case I could have it drilled out for a modern binder, but I’d rather not if I don’t have to. Annoying, I like it when bolts are easy standard items!

 

[Simon Masterson]

So now thinking the 7mm is too big- just doesn’t want to go. The 1/4” will with a bit of encouragement, but I don’t want to cross thread it...

 

[Midlife]

I just had the thought that a frame builder might have a selection to try?

Maybe splash a tenner on a 1/4 " tap. Does anything sit into the recess? Some of the receeses for the bolts were smaller than others. I had a look at ceeway but no help. Back in the 70's people like Ron Kitching brought in all sorts that ended up on frames.

Best of luck

 

[torqueless]

Re:

Afaict mine ('70s, English, mystery) is 1/4" x 26tpi and takes a 3/16" hex key.

A bit of research suggests the thread (on mine) could be 'Cycle Engineers' or BSF- slightly different thread forms but both 1/4" x 26tpi.

 

[Midlife]

I have cable tied the Allen key for the seat bolt to the frame, it's an odd non-metric size and don't want to lose it...... Perhaps it's a 3/16 th


One of the joys of being a child of the 70's lol

 

[bagpuss]

Re:

I have a Mercian with this type of recessed threaded seat bolt circa 1970's
It maybe worth trying them .

 

[Jonny69]

Try a 1/4" UNF as well as a UNC. Next size up is 5/16" which is nearly 8mm, so it should be pretty obvious.

 

[torqueless]

Re:

UNC 20tpi
UNF 28tpi

when I bought it secondhand c.35 years ago, my frame didn't have the bolt in either. Luckily for me back then my Dad had a stash of AK bolts, and keys. He wasn't ever in the cycle business, (i.e. they weren't dedicated bike bolts) so I'd deduce that my bolt is BSF, moreso since his remaining 1/4" taps are BSF.

I don't remember ever having had to re-tap the thread in the frame to accept the bolt. I flatter myself that I have a modicum of 'engineers feel', and even 35 years ago I don't think I'd have attempted to force a bolt into a hole with a totally incompatible thread.

Well, this morning I tried the 1/4" BSF tap against my binder bolt, and the threads seemed to interlock- (same tpi). Then I tried the tap in the binder bolt housing on the frame, freehand with no leverage. It wouldn't go far. It probably would have with leverage, but since the thing has been working fine as it is for thirty five years, I didn't feel like re-tapping the thread- "If it ain't broke..."

So instead I tried the bolt with a tap-verified 1/4" BSF nut and it worked fine.

So I'm pretty sure mine is 1/4" x 26tpi BSF, or even if it's not, it's close enough to have functioned for thirty five years without complaint.

None of that means that the OP's frame is the same thread as mine, of course..