Older seat bolt size query

[sjcprojects]

I have a 1960s frame which doesn't take the usual (i.e more 'modern') seat bolt size, and it's also not an M5 or M6 size either, it seems somewhere in-between. Imperial, perhaps? The thread is on the left-hand side, and the right-side has quite a large opening for the bolt head to fit into.

Here's how it originally was (I have now sadly lost this bolt):



Here's how it looks from the back, with the threaded part on the left:



I feel like this is probably obvious to someone used to older standards, but I'm more of an 80-90s era person so I'm a bit clueless with this one. Any ideas what size I should be looking for?

 

[bikeworkshop]

If it's not m5 or m6, it will be imperial

1/4" is close to 6mm...
5/16 not unknown.

Cycle thread was usually BSC,
26tpi iirc
But bsf is another possibility.

 

[LondonClassic]

You can consider a re-tap to metric (as @bikeworkshop says the thread is in all likelihood imperial). I retapped to 8mm in my case but I had to grind the cap head down on the metric bolt I had to fit in the cup on the R side of the cluster.

Sorry, just read you lost the bolt...you kind of have to go metric in that case.

 

[sjcprojects]

If it's not m5 or m6, it will be imperial

Yeah, I did kind of answer my own question, there, really...

But actually, I'm wrong. I've found an M6 bolt (from an old stem) which does fit the thread, seemingly - but the head is too small. So it looks like I need a specific length M6 with a large head...

 

[juvela]

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t'is common to discover that the binders on Brit produced cycles be Whitworth dimension

Raleigh for example employed Gripfast made binders of Whitworth dimension for decades


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[sjcprojects]

I should add that I took the bike to my local bike co-op and having tried a few they had in stock, they reckoned it was somewhere between an M5 and M6, so probably imperial - hence my post here. But it does seem to take an M6, after all. I guess I'll try the right length regular M6 bolt with a large enough washer inserted to tighten against, maybe?

 

[bikeworkshop]

Remember when the pubs shut at 3 on a Sunday and didn't reopen til 6?

 

[3wheeler7]

This Paramount I refurbished a couple of years ago had an M7 seatbolt.

 

[Foreigner]

that does not look right to me -the bolt should go all the way in or at least that is what most of such seat binders I had used before do.

BTW is this a Wilson frame?