Seat bolt question

[argonsixar]

I am currently building a late 70s (poss 1979/80) frame and I need to find a seat bolt for it. The frame is of the shot-in seat stay design with a single bolt into a thread on the opposite side of the lug. My question is, can I expect it to be imperial or metric? I can source either and have tentatively tried an M8 and a 5/16-18 UNC , both of which "catch " the first couple of threads, but I am now in two minds which one I use.
I believe the builder is Vic Edwards, so anyone with experience of what he typically used will help.

PS - any tricks to working out how to measure the thread would be most welcome

 

[hamster]

Can you run the bolt in from the wrong side? Then it will also clean the thread out. You could also try running in a tap the same way, to see if it goes. You sound like you have the delicacy of touch not to make things worse.

My guess is that it will be metric, using something like Prugnat or Bocama fittings.

 

[argonsixar]

Yes I tried that. Of course if I run the wrong size tap through...

I guessed M8 first but backed off and tried the imperial, which actually caught easier. But that isnt necessarilly a good thing.

 

[Nob]

Could be of the BSF variety as my Dave Russell has BSF threads…

 

[argonsixar]

I will look at that.

 

[Glug]

I had a similar issue with a 1961 Holdsworth.

From Doug Fattic https://www.bikeforums.net/21585236-post12.html

I am an American that learned how to build bicycle frames in England in 1975. Before that time I had ordered several British frames from well known builders like Bill Hurlow and Hetchins. They all used 5/16" X 22 teeth per inch bolts.

Advice was 5/16" BSF 22 tpi.
But it turned out I needed 5/16" BSCy 26tpi, which is an older standard.
Late 70s is probably metric, or possibly BSF, just thought I'd mention there is another 'standard'.

Maybe try a bolt with a torque wrench and stop turning if the torque gets too high.

 

[Midlife]

Just out of curiosity what frame is it

 

[bikeworkshop]

You should be able to count the number of thread turns with a bent pin - then measure the length those turns are in, do some maths, and get threads-per-inch.
then measure the opening as best you can - you already have a couple of candidates at m8 and 5/16.
M8 is pretty standard, so if the m8 bolt feels wrong, it probably is.

But if you look at tpi correlated with 5/16", and find a bolt, it will probably be the best fit.
Who knows which of the half dozen British threads it might be.

The ultimate fix is to just fit an m8 helicoil( and properly done, it's potentially stronger than the original)

 

[jim haseltine]

5/16"
UNC = 18tpi
BSW = 18tpi but the thread angle is different to UNC so one into the other is a bit tight
BSF = 22tpi
UNF = 24tpi
BSC = 26tpi

 

[argonsixar]

All helpful advice. Thanks chaps. I have found a couple of online sites that have most bolt types so I have ordered a few variants to try them out.
I have an inelegant solution with a quick release lever from an MTB, but I want to avoid that.