Help with stainless bolts and washers

[RonA]

I get the same result, scaling on screen, counting threads and using vernier calipers. BSW, UNC and UNF are too far out with the TPI, which comes out at 25.5tpi for me, thus 26 BSF.

It could also be M6x1mm....

Unlikely if it dates from the 70’s UNF all the rage & BSF beginning to phase out
RonA

 

[My Old Landy]

Unlikely if it dates from the 70’s UNF all the rage & BSF beginning to phase out
RonA

Phased out? I still use them

 

[graffian]

From a quick scale of the thread diameter and pitch relative to the battery and taking into account they are from the 1970's I would estimate these are 1/4" BSF. The real test though is accurate diameter and pitch measurement then relate those to Zeus (or similar) tables.

Looks coarser than 26tpi.

 

[Seadog]

I'd agree.

Edit: but I've measured it as 26.35tpi

 

[RonA]

Phased out? I still use them

So do I but the vehicle I use them on is 87 years old but not seen many on modern stuff RonA