650 central head bolt thread

[Caulky]

Hi
Anyone know what thread this 70-4771 5/16" bolt has?
Spares books say its a bolt, and if yer lucky a head-bolt.
It appears to be 26tpi.
I need to tap this bolt hole deeper in a new barrel.
The hole is as deep as the threaded part of the bolt (7/8ths), but only threaded to about 5/16ths deep.
I thought about shortening the bolt, but cant find a nut that will fit.
Maybe British cycle thread???
Went to a garage, but they were unable to help...sorry mate it might be a `special`.

[jimmy bush]

British Only has it listed as a CEIX26X3 in their ebay store.

[Caulky]

Thankyou Jimmy.
CEI is cycle engineers institute or BSC.
26 is the tpi and 3 is the length of the bolt in inches.