I was able to drill out the broken pins; they don't seem to be hardened
through the center (if at all).
I started with a sharp 1/16" drill bit (less than $12/dozen from MMC), and
held the drill motor at an angle to move the hole until it was centered on
the broken screw as well as I could manage. Drilled all the way through,
continuing to make corrections as necessary/possible to keep the hole
centered as well as I could. IIRC, I then used a Dremel & small stone to
clean up the entry hole, then followed with larger and larger bits until the
pin fell apart. Once the threaded portion separates from the taper portion,
you can remove the fork and tap out the tapered portion from the other side
with a punch. I picked the remains of the threaded portion out of the fork
with a dental pick, then chased the threads. Sorry, I don't recall the
threads offhand; it's possible they were British rather than SAE.
A brief search turned up replacement pins at several places; Canley was the
cheapest I found. They mostly specialize in the 'small' Triumphs but
they're good people.
https://www.canleyclassics.com/demoapp/?ptno=122653
Although that's not the usual TR4 number, they appear to be interchangeable
https://www.canleyclassics.com/triumph-tr6-late-top-cover-extension-and-sele
ctors
-- Randall
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