A little A104 problem...

[Nanozeugma]

Hi.
Currently restoring a venerable Murphy A104 baffle receiver.
Step 1. Get the chassis out of the case; small problem, the outer knob on the central dual concentric shaft won't shift... curses!
Half the steel grubscrew head sheared off when undoing it.

Applied much percussive maintenance, oil etc; still won't shift.

Last chance saloon; brute force method. large file blade underneath and heave... remarkably it pops off and no damage

The problem:
Grubscrew stuck in the threaded hole with half the head missing and can't get even the tiniest pair of needle nosed pliers down there to shift it.

Any ideas?

Photographs attached.

[Beardyman]

That's a toothsome problem!
I'd try filling the hole up with 3-in-1 or WD40 and letting it soak through for a few days. As there is no tension on the screw you may just be able to turn it. Failing that I would take a dental burr to the other half of the screw head to get it down flat then drill a hole through it of approximately half its diameter. The action of friction can sometimes be enough to free it up, if not then go up in drill sizes until you reach just shy of the screw diameter, then run a tap through to clear the remains. Long winded but worth it. Best of luck to you.

[Andrewausfa]

There's a rarity, a complete A104 fine tuning knob.

If you really get stuck I may have a spare. Might be worth trying to line the hole with a fine tube for the drill to pass through and turn but enough to stop it spinning off centre and widening the hole.

Andrew

[Herald1360]

Left handed drill bits can be useful for this, they'll heat it up and apply undoing torque. Battery operated drill/drivers conveniently supply the reverse rotation.

Forget 3in1 and WD40, Plus Gas formula A is what you want. Or a drop of diesel at a pinch.

Grind off the remaining bit of the grubscrew in the shaft hole, fill knob with epoxy, drill in a new place and re-tap using a helicoil if to hand.

[vidjoman]

Get a bit of steel rod that will fit nicely in the hole in the knob, file a small flat on one side to make it D shaped. You then have a “screwdriver” that will get the screw out assuming you have given it a bit of plusgas for a couple of days.

[Nanozeugma]

Originally Posted by Andrewausfa

There's a rarity, a complete A104 fine tuning knob.

If you really get stuck I may have a spare. Might be worth trying to line the hole with a fine tube for the drill to pass through and turn but enough to stop it spinning off centre and widening the hole.

Andrew

Thanks Andrew, I may well take you up on that if push comes to shove.
As regards the "intact" fine/bandspread tuning knob, yes it is unusual.
The hilarious thing is (to me anyway), I see people paying good money for A104 receivers very obviously sporting a non standard replacement - no doubt caused by someone tipping the case on it's face years ago - very obvious if you know what you're looking at.
This receiver (with original knobs) cost me exactly £1 (plus petrol getting it).

[Nanozeugma]

Thanks for the suggestions folks, some Plus Gas on order...

[Boater Sam]

Get a soldering iron onto the screw and cook it, if it still refuses to move, cook it again and drop it into diesel whilst hot.
Drill and tap another hole if nothing else works, grinding the old one away.

[Nanozeugma]

It's OUT!
Thanks to the restorative properties of PlugGas (!).
Managed to free it enough to gradually turn the grub screw with what remained of it's head using tiny flat nosed pliers.
Once out, filed the head flat and cut a new slot with a hacksaw.
Cleared the detritus out of the thread in the knob, now it turns smoothly.
All's well that ends well

Mods: If you would like to close the thread now please...

It sounds counter intuitive, use some Loctite on the screw, it holds it in and also stops the treads rusting as no air can get in. It will be a bit stiff upon removal and remain that stiff not getting worse.