[ARC5] Bowden tuning cables problem

[Kenneth G. Gordon]

On 26 Mar 2013 at 8:49, Richard Hankins wrote:

> I have just obtained some of those hard-to-find Bowden tuning cables

I am almost certain that "Bowden" is not the correct term. However, we know 
what you mean.

> used on SCR-274, etc.   Having just tried several on a set and control
> unit, I find they are unusable in their current form.

That is not unusual...
 
> The problem is that the inner is too long relative to the outer. I can
> assemble one end just fine, and it all works as expected. But the
> other end cannot be assembled because the locking ring doesn't get
> anywhere near the collar on the set/control unit.

Yes. That is a common problem. I recently sold three 78" long MC-215s for 
use in a restoration of a Curtiss P-40 fighter aircraft through one of our 
members here which looked like that...until I worked on them a bit. 

Mine still had the original yellow tags on them and the ends were still 
covered with the original chemically impregnated tape, and there was still the 
original grease in the ends and along the length of the inner cable.

> I presume I have cables intended for some other application, where the
> spline is buried much further into the equipment.

No. That is not the problem.
 
> Anyone know if its possible to modify these cables, and if so, how you
> do it?   I hope it can be done, because they are otherwise in good
> condition.

What you will need to do is as someone else here mentioned: 1) throughly oil 
the outer sheath, removing as much corrosion as possible in the process. 

Then, 2) work each "joint" in the spiral of the sheath back and forth several 
times, over the entire length of the sheath. While doing this, you will hear or 
feel something "letting go". Each "joint" may even snap or crack as the 
corrosion that locks each joint releases. 

3) When you have finally gotten the sheath as flexible as you possibly can 
make it, lock one end in something that you can pull against, and pull the 
outer sheath gently but firmly until its over-all length extends to the "correct" 
length. 

You may have to oil it and "work" it repeatedly until you get enough "stretch" 
(it isn't really "stretching", by the way). You may also wish to work the sheath 
up and down over its entire length as you pull on it.

When those cables were made, the outer sheath and the inner cable were 
made correctly and about the same length. Over the many years since, the 
natural springiness of the sheath caused them to become compressed or 
"pulled together", and corrosion then locked them in that position. What you 
are doing is restoring them to their natural length against their natural 
springiness and the corrosion.

But be careful: you don't want any of the spiral joints to completely separate.

Please don't "modify" those. They are getting almost impossibly difficult to 
find.

I have a product called "Evapo-rust" here that I have used to soak such 
cables in. It dissolves not only rust, but other forms of corrosion as well, and 
made my cables much easier to work with. When I finished with them, they 
looked almost new.

Ken W7EKB