Considering putting in a Zeus 2001 axles and cones into a 1979 Campagnolo Record. Original need for an axle is an extra 5mm to add an extra speed. Wonder if this would compromise the bearing/cone friction?

Can you just get a longer axle with the same thread?
I've been thinking about putting some thinner axles and matching cones into a pair of my favorite hubs so I can use them in my '58 Lenton.

No. As long as the bearing size is the same, the axle set and cones should work in the Campy hub.

I have a 126 mm hub and I need a 120. If you want to swap the axles, PM me.

This is not rocket science. Whatever the OLND you want, the total axle length has to be 9-10mm longer. Example: for 120mm OLD rear hub, you need a 130mm rear axle, that way you end up with 4-5mm of exposed thread on each end. I can post a picture if needed.

You can get one of these from wheels manufacturing, they are very tough and have never seen one break. You can cut them to whatever length you need.

Wheels Manufacturing Hub Axles

Nice link, MA. Good resource. But I notice they don't offer a 130 mm axle in 10 X 26. Is that considered obsolete now?

Cut to desired length.

A few hours after I posted I was questioning the cone/dust cap interface? The cone perimeter size would have to match the Campy or the dirt would get in or rub against the dust cap?