Cap sub
 

Can a 1000pf mica be used in place of .001nf 600v in a horizontal circuit ? :scratch2:

Are you sure that's in nanofarads? .001 nf = 1pf. .001 uf = 1000pf.

Yes I have a typical .001 600 v cap that measures to high in NF but the only other cap I have is a 1000pf mica.

I would be cautious about the mica not being able to handle the possibly high current, depending on where in the circuit the cap is. Better to get the original type just to be safe.

I actually posted the wrong voltages, there are 2 .001 caps that are 200v and 400 volt so the Micas I have are 500v so I'm just wondering if it may be ok to use the Micas. These caps I want to use are going in the sweep section and besides I think it's better using Micas instead of ceramic disc which change with temps.

This may be correct, but if the ceramic has a particular temperature coefficient to compensate a coil drift, you should use that.
Edit: if it's just a coupling cap and not a frequency determining part, it makes no difference what the tempco is.
What are the markings on the ceramics?

The 2 caps are not ceramics they are the typical caps you would change when recapping a set they are black or sometimes green or red color.

Then I'd use ordinary 630V yellow film caps. Orange if you need radial leads.