Foundations of Amateur Radio

If you've ever found yourself in the position of attempting to screw a PL259 into an SO239, or an N-type plug into an N-type socket you'll have likely come across the situation where the thread doesn't quite fit. If it does, you might have issues attempting to undo the connection, even if you didn't particularly do anything strenuous in relation to mating the two in the first place.

This kind of situation happens to me more than I think is reasonable. It happens on cheap connectors, on expensive ones, on the back of radio gear, on adaptors, patch leads and the like.

Initially I put this down to cheap vs. expensive, but that really doesn't add up if you're attempting to connect an expensive plug into an expensive radio.

If you're into machining you'll know about swarf. If not, think metallic dust. Of course it doesn't have to be metallic, it could be a single grain of sand, or it could be a slightly damaged thread.

A couple of months ago I went on the hunt for a tap and die set that would solve this issue once and for all. If you're not familiar with the terms, a tap is like a long bolt with a square head and a die is like a thick washer with holes cut out.

In addition to being hardened, they each have cutting edges, which allows these two tools to do their job, the job of cutting threads.

Normally you'd use a tap to make a thread into a hole that you've drilled. You'd use a die to make a thread onto a rod that you have. There's lots of technique associated with this, cutting fluids, alignment, pressure and the like. Plenty of relaxing YouTube videos around - which is how I came upon this idea in the first place.

You can also use a tap or a die to cut across an existing thread and you can do this with connectors.

A die, threaded over a socket, will clean up the socket threads. Similarly a tap screwed into a plug will clean up the plug thread. There's a disclaimer coming for that last point, but stick around.

Trying to find a tap and die to match can be a challenge. The PL259, SO239 and N-type connectors are all 5/8th size threads. They're 24 turns per inch, and also known as UNEF (Uniform November Echo Foxtrot) threads, or Unified Extra Fine.

So if you start on your hunt, you'll be looking for 5/8th, 24 TPI, UNEF taps and dies.

I found mine online at $15 or so from a US supplier. Got to me in about a week.

When they arrived I immediately set about cleaning up all my sockets. This was amazing, all of a sudden stuff started fitting well. Unfortunately I couldn't use the tap. The centre hole in a standard tap isn't big enough for the pin of a PL259, let alone an N-type connector, but a friend of a friend has access to machine tools and made the centre hole bigger. Word of warning, this is hardened steel. A hand-drill won't cut it.

I must mention that this won't allow you to use the tap inside an N-type plug, but you can use a die on the socket.

I'll also point out that if you need to use a tap wrench or a die holder, you're doing it wrong. We're cleaning up the thread, not making a new one. If you need extra force the most likely scenario is that you've cross threaded the tool onto the connector.

Of course if you've got a completely stuffed connector thread, then these tools can help, but you might want to consider replacing the connector.

My tap and die live in my go-kit right next to the coax adaptors. On my next field-day I won't be having to deal with poor connections, nor will I have to worry about unscrewing them after the event.

A tap and die, great simple tools to fix a recurring issue.

I'm Onno VK6FLAB