Why are 4mm shrouded banana plugs not a universal fit?

[RedGrittyBrick]

I bought an old Fluke on eBay, the test probes that came with it were pretty worn out (blunt, crusty, bent) so I looked at prices of replacement sets with a few useful accessories (crocs, ic clips). The Fluke sets are expensive (more then I paid for the meter) so I bought an Agilent set.

I can force the Agilent leads in but it takes several minutes of brute force to extract them - I worry about damaging the meter! So they are useless to me (the add-on clips etc are fine though). I tried some cheap unbranded leads (shrouded banana to banana) and these were only slightly better.

Why is this? Is it a common problem? Is there anything I can do to make the leads fit?

[Rerouter]

perhaps its a case of the fluke was not intended to have the standard spacing but rather the designers felt that was ideal spacing (chicken and egg) and it turned out to be so close to the now standard

[codfish28]

You could alway just sand the shroud on the leads down just a bit to help it fit. I had the same problem with a Fluke 8020a that I bought on ebay. I sanded about 0.001 inch off with a very fine sandpaper and now it is a firm fit
but I do not have to fight the leads to remove them.

[BurningTantalum]

If it is any consolation, RGB- The red Fluke lead supplied new with my 1507 insulation tester is such a tight fit in the prod and the croc clip that it is very difficult to insert and remove it.
The black one is fine. I should have returned it under warranty but didn't.
Just today I had an odd test result which I realised was due to the plug not being inserted far enough into the croc clip, so it is a safety issue. I will try the sandpaper method !
BT

[ywara]

There are two different sizes of banana jack. The standard is .166/4.2mm, and has been around since 1924. There are also newer 4mm jacks which are an extremely tight fit for standard plugs. I suspect this was a result of someone rounding it off to 4mm for naming purposes and people took it literally. These are becoming increasingly common as many of the low cost connectors from China appear to have these dimensions.

If you have a plug with the normal four-sided stamped spring members and want to stick it in a 4mm jack, try squeezing each pair of springs in a vise - but not too hard. This usually allows them to mate readily.

edit: This is almost certainly not the problem with an old fluke meter. Oops. If the spring is stiff you may be able to mate/demate the connectors a couple dozen times with any suitable receptacle. If it's the plastic shroud that's interfering, remove any leftover molding flash or just sand it down.

[BurningTantalum]

ywara-  I think that RedGrittyBrick is referring to the plastic shroud having an outer diameter that is slightly too large, not the actual spring contacts. The internal diameter being too small would also cause a problem as it would be tight on the OD of the 'cylinder' contact in the meter body.
Regards, BT

[coppice]

The only aspect of these plugs that is in anyway standardised is the thickness of the pin. Even its length is rather woolly. The plastic pin shield commonly used these days doesn't follow any standards at all. The spacing between pins, for two pin plugs, is not standardised, either.

Although makers have somewhat converged around common dimensions, its still very common for test leads to not fit - sometimes even among models in one maker's range.

[Shock]

Why are my leads so loose... oops wrong thread!

[Techoisseur]

I believe that the basic premise in the topic question is incorrect.
There is no such thing as a "4mm shrouded banana plug".
There are "Banana Plugs" and there are "4 mm Plugs" (shrouded and unshrouded).
"Banana Plugs" are supplied and used on equipment for use in USA.
"4mm Plugs" are supplied for use elsewhere (e.g. Europe).
A "4mm Plug" should never be described as a "Banana Plug".
I suspect that, as ywara points out, "Banana Plugs" have a diameter of 4.2 mm.
"4mm Plugs" have a pin diameter of 4 mm.

There should be no problem if you don't confuse or intermate the two types.

[Gromitt]

I believe that the basic premise in the topic question is incorrect.
There is no such thing as a "4mm shrouded banana plug".
There are "Banana Plugs" and there are "4 mm Plugs" (shrouded and unshrouded).
"Banana Plugs" are supplied and used on equipment for use in USA.
"4mm Plugs" are supplied for use elsewhere (e.g. Europe).
A "4mm Plug" should never be described as a "Banana Plug".
I suspect that, as ywara points out, "Banana Plugs" have a diameter of 4.2 mm.
"4mm Plugs" have a pin diameter of 4 mm.

There should be no problem if you don't confuse or intermate the two types.

Sorry, but "banana plug" is used for 4mm plugs at least in Sweden, Germany and other European countries.

/stefan

[Techoisseur]

I stand corrected  .
I have just discovered that I am using 4mm Banana Plugs (unshrouded) for my loudspeaker connections.
See attached image:
4mm Bananas.jpg

The diameter at the tip of the pins is about 3.96 mm, and the plugs make a good connection to standard 4mm sockets.
I guess the term "Banana Plug" should be reserved for plugs using "banana" shaped spring contacts, and you need to be careful that matching standards are being used for the mating plug and socket contacts (whether imperial 0.166 inch or metric 4.0 mm).