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An introduction to, and resources reference for - using Kadee couplings on UK 4mm rolling stock

Compiled by Tim Pearson

Why would you want to use Kadees?

A lot of people now use Kadee couplers in UK modelling - Kadees have mostly been used in America and are a fairly big commitment to change to if you are a UK modeller - so why would you want to? what are the pros and cons?

  • Much less visibly obtrusive than tension lock couplers
  • Easier to lift models from the layout without complex uncoupling
  • Closer coupling of stock
  • Ability to use delayed uncoupling - which is helpful e.g. in shunting
  • Wide range of coupler fittings and wide range of uncouplers
  • Widely available
  • A growing amount of experience documented on conversion

  • It's not cheap - adding around £2 per item of rolling stock
  • Some UK stock can be hard to convert
  • The coupling is quite height sensitive and so the not all modern NEM pocket fittings are suitable
  • The couplings do need some maintenance from time to time
  • The couplers are designed on an American not a British prototype
  • There are a bewildering array of couplers in the Kadee range and it takes a little time to understand them all
  • They are less good at coupling on a bend than tension lock couplers

For me though the major reason for changing is that tensions lock couplers scream "TOY!" - Look at the Hornby CCT in the following photograph - it's an OK model, one that I am told Hornby have reworked from the Lima tooling that they bought, the paintwork is excellent and whilst there is plenty you could improve it - the coupling is the thing which, to my mind, most offends the eye.

Train Wheel Vehicle Rolling stock Railway

Hornby CCT R6364 as released spring 2007

Introducing the Kadee knuckle coupler

The kadee has a spring loaded knuckle which will couple automatically when two pieces of stock are pushed together. This autocoupling is no better than tension lock couplers. Kadees have a descending curved bend of metal known as a 'trip pin'. When two couplers are joined the trip pins can be forced apart magnetically to allow automatic decoupling. They have a shank which runs backwards and connects to the rolling stock. There are several different types of connection to the rolling stock as described below. The operation of the couplers also allows for 'delayed uncoupling' i.e. you push the trip pins apart at one point in the layout but the actual uncoupling will only take place at a later point where you choose - based on when you finish pushing the stock - see the Kadee site for an animation -

Kadee - The Coupler People

There are lots a and lots of different types of Kadee couplings, so many it can become seriously confusing. For UK modellers, using modern stock, there is one key differentiation between the types that it is worth starting with - what type of shank (connection to the rolling stock) the coupler has. There are 4 Kadee varieties (#17,#18,#19,#20) that come with a (swallowtail) shank suitable to plug straight into the NEM pocket that are often now provided by manufacturers. Here is a #20 -

Sculpture Wood Art Fashion accessory Metal

A Kadee with a 'Swallowtail' fitting suitable for an NEM pocket - in this case a #20

The only differences between each of the four types of NEM Kadee couplers are the lengths of the shank - so #17 is the shortest and #20 is the longest.

The majority of the other types of kadee expect to be screwed or glued onto the rolling stock body - these typically come in four components that need to be assembled and fixed to the stock - the shank fits inside the 'gearbox' along with the centering spring (this is the metal plate shown in the picture and not to be confused with the miniscule springs on the knuckle itself).

This 'gearbox' arrangement allows the coupler to have an amount of rotation, left and right, to allow the stock to negotiate curves.

There are a range of couplers to allow you to choose one which has the right shank length and the right height for the particular stock you are fitting - There are also some choices in shape and style of gearbox which further adds to the complexity of the range. A more recent addition to the kadee range is the #58 which works on exactly the same principle but is even smaller and so less obtrusive still.

If there is a 'default' choice it is the number 5, this is the most widely available and cheapest, and people often describe others in the Kadee range in comparative terms to the #5.

Below you can see the #5 Kadee with each of it's four major components -

Font Gas Auto part Circle Nut

The four components to mount a screw or glue fit Kadee - in this case a #5

You can download the fitting instructions for all Kadees - here is a link for the number 5

The instruction sheet for fitting a Kadee #5

Getting the height and the shank length correct

If it is NEM or not is largely decided by the fitting provided by the RTR manufacturer - that leaves two main variables to sort out - getting the right height and getting the right shank length.

Getting the right shank length is helped by a rule of thumb that seems to work well - you need to line up the rear face of the coupler with an imaginariy line drawn between the leading faces of the buffers. The following photo shows you the line you need to make -

Font Audio equipment Auto part Circle Balance

Rule of thumb for choosing the shank length

In general terms the further your coupler is mounted forward the easier it is for the stock to negotiate bends without causing buffer lock. (American stock, for which the Kadees were originally designed, typically doesn't have buffers and therefore buffer lock is obviously not a problem for them.) Needless to say the downside of mounting too far forward is that you don't get close coupling - one of the things that you were probably trying to achieve - the rule of thumb is a sensible compromise.

Height is where the problems begin - if you are fitting a coupler which has a 'gearbox' like the number 5 then there are a number of choices for getting the correct height - the two obvious ones are (i) to choose a kadee that has a shank offest up or down, and (ii) to fit shims in between the gearbox and rail - many people cut a piece of plastikard of appropriate thickness to use as a shim. To assess if the coupler is at the right height you will need a simple height guage that kadee sell. see below -

Train Wheel Vehicle Motor vehicle Mode of transport

The #205 Height Guage - An essential tool

Where things should be easy, is where stock has been provided with NEM pockets, unfortunately the NEM pockets fitted on some stock is at the wrong height. The relevant standard is the NEM 362 standard - which, although written in German, you can see gives the necessary height dimensions -

MIBA Miniaturbahnen

Now this is all well and good but the NEM 362 is an HO and not an OO standard, so the height isn't really perfect. Nonetheless, it is the closest we have got to a standard, and more recently on Model Rail Forum, the Double O Gauge Association have supported the NEM standard height -

DOGA statement on 362 pocket positioning

So feel free to give RTR manufacturers pain if they provide NEM 362 pockets at anything other than the 8.5mm above the rail height.

Of course what the Kadee range lacks is some NEM 362 shanks which have heights offset in either direction - so perhaps we could encourage Kadee to help by asking them for the same - even though that is a perverse way of fixing the problem.

A collection of useful internet resources for 4mm UK modellers

General Reference -

The list of downloadable instruction sheets from Kadee
American thread pitch and screw diameter sizes explained (often referred to in Kadee documents)

Other guides -

MRMs two comprehensive articles and well illustrated articles on the subject are available free online
The platelayers Society powerpoint presentation one clubs complete switchover - beware big file
A site on converting UK stock, giving a good explanation of different coupler types
A site giving a number of conversions - particular of some slightly older models

Where to buy -

MG sharp
Totally Trains
Express models

A list of sites/forum postings that give specific conversion information for one or more models -

Bachmann locos

Bachmann wagons

Bachmann coaches

Hornby locos

Hornby wagons

Hornby coaches

Heljan locos

Other locos
Other wagons
Other coaches

A forum post from Doug on the latest version of the Kadee electrically operated auto uncoupler - Kadee 309 electrically operated uncoupler

Your Patience and Correspondence Please

I have been away from modelling for 30 years, I have just begun again, I do not claim to be an expert on this (or any) subject... I just have been collecting some resources together, and thought others might find them useful. Please let me know about corrections or additions, especially links to other conversion advice, by commenting below or sending me a message. - Thanks - TimP - August 2007

All text, photos & graphics ©2007 Tim Pearson - All rights reserved.
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