I was using Symoba close coupler mechanisms for cars (
http://www.symoba-schniering.de/ ), replacing original couplers and super glue the mechanism. There are several pocket lengths available. I bought my stuff from Axel Schnug, friendly and dependable (
http://www.schnug-modellbahn.de/index.html ).
In a series of MM articles, Axel Schnug describes also the retrofitting of earlier locomotive models with close couplings:
https://www.maerklin.de/...emen-specials-und-serienSearch for: "Alte Modelle - kurz gekuppelt (MM 04/2002 bis 04/2006)".
Inter alia, you searched for Märklin 3054, the 103 electric loco.
Axel Schnug's MM article "Alte Modelle kurz gekuppelt", episode 2, MM 5/2002, page 50 describes the conversion of such an engine. Link:
http://medienpdb.maerkli...ls/Kurz_gekuppelt_02.pdfTranslation of the German description (
translated with DeepL.com (free version)):
"Conversion of the 103 series
Although the work on the 110 and 140 locomotives is pleasingly simple, the BR 103 locomotive, on the other hand, unfortunately does not make it easy for the prospective rebuilder. Although both the matching bogie frames (440640 in black) and the required drawbar (440630) are available as spare parts, if you fit these to an older locomotive, the installation height of the coupling will no longer fit. A connection with wagons is then not possible. The reason for this is the modified motor bogie and the bogie frame: The illustration shows the difference. Three conversion options were examined: The bogie frame with the drawbar 404020, on which the springs were cut off, can be seen on the left in the picture (Fig. below). Although the reduced coupling distance has a positive visual effect, the coupling sits too low and therefore does not couple well on all wagons. The example on the right shows the conversion with the old frame and a 344890 drawbar fitted. To do this, the springs on the frame have to be cut off. The locomotive thus couples very tightly, but the coupling height is also too low, which is particularly annoying when changing locomotives frequently; nevertheless, this is a labor-saving and cost-effective alternative. In the center of the picture is the bogie frame 440640, which has been adjusted to the required support dimension using a grinding wheel. This conversion requires a little patience, as it is necessary to repeatedly install and remove the frame to check the height during the grinding process. As the frame and drawbars have to be procured from scratch in this case, this method is more expensive than the other two solutions, but it undoubtedly leads to the best result because it both minimizes the coupling distance and leads to the correct height. If a particularly close vehicle connection is required, the drawbar 344890 intended for the 110/140 can also be installed. However, this requires an extensive solo test run of the intended train set in all conceivable directions before it is put into regular service on your own layout! The downer, however, is that in this case the stability of the newly fitted coupling does not match that of the old coupling hook.[...]
Axel Schnug, Holzminden"
Then, you mentioned the Märklin 3021, the V200 diesel. I don't know if this applies by analogy, but Axel Schnug reports on the conversion of a class 216 diesel - scroll down to last picture at the bottom of the page.
http://www.schnug-modell...bauten/adapter671860.htmTranslation of the German description (
translated with DeepL.com (free version)):
"Conversion tip for locomotives with a pivot point: adapter 671860
Conversion of locomotives with a pivot point mount to NEM shaft
Some Märklin locomotives have a coupler pocket with a pivot point. Either a close coupling or a Relex coupling made of plastic with a metal bracket and front uncoupling is fitted here ex works. Often, however, you will only find a simplified coupling without a pre-decoupling function or even just a coupling hook. In any case, this coupling is held in the middle position with 2 springs. On many of these locomotives, the old coupling can be replaced by the adapter 671860 and a coupling head according to NEM. The advantages:
Shortening of the coupling distance
Free choice of coupling head
Option to install a current-carrying coupling.
The adapter can be fitted to these two locomotives without any problems. However, the return springs must be at the bottom when inserting the adapter. If the adapter is inserted the wrong way round, the spring effect is not effective.
However, the adapter does not fit in all locomotives. The BR 515 battery-powered railcars, for example, can only be converted with an exchange coupling from the 7205 pack. I supply this coupling separately under the number 263730. The adapter cannot be fitted to very old models either because the coupling mount is too narrow. This mainly affects class 151 and 216 locomotives. Alternatively, a replacement bogie frame could of course be installed here. The photos below show how to distinguish between the individual types:
[...]
Class 216
Locomotives of this series can usually be optimized with 671860. Very old versions such as this 3075 still have a metal plate on the coupling bracket of the bogie frame. In this case, the frame must be replaced. The individual part numbers are 469620 for the bogie with slider and 393990 for the drive bogie shown. "