As mentioned in Part 1 of the Couplers posting, the standard-length Sergent Type E couplers are too short for coupling the second sections together (back-to-back) when American Limited diaphragms are mounted.  I imagine the same problem is encountered when using standard-length Kadee couplers, although I did not try them.

Fortunately, Sergent Engineering offers extended-length couplers: in .100” and .130” sizes.  I ordered one set of each; there are six couplers in each set.  They need to be assembled.  My previous orders have been for factory-assembled standard-length couplers (and I see that they are no longer offer this option).

This will give me three extended-length options over a .060” range to try: .100”+.100”, .100”+.130”, & .130”+.130”.  I was not sure which combination would work best with respect to having good working clearance for the diaphragms when negotiating curves while keeping the diaphragm striker faces closed.  In addition, there is a need to put in some slack between the coupler jaws in order to allow for uncoupling.  So, I made up two of the .100” and two of the .130” variety.

I followed the Sergent instructions for assembling them. It is a pretty straight forward process, although the small size of the parts makes assembly naturally challenging.  In particular, there is a tiny steel ball that needs to be placed before the jaw and then the knuckle halves are glued, capturing the jaw.  Tough enough to see, tough to pick up too.  A pair of non-magnetic tweezers is suggested by Sergent.

The items needed for assembly are seen below.  The No. 11 blade is for scraping away at a surface on the main knuckle where a mold ejector pin mark is, as well as a similar mark on the “wing” of the jaw.  My samples looked quite good: flash-free although I did scrape those pin marks to smooth-out the surfaces.  The tweezers are non-magnetic.  The tooth pick is to burnish the blind hole where the steel ball sits.  I used medium CA, all I had (the instructions call for thick CA).  Oh yes, and the magnifiers…


Although Sergent offers an assembly jig, it isn’t made for the extended couplers, but I managed very well without one.  The knuckle halves are glued using CA.  After assembly, I worked-in the couplers using the method detailed by Sergent.  Without having done this before, I very quickly had sets of couplers made up.


I temporarily attached the diaphragms to the body ends using a little double-sided tape.  Then I installed the extended couplers, in combinations starting with .100”+.100”.   I pushed these two B-units around Fillmore by hand.  The tightest curve I have is 32”.  In the end though, I found that one .100″ extended coupler on one end with a standard length coupler on the other gave very good results.



Too bad that .050″ extended couplers are not offered, so that I could have both couplers the same length.  However the difference on the models is not that noticeable even when the two locomotives are uncoupled.  Life is full of compromises.


I typically airbrush my couplers with Polly Scale Rust under low pressure to minimize the “wetness” of the paint that may foul the working surfaces.  I then apply a shot of thinned Engine Black to tone this down.  Finally, a graphite pencil is run over the contact faces, as per Sergent instructions.  I have yet to do all of this though.

Fancy huh, but how do they work?  Here we have the two second sections coupled…


…the Sergent passenger car uncoupling tool tab end is placed above the couplers and below the diaphragms…


…and, magic!  This is what I love the most about Sergents (besides the scale look, of course) – the uncoupling is simply magic!


Under operations, uncoupling the two locomotives should be easier to do than a locomotive and passenger car.  The reason for this is when one locomotive is backed into the other to put in some coupler slack, the other locomotive won’t roll away due to stiff diaphragm springs or a grade on the road.

FT Tidbit: I am sure that the Demonstrator returned to La Grange at times to have corrections and improvements made over the duration of the tour.  I know of one case when one locomotive returned to the shop while testing on the ATSF (I don’t know which one).  The Santa Fe, though, continued testing the other locomotive as a 2700 hp A-B unit.  They were impressed with the performance, but much preferred to have the A-B-B-A configuration.