I recently replaced the couplers on a model of an X class loco with Kadee #156 long-shank "Whisker" couplers, but it didn't work very well because the mounting hole for the draft gear box supplied with the Whisker couplers needs to be closer to the face of the buffer beam.
I contacted the Kadee company by e-mail to point out the discrepancy with the mounting holes, and they suggested I try the #262 draft gear box, which is designed to work with the range of "Whisker" couplers but has the same mounting hole dimensions as the older #5 and similar couplers.
I recently obtained some of the #156 draft gear boxes (20 to a pack, couplers not included) and have now completed the coupler conversion on the X class.
The #262 box has several useful features including:
* narrower than standard draft gear boxes which provides greater clearance for bogie swing on tightish curves,
* mounting hole is the same distance from the front face as standard draft gear boxes, so it is easy to fit or retrofit Whisker couplers to rolling stock designed for #5 couplers or similar,
* minimal vertical slop, so coupler height is more precise,
* snap-together design, so no gluing required.
If you are not familiar with Kadee Whisker couplers, these have very fine wire springs permanently attached to the coupler shank to provide centering, rather than the spring formerd from phosphor bronze sheet which is standard for #5 and similar couplers. The "Whisker" centering springs require less force to move the couplers sideways, so they work better with magnetic uncouplers, especially for delayed uncoupling.
Whisker couplers are easier to assemble as they don't require a separate centering spring, and the coupler boxes, either the supplied type (#242) or #262 type, snap together so the coupler stays together without gluing. Whisker couplers are available in both standard size heads (same as #5) and "scale" size heads, as well as several different shank lengths. For example, #158 has the "scale" head with the common, medium length shank, and #156 has the "scale" head with a long shank. The long shank couplers are particularly useful for WAGR steam and diesel locos with cowcatchers as they space adjacent wagons far enough away so the opposing "glad hand" does not get caught in the cowcatcher which can cause uncoupling or derailment.
Also, the rear of the draft gear box can be shortened by about 3 mm, without affecting operation, which is often useful to provide greater clearance for the front bogie of a steam loco, or for long bogie wagons or coaches. In these situations, there is an even smaller draft gear box, the #252, which can also be used.
The #252 and #262 draft gear boxes will not work with non-Whisker couplers, as there is insufficient room for the phosphor bronze centering spring. The #242 draft gear box will work with #5 and similar couplers, as well as with whisker couplers. Incidentally, the shanks on the Whisker couplers are slightly thicker so they are equivalent to a #5 shank plus the thickness of the #5 style phosphor bronze centering spring, which is why the #242 draft gear box will work with either style of coupler.
If you are a prolific wagon builder, it is also possible to buy the common Kadee couplers such as #158 in bulk packs of 50 or 100, without draft gear boxes, and buy the desired draft gear boxes separately. This actually works out cheaper than buying them 4 at a time.