The HO KD knuckle has been around for 30 plus years and is considered the defacto standard of the hobby.
Over these years KD has developed a comprehensive system of couplers, mounting methods and hardware to fit virtually every model locomotive or car ever built. Not all locos and cars accept the same coupler.
KD presently produces 40 different coupler and hardware sets in both metal and acetal plastic, conversion kits, shims, washers, coupler height gauge, trip pin pilars, wheelsets, uncouplers and freight cars.
The KD # 5 is the most popular but there is also the new # 58, overall smaller in size and closer to scale dimensions as the common prototype E coupler except for its overall length over the knuckle.The #58 works well with other KD couplers.
Here’s a few sites.
For conversions, www.kadee.com/conv/list.htm
For applications, http;//www.nmra.org/beginner/couplers.html
To see a specific KD product, couplers go to www.kadee.com and scroll down to product # 1-79 and click.
Then click on the specific product #, such as the #58 and it’s picture and specifications will appear.
Should you go KD there are two KD tools that you will need
Trip pin pilars, product # 237, $11.00
Coupler height gauge # 205, $4.00
Also a lubricant,"Greas-em, product # 231, $2.00
You can also see these products by going back to www. kadee.com and scrolling down the product range #s and click.