Frequently Asked Questions - Compatibility

Kadee and Sergent Coupler

Q: Do Prototype Couplers mate with conventional HO scale knuckle couplers?

A: No. The knuckle design of the Prototype Couplers coupler is precisely scaled from the prototype. The knuckle profile of a conventional HO scale coupler is simple too far off to accommodate without seriously compromising the authenticity of the design.

Q: Are Prototype Couplers compatible with my 18" radius curves?

A: Yes. The minimum radius is controlled by the amount of side play the coupler has within its draft gear box. Prototype Couplers EC87 couplers fit into existing coupler pockets. Therefore rolling stock equipped with Prototype Couplers will not require curves any broader than with conventional HO scale couplers.

Traditional knuckle couplers that incorporate centering springs require a large gathering range in order to successfully couple on a curve. The gathering range of a coupler indicates the amount by which the couplers can be misaligned and still successfully hook up. Couplers that use centering springs need lots of gathering range because the springs push the couplers out of alignment on curves. The gathering range of Prototype Couplers is about half that of conventional knuckle couplers, but since there are no centering springs to push the couplers out of alignment, curves aren't a problem.

Alignment of Prototype Couplers before a tie is a manual operation, just as it is with the prototype. Because the gathering range is small, this job takes a little practice. This is especially true for track and wheels built to normal NMRA standards, which allow quite a bit of slop between the wheel and track gage. This situation improves when using NMRA fine scale recommended practices and practically disappears with the much tighter P87 standards.

Q: It seems like smaller, true scale size couplers would be likely to slip over one another and disconnect in operation. Is coupler height more critical with true scale size couplers? Will I have to pay more attention to my trackwork where changes in grade occur?

A: No. Actually, the prototypical height of the knuckle on Prototype Couplers is not very different from the height used on traditional HO scale knuckle couplers. This means that Prototype Couplers are no worse than traditional knuckle couplers at handling errors in coupler heights and abrupt grade changes. In fact, Prototype Couplers are actually better at dealing with these issues because they grip each other more tightly and they fit more snugly in the draft gear boxes.

Q: Will your couplers fit in the "scale size" draft gear boxes offered by other manufacturers?

A: The EN87 fits perfectly into the Accurail® Proto:HO box -- and in fact is supplied with that box. An EC87A-135 will fit into the Kadee® #78 type box included on their excellent line of rolling stock by thinning the coupler shank to 0.057" with a file, and slightly and trimming the point that is cast into the mounting post. The shank thinning operation is easy if an AFC assembly fixture is used to hold the coupler while filing. The EC87K can also be adapted to this box, but its mounting hole will have to be enlarged to 0.135" with the RM20 tapered reamer.

Q: Will Prototype Couplers mate with conventional HO scale dummy couplers?

A: Sometimes. Most (but not all) dummy couplers are designed using the profile defined by the NMRA's RP-21.1 issued in 1957. This contour is about 25% larger than an actual scaled down version of the prototype. A little creative work with a file can usually result in a dummy knuckle that will reliably mate with the prototype contour used by the Sergent Engineering Couplers. The EC87DC dummy couplers offered by Prototype Couplers do however mate with Prototype Couplers operational couplers with no modifications.